The Role of Downhole Measurements in Marine Geology and ...

The Role of Downhole Measurements in Marine Geology and ...

The Role of Downhole Measurements in Marine Geology and ...


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David Goldberg<br />

Borehole Research Group, Lamont-Doherty Earth<br />

Observatory, Palisades, New York<br />

Abstract. Dur<strong>in</strong>g the last 25 years, downhole measure- core data with regional geophysical surveys <strong>and</strong> <strong>in</strong> proments<br />

have been <strong>in</strong>creas<strong>in</strong>gly used for scientific applica- vid<strong>in</strong>g data where core sections could not be obta<strong>in</strong>ed.<br />

tions <strong>in</strong> mar<strong>in</strong>e geology <strong>and</strong> geophysics, particularly <strong>in</strong> Examples <strong>of</strong> recent scientific applications <strong>and</strong> apdeep-sea<br />

drill<strong>in</strong>g operations. Used mostly by the oil proaches are presented that address previous problems<br />

<strong>in</strong>dustry to map promis<strong>in</strong>g formations for exploration with data quality <strong>and</strong> changes <strong>in</strong> properties over time<br />

<strong>and</strong> production <strong>of</strong> hydrocarbons, a variety <strong>of</strong> <strong>in</strong>stru- after a hole is drilled. <strong>The</strong> role <strong>of</strong> downhole measurements<br />

have been developed that can be lowered down ments is discussed for two broad areas <strong>of</strong> research: the<br />

drill holes to extract <strong>in</strong>formation about the subsurface structure <strong>and</strong> composition <strong>of</strong> the Earth's crust, most <strong>of</strong><br />

geology. In the last decade, advances <strong>in</strong> computers, which is formed at mid-ocean ridges, <strong>and</strong> past changes <strong>in</strong><br />

s<strong>of</strong>tware, <strong>and</strong> data transmission have greatly <strong>in</strong>creased Earth's environment recorded <strong>in</strong> the deep-sea sediments<br />

the amount <strong>and</strong> quality <strong>of</strong> data that such <strong>in</strong>struments can overly<strong>in</strong>g the crust. F<strong>in</strong>ally, new emerg<strong>in</strong>g technologies<br />

provide. Relatively new <strong>in</strong>struments that image the bore- <strong>and</strong> experiments that promise significant advantages<br />

hole wall with high resolution can reveal layers <strong>and</strong> over current methods for downhole measurements <strong>in</strong><br />

faults that previously could be seen only <strong>in</strong> core sections. mar<strong>in</strong>e geology <strong>and</strong> geophysics are discussed.<br />

<strong>Downhole</strong> measurements play a crucial role <strong>in</strong> l<strong>in</strong>k<strong>in</strong>g<br />


<strong>The</strong> scientific use <strong>of</strong> downhole measurements <strong>in</strong> ma-<br />

r<strong>in</strong>e geology <strong>and</strong> geophysics has become <strong>in</strong>creas<strong>in</strong>gly<br />

important <strong>in</strong> recent years. <strong>The</strong> methods <strong>and</strong> tools are<br />

derived largely from those developed for oil <strong>and</strong> gas<br />

exploration <strong>and</strong> are applied to recent scientific problems<br />

<strong>in</strong> the Earth's' oceans. <strong>The</strong> purpose <strong>of</strong> this review is<br />

use <strong>of</strong> logg<strong>in</strong>g is due to several causes, <strong>in</strong>clud<strong>in</strong>g permanent<br />

shipboard systems for rout<strong>in</strong>e operations, vast improvements<br />

<strong>in</strong> downhole <strong>in</strong>strumentation technology<br />

<strong>and</strong> drill<strong>in</strong>g methods, <strong>and</strong> new measurements made on<br />

core samples that allow for one-to-one correlation with<br />

similar measurements made downhole.<br />

<strong>The</strong>re have been numerous published discussions <strong>of</strong><br />

the scientific goals <strong>and</strong> overviews <strong>of</strong> DSDP <strong>and</strong> ODP<br />

tw<strong>of</strong>old: first, to present applications <strong>of</strong> state-<strong>of</strong>-the-art that <strong>in</strong>clude elements <strong>of</strong> specific downhole measurement<br />

downhole measurements <strong>in</strong> recent mar<strong>in</strong>e science prob- capabilities, their successes, <strong>and</strong> their failures. For genlems,<br />

<strong>and</strong> second, to review both exist<strong>in</strong>g <strong>and</strong> new meth- eral <strong>and</strong> historical background <strong>of</strong> the DSDP <strong>and</strong> ODP,<br />

odologies for downhole measurements with new scien- the <strong>in</strong>terested reader is referred to Revelle [1981] <strong>and</strong> the<br />

tific directions. Dur<strong>in</strong>g the last 25 years, the Deep-Sea proceed<strong>in</strong>gs <strong>of</strong> the International Conference on Scien-<br />

Drill<strong>in</strong>g Project (DSDP) <strong>and</strong> its successor s<strong>in</strong>ce 1984, the tific Ocean Drill<strong>in</strong>g (COSOD) <strong>in</strong> 1987 [Jo<strong>in</strong>t Oceano-<br />

Ocean Drill<strong>in</strong>g Program (ODP), have been progressively graphic Institutions for Deep Earth Sampl<strong>in</strong>g <strong>and</strong> Jo<strong>in</strong>t<br />

exp<strong>and</strong><strong>in</strong>g the role <strong>of</strong> downhole measurements. <strong>The</strong>se Oceanographic Institutions, Inc., 1987]. For more deprograms<br />

have successfully fulfilled their scientific mis- tailed <strong>in</strong>formation on specific drill<strong>in</strong>g locations, the Inisions<br />

by drill<strong>in</strong>g holes <strong>in</strong> nearly all the different geologic tial Reports <strong>and</strong> Scientific Results <strong>of</strong> the ODP <strong>and</strong><br />

sett<strong>in</strong>gs <strong>of</strong> the world's oceans. A total <strong>of</strong> 170 drill<strong>in</strong>g DSDP <strong>of</strong>fer summaries, <strong>and</strong> various monographs <strong>of</strong> the<br />

expeditions, or "legs," around the world's oceans have American Geophysical Union present synthesis articles<br />

been successfully completed at the time <strong>of</strong> this writ<strong>in</strong>g. for sites around the globe. For a more detailed discus-<br />

More than 1000 drill holes have been cored, after which sion <strong>of</strong> particular downhole measurements, a number <strong>of</strong><br />

many have <strong>in</strong> turn been logged us<strong>in</strong>g downhole <strong>in</strong>stru- smaller proceed<strong>in</strong>gs outl<strong>in</strong>e various specific applications<br />

ments. While downhole measurements were conducted [e.g., Worth<strong>in</strong>gton et al., 1987; Hyndman, 1991; Cullen,<br />

<strong>in</strong> less than 14% <strong>of</strong> all mar<strong>in</strong>e holes drilled dur<strong>in</strong>g the 1994]. Reviews <strong>of</strong> current downhole technologies are<br />

DSDP, they have been made <strong>in</strong> more than 56% <strong>of</strong> the also periodically published <strong>in</strong> <strong>in</strong>dustry journals [e.g.,<br />

holes drilled by ODP. This dramatic <strong>in</strong>crease <strong>in</strong> ODP's Snyder <strong>and</strong> Flem<strong>in</strong>g, 1985; Prensky, 1994].<br />

Copyright 1997 by the American Geophysical Union.<br />

8755-1209/97/97 RG-00221 $15.00<br />

e315e<br />

Reviews <strong>of</strong> Geophysics, 35, 3 / August 1997<br />

pages 31 5-342<br />

Paper number 97RG00221


Cont<strong>in</strong>ental scientific drill<strong>in</strong>g programs have also suc- "an ODP borehole is a scientific legacy; it is not a mere<br />

cessfully used downhole measurements to achieve their relic <strong>of</strong> a core acquisition procedure. Scientific measureobjectives.<br />

<strong>Downhole</strong> experiments support <strong>and</strong> enhance ments <strong>in</strong> boreholes <strong>and</strong> on recovered core should be<br />

core-related studies <strong>of</strong> the subsurface <strong>in</strong> almost every planned on the basis <strong>of</strong> their <strong>in</strong>corporation <strong>in</strong>to a reenvironment<br />

<strong>and</strong> <strong>in</strong> nearly every scientific discipl<strong>in</strong>e. gional or global model, their future re<strong>in</strong>terpretation <strong>and</strong>,<br />

Dur<strong>in</strong>g the last decade, several cont<strong>in</strong>ental scientific <strong>in</strong> some cases, the reoccupation <strong>of</strong> the drill site for<br />

drill<strong>in</strong>g programs conducted by Germany, Japan, Swe- further <strong>in</strong>vestigations." Given these views <strong>of</strong> the future,<br />

den, the United States, Russia, <strong>and</strong> Ukra<strong>in</strong>e have relied the summary provided here <strong>of</strong> recent scientific applicaon<br />

the extensive use <strong>of</strong> downhole measurements tions <strong>and</strong> new methods for downhole measurements<br />

[Zoback et al., 1994]. Often the results <strong>of</strong> these efforts leads to the conclusion that downhole data are irreplacecomplement<br />

the goals <strong>of</strong> the ODP <strong>in</strong> advanc<strong>in</strong>g new able assets <strong>in</strong> mar<strong>in</strong>e geology <strong>and</strong> geophysics; plann<strong>in</strong>g<br />

scientific applications <strong>of</strong> downhole measurements, <strong>and</strong> <strong>and</strong> use <strong>of</strong> downhole experiments <strong>in</strong> the future should<br />

together, these programs are mov<strong>in</strong>g researchers toward exp<strong>and</strong>, both while drill<strong>in</strong>g <strong>and</strong> <strong>in</strong> exist<strong>in</strong>g holes. <strong>The</strong><br />

a global scientific <strong>in</strong>vestigation us<strong>in</strong>g downhole <strong>in</strong>stru- scientific legacy <strong>of</strong> new downhole data will undoubtedly<br />

ments to measure <strong>in</strong> situ properties <strong>of</strong> the Earth. cont<strong>in</strong>ue to advance mar<strong>in</strong>e science <strong>in</strong> several disci-<br />

<strong>The</strong> scope <strong>of</strong> this review focuses on past, present, <strong>and</strong><br />

future scientific applications that have used or will use<br />

"short-term" experiments, that is, measurements that<br />

pl<strong>in</strong>es.<br />

themselves do not require <strong>in</strong>struments to be deployed<br />

for more than several hours or days <strong>in</strong> the subseafloor.<br />


Such measurements can be repeated over longer periods Dur<strong>in</strong>g the previous decade, downhole measurements<br />

for time series studies. This class <strong>of</strong> downhole measure- have become viewed as essential <strong>and</strong> complementary to<br />

ments is commonly referred to as "logg<strong>in</strong>g" <strong>and</strong> is dis- measurements made on recovered core. <strong>The</strong>y are critical<br />

t<strong>in</strong>ct from the class <strong>of</strong> <strong>in</strong>struments deployed below the for measurements, such as temperature, that must be<br />

seafloor for long-term studies. <strong>The</strong> latter class may be made <strong>in</strong> situ. <strong>The</strong> scientific objectives at most study sites<br />

referred to as <strong>in</strong> situ "observatories" <strong>and</strong> is discussed <strong>in</strong> are addressed with a multidiscipl<strong>in</strong>ary strategy, <strong>in</strong>tegratless<br />

detail (the reader is referred to recent workshop <strong>in</strong>g measurements made on core samples with those<br />

reports by Hyndman [1991] <strong>and</strong> Carson et al. [1994] for made downhole <strong>and</strong> plac<strong>in</strong>g both <strong>in</strong> the context <strong>of</strong> rean<br />

overview <strong>of</strong> <strong>and</strong> further references to a variety <strong>of</strong> gional geophysical <strong>and</strong> seismic studies. Worth<strong>in</strong>gton et al.<br />

downhole observatory applications <strong>in</strong> mar<strong>in</strong>e research). [1991] summarize this multiscale approach to drill<strong>in</strong>g<br />

<strong>The</strong> reason for this dist<strong>in</strong>ction is methodological, not <strong>in</strong>vestigations <strong>and</strong> suggest that data <strong>in</strong>tegration will be<br />

scientific, as both types <strong>of</strong> deployments are complemen- the hallmark <strong>of</strong> the geosciences <strong>in</strong> the 1990s. Figure 1<br />

tary. <strong>The</strong> short-term logg<strong>in</strong>g measurements that are pri- shows their illustration <strong>of</strong> the range <strong>of</strong> experimental<br />

marily addressed <strong>in</strong>clude (1) measurements made <strong>in</strong> a scales <strong>of</strong> <strong>in</strong>vestigation used today. <strong>The</strong> multiple scales <strong>of</strong><br />

borehole by <strong>in</strong>struments lowered on a wirel<strong>in</strong>e, (2) mea- <strong>in</strong>vestigation used with seismic, downhole, <strong>and</strong> core data<br />

surements made while drill<strong>in</strong>g, <strong>and</strong> (3) measurements acquired <strong>in</strong> the same geological environment complerepeated<br />

over time. A short background is given first to ment each other extremely well. Seismic sections are the<br />

def<strong>in</strong>e the current types <strong>and</strong> range <strong>of</strong> measurements that basis for a regional description, downhole measurements<br />

can be made by logg<strong>in</strong>g. <strong>The</strong>n a discussion <strong>of</strong> strategies are <strong>of</strong> an <strong>in</strong>termediate scale <strong>and</strong> give cont<strong>in</strong>uous <strong>in</strong>forfor<br />

downhole measurements is presented, followed by a mation <strong>in</strong> the region surround<strong>in</strong>g the borehole, <strong>and</strong> core<br />

discussion <strong>of</strong> various scientific achievements <strong>of</strong> down- samples provide detailed <strong>in</strong>formation on physical prophole<br />

measurements over the past several years. With<strong>in</strong> erties <strong>and</strong> age. <strong>Downhole</strong>, core, <strong>and</strong> seismic data used<br />

this last section, two broad discipl<strong>in</strong>ary areas are dis- jo<strong>in</strong>tly also contribute to the confidence <strong>in</strong> each <strong>in</strong>dividcussed:<br />

the Earth's crust <strong>and</strong> Earth's environment. <strong>The</strong> ual data set. Unlike measurements on core samples,<br />

applications <strong>of</strong> many different downhole measurements which are <strong>of</strong>ten disturbed dur<strong>in</strong>g the process <strong>of</strong> recovery,<br />

fall <strong>in</strong>to these two categories, both <strong>of</strong> which conta<strong>in</strong> downhole data provide a set <strong>of</strong> cont<strong>in</strong>uous logs <strong>of</strong> <strong>in</strong>forseveral<br />

subdiscipl<strong>in</strong>es that illustrate the broad range <strong>of</strong> mation <strong>and</strong> sample a larger volume <strong>of</strong> rock than core<br />

questions which can be addressed us<strong>in</strong>g downhole meth- measurements. Because logs have much greater vertical<br />

ods.<br />

resolution than surface data but little lateral resolution,<br />

<strong>The</strong> scientific objectives for the future <strong>of</strong> mar<strong>in</strong>e the comb<strong>in</strong>ation <strong>of</strong> the two def<strong>in</strong>es subsurface geological<br />

scientific drill<strong>in</strong>g have recently been outl<strong>in</strong>ed <strong>in</strong> the ODP structures far better than either data type can alone. <strong>The</strong><br />

Long Range Plan [Jo<strong>in</strong>t Oceanographic Institutions, Inc. difference <strong>in</strong> the scale <strong>of</strong> physical phenomena affect<strong>in</strong>g<br />

(JOI), 1996]. This plan describes the directions for ma- each type <strong>of</strong> measurement may be extreme. <strong>The</strong> scale<br />

r<strong>in</strong>e scientific drill<strong>in</strong>g from now through 2008 <strong>and</strong> the ratio from core to log may be greater than 2 x 103; the<br />

new technologies that will be required to accomplish ratio from log to seismic section may be 106 to 107 times<br />

them. <strong>The</strong> plan relies heavily on downhole experiments larger. In most <strong>in</strong>tegrated scientific applications, thereto<br />

achieve its scientific objectives. It also ma<strong>in</strong>ta<strong>in</strong>s the fore, downhole measurements provide three complehistorical<br />

premise that [Worth<strong>in</strong>gton et al., 1987, p. 135] mentary advantages: (1) data are acquired under <strong>in</strong> situ


Figure 1. Schematic diagram illustrat<strong>in</strong>g the different scales <strong>of</strong> measurement <strong>in</strong> geophysics [after Worth<strong>in</strong>gton<br />

et al., 1991]. <strong>The</strong> span <strong>of</strong> measurements from core samples to seismic survey<strong>in</strong>g is greater than 10 4,<br />

complicat<strong>in</strong>g the <strong>in</strong>terpretation <strong>of</strong> data from samples to regional geology without <strong>in</strong>termediate-scale logg<strong>in</strong>g<br />

<strong>and</strong> borehole measurements.<br />

conditions, (2) data are acquired <strong>in</strong> cont<strong>in</strong>uous pr<strong>of</strong>iles<br />

measured throughout the <strong>in</strong>terval with no miss<strong>in</strong>g sections,<br />

<strong>and</strong> (3) data are sampled at a larger scale, <strong>in</strong>termediate<br />

between core <strong>and</strong> seismic measurements. 460<br />

In most drill<strong>in</strong>g environments, cont<strong>in</strong>uous cor<strong>in</strong>g does<br />

not result <strong>in</strong> cont<strong>in</strong>uous core recovery. In fact, core<br />

recovery by techniques other than piston cor<strong>in</strong>g is less 470<br />

than 50% on average, <strong>and</strong> this proportion is <strong>of</strong>ten disturbed<br />

by the drill<strong>in</strong>g process [Hyndman, 1991]. As a<br />

result, the true core depth becomes ambiguous. Drill<strong>in</strong>g 480<br />

disturbances with<strong>in</strong> a recovered section can be cor-<br />

rected, however, by correlation with cont<strong>in</strong>uous logs, so<br />

that preferential recovery <strong>of</strong> particular rock types can be<br />

documented. In Figure 2 a nonrepresentative section <strong>of</strong><br />

<strong>in</strong>terlayered basalt <strong>and</strong> sediment near the Juan de Fuca<br />

490<br />

ridge <strong>in</strong> the northeastern Pacific would have been <strong>in</strong>terpreted<br />

from the recovered core alone, but the cont<strong>in</strong>uous<br />

log pr<strong>of</strong>iles reveal layers <strong>of</strong> sediment between basalt<br />

s00<br />

that were not recovered through cor<strong>in</strong>g. Despite such<br />

successes, complete recovery <strong>of</strong> cont<strong>in</strong>uous log pr<strong>of</strong>iles<br />

is still not possible. With current wirel<strong>in</strong>e technology, the<br />

s0<br />

<strong>in</strong>terval immediately below the seafloor is not logged<br />

because the drill pipe must be lowered 80-100 m to<br />

ensure hole stability <strong>in</strong> the s<strong>of</strong>test sediments for logg<strong>in</strong>g<br />

s20<br />

to beg<strong>in</strong>. To benefit from core-log correlation, the stratigraphic<br />

<strong>in</strong>terpretation from logs is limited to below this<br />

depth <strong>and</strong> only where high-quality data are recorded.<br />

s30<br />

ß<br />

<strong>Downhole</strong> <strong>Measurements</strong>: Logg<strong>in</strong>g<br />

In 1927, C. <strong>and</strong> M. Schlumberger made the first well<br />

log near Paris. It was a simple electrical current experiment<br />

that used an electrode placed at a series <strong>of</strong> horizontal<br />

po<strong>in</strong>ts on the ground to make measurements <strong>and</strong><br />

detect variations <strong>in</strong> geological structure below the sur-<br />

Resistivity Gamma-ray<br />

Intensity<br />

(ohm m) (raw counts)<br />

0.1 1.0 10.0 0 20 40<br />

....<br />

-"::i i<br />

....<br />

::=:<br />

::::<br />

:=::<br />

::::<br />

.........<br />

: : :: .. ::::<br />

: : ::: ::::<br />

: : : ::::.<br />

: : : ::::<br />

: : : ::::<br />

: : : ::::<br />

Hole 857C<br />

Figure 2. Logs <strong>of</strong> electrical resistivity <strong>and</strong> natural gamma<br />

radiation <strong>in</strong> a layered basalt-sediment sequence <strong>in</strong> ODP Hole<br />

857C near the Juan de Fuca ridge <strong>in</strong> the north eastern Pacific<br />

[from ODP Leg 139 Scientific Drill<strong>in</strong>g Party, 1992]. Core recovery<br />

(black zones) is partial <strong>and</strong> is arbitrarily set at the top <strong>of</strong><br />

each core section, bias<strong>in</strong>g any subsequent geologic <strong>in</strong>terpretation.<br />

<strong>The</strong> complete <strong>in</strong>terpretation (dotted zones are basalt) is<br />

based on the logs. Depth is <strong>in</strong> meters below sea floor (bsf).<br />



lOOO<br />

lOO<br />

n- 10<br />

E 1<br />

E<br />

i i i i<br />

,..,,,,, DSI 500<br />

_/- m 200<br />

FMS 65<br />

_ ß _ _<br />

ß HDT 1.0 ß Cable telemetry (80 Kbits/sec) -<br />

ß Digital telemetry (66 Kbits/sec)<br />

0.1 I I I I<br />

1970 1975 1980 1985 1990 1995<br />

Year Tool Introduced<br />

face. Dur<strong>in</strong>g the 1930s, they evolved this method <strong>in</strong>to a<br />

simple tool consist<strong>in</strong>g <strong>of</strong> an electrode <strong>and</strong> a current<br />

source that made cont<strong>in</strong>uous measurements <strong>of</strong> the sub-<br />

surface resistivity with each lower<strong>in</strong>g <strong>in</strong>to a borehole.<br />

Depth was determ<strong>in</strong>ed by measur<strong>in</strong>g the length <strong>of</strong> the<br />

cable run <strong>in</strong>to the hole, as it is today. <strong>The</strong> <strong>in</strong>terpretation<br />

<strong>of</strong> these cont<strong>in</strong>uous logs for oil exploration helped to<br />

determ<strong>in</strong>e bed thicknesses, identify clay-rich <strong>and</strong> hydrocarbon-bear<strong>in</strong>g<br />

zones, <strong>and</strong> provided a rough estimate <strong>of</strong><br />

formation permeability, all qualities that could be correlated<br />

between holes. In the 1940s, Archie [1942] l<strong>in</strong>ked<br />

laboratory measurements <strong>of</strong> resistivity to the amount <strong>of</strong><br />

water <strong>and</strong> hydrocarbon <strong>in</strong> pore space, which greatly<br />

so that decisions can be made on site to enhance data<br />

Figure 3. Telemetry rates for common wirel<strong>in</strong>e<br />

tools show<strong>in</strong>g the steady <strong>in</strong>crease <strong>in</strong> data rates<br />

over the past 25 years (after Prensky [1994];<br />

data from Schlumberger). See Table 1 for list<br />

<strong>of</strong> abbreviations.<br />

<strong>of</strong> a number <strong>of</strong> commercial <strong>in</strong>struments over the past 10<br />

years. Advances <strong>in</strong> acquir<strong>in</strong>g scientific data for mar<strong>in</strong>e<br />

geology <strong>and</strong> geophysics have followed closely from these<br />

new technological developments.<br />

Prensky [1994] provides an excellent summary <strong>of</strong> current<br />

downhole technologies; Doveton [1986], Serra<br />

[1987], Ellis [1987], <strong>and</strong> the Borehole Research Group<br />

[1990], among others, have summarized the pr<strong>in</strong>ciples <strong>of</strong><br />

downhole measurements <strong>and</strong> the <strong>in</strong>terpretation <strong>of</strong> logg<strong>in</strong>g<br />

data <strong>in</strong> scientific applications. <strong>The</strong> <strong>in</strong>terested reader<br />

is referred to these publications for a more detailed<br />

discussion. <strong>The</strong> follow<strong>in</strong>g summary <strong>of</strong> downhole logg<strong>in</strong>g<br />

improved log <strong>in</strong>terpretation. In various modifications his<br />

measurements is <strong>in</strong>cluded only to present a general<br />

overview <strong>of</strong> the methods used <strong>and</strong> the range <strong>of</strong> data <strong>and</strong><br />

empirical relationship is still used to calculate porosity their measurement resolutions.<br />

<strong>and</strong> fluid saturation from resistivity logs. In the mid- Typically, logs made downhole fall <strong>in</strong>to three general<br />

1940s through the 1970s, rapid technological advances categories: electrical, nuclear, <strong>and</strong> acoustic. In addition,<br />

made it possible to estimate porosity us<strong>in</strong>g nuclear acti- borehole imag<strong>in</strong>g, temperature, <strong>and</strong> various other <strong>in</strong> situ<br />

vation, gamma radiation, <strong>and</strong> acoustic techniques. <strong>The</strong> properties can be measured downhole us<strong>in</strong>g wirel<strong>in</strong>e<br />

resolution <strong>of</strong> the logg<strong>in</strong>g measurements also improved logg<strong>in</strong>g tools. Although the accuracy, resolution, <strong>and</strong><br />

with the development <strong>of</strong> sensors that extended close to applications differ for each type <strong>of</strong> measurement, tothe<br />

borehole wall. In the 1980s, cont<strong>in</strong>ued advances <strong>in</strong> gether they provide a comprehensive data set that can be<br />

digital data acquisition <strong>and</strong> signal process<strong>in</strong>g have made used as a proxy for the subsurface geology. Each type <strong>of</strong><br />

it possible to evaluate logg<strong>in</strong>g data as they are collected, measurement is discussed briefly below; a summary <strong>of</strong><br />

the vertical resolutions <strong>of</strong> several common devices which<br />

quality <strong>and</strong> <strong>in</strong>terpretation. With these improvements, range from a few millimeters to over a meter is prelogg<strong>in</strong>g<br />

has become the st<strong>and</strong>ard for evaluat<strong>in</strong>g subsur- sented <strong>in</strong> Figure 4 (see Table 1 for abbreviations). Most<br />

face geology <strong>in</strong> the oil <strong>in</strong>dustry; it is now <strong>of</strong>ten the sole typical logg<strong>in</strong>g devices have a vertical resolution <strong>of</strong> at<br />

source <strong>of</strong> data used for geological <strong>in</strong>terpretation because least 0.5 m, so that beds th<strong>in</strong>ner than this are difficult to<br />

<strong>of</strong> the difficulty <strong>and</strong> expense <strong>of</strong> rout<strong>in</strong>e cor<strong>in</strong>g. study [Allen et al., 1988; Tittman, 1991].<br />

Over the past 10 years, researchers <strong>in</strong> academia <strong>and</strong> To maximize the vertical resolution <strong>of</strong> logg<strong>in</strong>g data, it<br />

<strong>in</strong>dustry have steadily improved the accuracy <strong>and</strong> sophis- is important to m<strong>in</strong>imize effects that may <strong>in</strong>troduce<br />

tication <strong>of</strong> geophysical <strong>and</strong> geochemical logg<strong>in</strong>g mea- additional uncerta<strong>in</strong>ties <strong>in</strong> the correlation between the<br />

surements. Most downhole experiments use technology recorded data <strong>and</strong> depth, such as motion due to ship<br />

developed by <strong>in</strong>dustry; a small, yet significant, number heave. In 1988 a hydraulic heave compensator that<br />

use technologies developed for scientific research. <strong>The</strong> moves the wirel<strong>in</strong>e opposite to heave motion while logrecent<br />

history <strong>of</strong> advances <strong>in</strong> all <strong>of</strong> these technologies g<strong>in</strong>g was first developed <strong>and</strong> used on the ODP drill ship.<br />

parallels advances <strong>in</strong> data transmission <strong>and</strong> computer Goldberg [1990] measured the effectiveness <strong>of</strong> this wirecapabilities,<br />

which have led to a remarkable <strong>in</strong>crease <strong>in</strong> l<strong>in</strong>e heave compensator by compar<strong>in</strong>g measurements <strong>of</strong><br />

the quantity <strong>and</strong> speed <strong>of</strong> downhole data acquisition <strong>and</strong> the acceleration <strong>of</strong> a downhole tool with the displaceprocess<strong>in</strong>g<br />

[Prensky, 1994]. Figure 3 shows how <strong>in</strong>creases ment <strong>of</strong> the motion compensator on the ship. <strong>The</strong> mea<strong>in</strong><br />

data transmission have accompanied the <strong>in</strong>troduction surements <strong>in</strong>dicate that heave amplitudes reach about


2.5 m with vertical accelerations <strong>of</strong> up to 8% <strong>of</strong> G <strong>in</strong> seas<br />

TABLE 1. Abbreviations for <strong>Downhole</strong> Tools <strong>and</strong> Logs<br />

with 3-4 m waves. As the ship's dom<strong>in</strong>ant period was<br />

about 8.5 s, the maximum downhole displacement <strong>and</strong><br />

Abbreviation Def<strong>in</strong>ition<br />

velocity due to heave were reduced to 1.5 m <strong>and</strong> 1.0 m/s,<br />

respectively, us<strong>in</strong>g the wirel<strong>in</strong>e compensator. S<strong>in</strong>ce typ-<br />

AIT*<br />

BHTV<br />

Array Induction Imager Tool<br />

Borehole Televiewer<br />

ical logg<strong>in</strong>g speeds are 0.2 m/s or less, <strong>in</strong> high seas it is CNL*<br />

Compensated Neutron Log<br />

possible that a tool could move downward even when it<br />

is be<strong>in</strong>g pulled uphole on the wirel<strong>in</strong>e. Much <strong>of</strong> this<br />

motion is probably stopped by friction between the tool<br />

<strong>and</strong> the borehole wall; however, recent corrections <strong>in</strong> the<br />

control <strong>of</strong> the heave compensator further reduce the<br />

DIL*<br />

DSI*<br />

FMS*<br />

FMI*<br />

GST*<br />

HDT*<br />

Dual Induction Log<br />

Dipole Shear Sonic Imager<br />

Formation MicroScanner*<br />

Fullbore Formation MicroImager<br />

Gamma Ray Spectroscopy Tool<br />

High Resolution Dipmeter Tool<br />

effects <strong>of</strong> heave by a factor <strong>of</strong> 2-3, elim<strong>in</strong>at<strong>in</strong>g the ILD<br />

Induction Log, Deep<br />

possibility that wirel<strong>in</strong>e motion could be reversed for<br />

most logg<strong>in</strong>g operations. As a result, downhole logs<br />

obta<strong>in</strong>ed at different times are consistent under most sea<br />

conditions, <strong>and</strong> the residual heave effects on vertical<br />

ILM<br />

IPLT*<br />

LDT*<br />

LSS*<br />

MDT*<br />

Induction Log, Medium<br />

Integrated Porosity Lithology Tool<br />

Litho-density* Tool<br />

Long Spac<strong>in</strong>g Sonic Tool<br />

Modular Formation Dynamics Tester<br />

resolution are negligible for most downhole tools.<br />

Electrical resistivity logg<strong>in</strong>g tools. Devices that<br />

gather data on a formation's electrical properties measure<br />

currents that propagate through the borehole <strong>and</strong><br />

pore fluids <strong>in</strong> the surround<strong>in</strong>g rock <strong>and</strong> sediment layers.<br />

NGT*<br />

NMR<br />

RAB*<br />

SDT*<br />

SFL*<br />

SHDT*<br />

Natural Gamma Ray Tool<br />

Nuclear Magnetic Resonance Tool<br />

Resistivity-At-Bit Tool<br />

Sonic Digital (Array-Sonic*) Tool<br />

Spherically Focussed Resistivity Tool<br />

Stratigraphic Dual-Dipmeter* Tool<br />

Water is a ubiquitous <strong>and</strong> conductive fluid underground;<br />

its electrical conductivity <strong>in</strong>creases with the concentration<br />

<strong>of</strong> Na + <strong>and</strong> C1- ions <strong>and</strong> with temperature, which<br />

*Trademark <strong>of</strong> Schlumberger.<br />

<strong>in</strong>creases the mobility <strong>of</strong> the ions. Electrical resistivity (laterologs) are used to measure higher resistivities<br />

measurements <strong>in</strong> a formation therefore allow one to (>100 ohm-m) that may occur <strong>in</strong> the calcareous <strong>and</strong><br />

estimate its porosity, fluid content, <strong>and</strong> <strong>of</strong>ten the degree igneous rocks encountered <strong>in</strong> ocean drill<strong>in</strong>g. Most mea<strong>of</strong><br />

fractur<strong>in</strong>g. Clays also contribute to the measured surements <strong>of</strong> electrical resistivity are made by <strong>in</strong>duction<br />

electrical conductivity because <strong>of</strong> the negative ions com- tools, with vertical resolutions rang<strong>in</strong>g between 0.5 m<br />

monly associated with the molecular structure <strong>of</strong> various <strong>and</strong> 2.0 m. <strong>The</strong> vary<strong>in</strong>g spac<strong>in</strong>g between electrodes on<br />

Al-bear<strong>in</strong>g m<strong>in</strong>erals found <strong>in</strong> many clays [Ellis, 1986]. these tools measures resistivity at different depths <strong>in</strong> the<br />

Self-potential devices measure the electrical potential borehole wall. If drill<strong>in</strong>g fluids have <strong>in</strong>vaded significantly<br />

generated by ions flow<strong>in</strong>g between the borehole <strong>and</strong> <strong>in</strong>to the formation, these measurements are not equal,<br />

pore fluids. This measurement is related to clay content <strong>and</strong> their difference allows one to make an estimate <strong>of</strong><br />

<strong>in</strong> a formation; it is high where resistivities are low. In formation permeability.<br />

most ocean drill holes, however, seawater fills both bore- Acoustic logg<strong>in</strong>g tools. Acoustic tools record comhole<br />

<strong>and</strong> pore space, <strong>and</strong> the self-potential measurement pressional, shear, <strong>and</strong> surface waves <strong>in</strong> the borehole<br />

is poor. Induction devices are used to measure lower environment, much like a seismic refraction experiment<br />



<strong>in</strong>dicate fractur<strong>in</strong>g <strong>and</strong> heterogeneity at the scale <strong>of</strong> is transferred to slow the neutrons down enough to drop<br />

centimeters to meters <strong>in</strong> the formation. Most acoustic below 0.1 eV energy, constitut<strong>in</strong>g the epithermal-thermal<br />

logg<strong>in</strong>g tools measure compressional waves which pen- neutron transition. Neutron responses are therefore<br />

etrate 0.1-0.5 m <strong>in</strong>to the borehole wall, with vertical strongly affected by the porosity <strong>and</strong> pore fluids <strong>in</strong> the<br />

resolutions <strong>of</strong> 0.5-1.5 m. Other <strong>in</strong>struments that clamp formation <strong>and</strong> by m<strong>in</strong>erals such as clay that have congeophones<br />

downhole <strong>and</strong> record energy from seismic siderable water bound <strong>in</strong> their molecular structure [Brosources<br />

on the surface penetrate through hundreds <strong>of</strong> glia <strong>and</strong> Ellis, 1990]. Some neutron tools utilize a pulsedmeters<br />

<strong>of</strong> the formation. Such vertical seismic pr<strong>of</strong>iles neutron accelerator to bombard the nuclei <strong>of</strong> m<strong>in</strong>erals <strong>in</strong><br />

(VSPs) typically provide acoustic velocity <strong>and</strong> amplitude the formation. <strong>The</strong> spectrum <strong>of</strong> gamma ray energies<br />

pr<strong>of</strong>iles with vertical resolution <strong>of</strong> 5.0-50 m.<br />

emitted by these <strong>in</strong>teractions is recorded us<strong>in</strong>g a crystal<br />

Nuclear logg<strong>in</strong>g tools. This class <strong>of</strong> <strong>in</strong>struments detector <strong>and</strong> provides a measure <strong>of</strong> the abundance <strong>of</strong> the<br />

measures naturally occurr<strong>in</strong>g radioisotopes <strong>and</strong> m<strong>in</strong>eral major m<strong>in</strong>eral-form<strong>in</strong>g elements, such as Fe, Si, Ca, <strong>and</strong><br />

constituents <strong>of</strong> the formation, as well as the fluid content A1 [Herron et al., 1993]. <strong>The</strong> elemental abundances may<br />

<strong>in</strong> pore space. <strong>The</strong>y are sophisticated devices that rely on be used to estimate m<strong>in</strong>eral concentration when core<br />

statistical count<strong>in</strong>g <strong>of</strong> subatomic particles <strong>and</strong> advanced materials can be comb<strong>in</strong>ed to provide sufficient calibracomputer<br />

analysis for <strong>in</strong>terpretation. Three types <strong>of</strong> nu- tion [Kerr et al., 1992; Myers, 1992]. Nuclear magnetic<br />

clear measurements are typically used <strong>in</strong> mar<strong>in</strong>e scien- resonance (NMR) tools measure the H present <strong>in</strong> the<br />

tific applications: natural gamma ray activity, gamma ray formation <strong>and</strong> pore fluid by <strong>in</strong>duc<strong>in</strong>g proton movement<br />

scatter<strong>in</strong>g, <strong>and</strong> neutron scatter<strong>in</strong>g.<br />

around a pulsed magnetic field [Brown <strong>and</strong> Gamson,<br />

Gamma ray activity tools are perhaps the most com- 1960; Jackson, 1984]. <strong>The</strong> time decay <strong>of</strong> the resonance<br />

mon nuclear measurement <strong>in</strong>struments <strong>and</strong> detect the signal is also directly related to the pore size distribution<br />

radioactive decay <strong>of</strong> natural isotopes <strong>of</strong> potassium, ura- <strong>and</strong> can be used to <strong>in</strong>dicate formation permeability.<br />

nium, <strong>and</strong> thorium us<strong>in</strong>g a sc<strong>in</strong>tillation counter <strong>and</strong> a Neutron measurements penetrate 0.5-1.0 m <strong>in</strong>to the<br />

crystal detector. <strong>The</strong> response <strong>of</strong> the detector is a simple formation, <strong>and</strong> the vertical resolution <strong>of</strong> the measurefunction<br />

<strong>of</strong> the concentration by weight <strong>of</strong> the radioiso- ment is approximately 0.4 m.<br />

topes <strong>and</strong> the formation density. <strong>The</strong> average depth <strong>of</strong> Borehole imag<strong>in</strong>g tools. Imag<strong>in</strong>g tools deliver highpenetration<br />

<strong>of</strong> the measurement <strong>in</strong>to the borehole wall resolution pictures <strong>of</strong> the wall <strong>of</strong> a borehole us<strong>in</strong>g preis<br />

about 0.5 m, <strong>and</strong> vertical resolution is approximately cision measurements <strong>of</strong> either electrical conductivity,<br />

0.3 m [Allen et al., 1988]. <strong>The</strong> natural gamma ray log optical variation, or acoustic reflectivity. <strong>The</strong>se three<br />

usually responds to clay content <strong>in</strong> a formation, where imag<strong>in</strong>g techniques are complementary, s<strong>in</strong>ce conductivnaturally<br />

radioactive elements concentrate, or to alter- ity (electrical), color (optical), <strong>and</strong> reflectivity (acoustic)<br />

ation m<strong>in</strong>erals that have these m<strong>in</strong>erals present with<strong>in</strong> are controlled by different physical <strong>and</strong> chemical charoxides<br />

<strong>and</strong> other compounds. <strong>The</strong> concentration <strong>of</strong> the acteristics <strong>of</strong> the rock. <strong>The</strong> images are always oriented to<br />

natural radioisotopes is therefore largely controlled by a magnetic reference measured downhole. Electrical<br />

depositional environments <strong>and</strong> diagenesis.<br />

imag<strong>in</strong>g provides approximately 5-mm resolution <strong>of</strong> the<br />

Density tools use a gamma ray source, usually 37Cs, borehole wall by sens<strong>in</strong>g contrasts between high- <strong>and</strong><br />

to bombard the formation with gamma rays that are low-conductivity features, such as water-filled fractures<br />

scattered through the rock <strong>and</strong> gradually lose energy. or f<strong>in</strong>e-scale bedd<strong>in</strong>g variations [Serra, 1989]. Devices<br />

Sensors pressed aga<strong>in</strong>st the borehole wall measure the such as the Formation MicroScanner TM (FMS) <strong>and</strong> Fullenergy<br />

flux <strong>of</strong> gamma rays returned to a sc<strong>in</strong>tillation bore Formation MicroImager TM (FMD) measure the<br />

counter <strong>and</strong> crystal detector which captures photons borehole's surface conductivity on four pads pressed<br />

emitted by Compton scatter<strong>in</strong>g. <strong>The</strong> radiation returned aga<strong>in</strong>st the borehole wall with vertical resolution 102<br />

is directly related to the electron density <strong>in</strong> the forma- times f<strong>in</strong>er than most other downhole measurements<br />

tion, which <strong>in</strong> turn is related to the bulk density <strong>of</strong> the (see Figure 4). Optical imag<strong>in</strong>g also <strong>of</strong>fers high vertical<br />

rock [Doveton, 1986; Ellis, 1987]. <strong>The</strong> electron density is resolution but is limited to holes conta<strong>in</strong><strong>in</strong>g transparent<br />

low for most pore-fill<strong>in</strong>g fluids <strong>and</strong> can therefore be used borehole fluids. Ultrasonic imag<strong>in</strong>g devices generate a<br />

as an <strong>in</strong>dicator <strong>of</strong> rock composition. <strong>The</strong> depth <strong>of</strong> <strong>in</strong>ves- complete 360 ø image <strong>of</strong> the reflectivity <strong>of</strong> the borehole<br />

tigation <strong>in</strong>to the borehole wall <strong>of</strong> density tools depends wall, an advantage over the four-pad electrical method,<br />

on the density <strong>of</strong> the formation; greater density reduces with a vertical resolution <strong>of</strong> 2-3 cm. <strong>The</strong> dip, strike,<br />

the penetration <strong>of</strong> emitted gamma rays <strong>in</strong>to the borehole width, <strong>and</strong> depth <strong>of</strong> geological features <strong>in</strong>tersect<strong>in</strong>g a<br />

wall. In porous <strong>and</strong> permeable formations, densities are borehole may be measured us<strong>in</strong>g any <strong>of</strong> these imag<strong>in</strong>g<br />

typically measured to approximately 0.5 m <strong>in</strong>to the bore- devices [e.g., Paillet et al., 1990; Luthi <strong>and</strong> Souhaite,<br />

hole wall, <strong>and</strong> the vertical resolution <strong>of</strong> the measure- 1990]. Images can be visually used to compare logs with<br />

ment is approximately 0.4 m.<br />

cores for bedd<strong>in</strong>g orientation <strong>and</strong> to study fractur<strong>in</strong>g,<br />

Neutron tools employ either a Am-Be radioisotope or structure, <strong>and</strong> borehole shape.<br />

an electrical generator source to bombard the formation Temperature tools. Temperature logg<strong>in</strong>g typically<br />

with neutrons. After the neutrons collide with molecules <strong>in</strong>volves measur<strong>in</strong>g a cont<strong>in</strong>uous pr<strong>of</strong>ile <strong>of</strong> borehole<br />

<strong>of</strong> like mass <strong>in</strong> the formation, such as H, sufficient energy fluid temperature as a proxy for the <strong>in</strong> situ formation


temperature by read<strong>in</strong>g resistance variations due to temperature-sensitive<br />

electrodes located <strong>in</strong> the borehole.<br />

Such measurements must be made after the fluid tem-<br />

perature has recovered from drill<strong>in</strong>g-<strong>in</strong>duced distur-<br />

bance, or must be extrapolated between several <strong>in</strong>ter-<br />

mediate times, to estimate undisturbed formation<br />

temperature. Erickson et al. [1975] describe a method to<br />

measure <strong>in</strong> situ temperature directly while cor<strong>in</strong>g that<br />

was used to compute heat flow through the ocean floor.<br />

<strong>The</strong>se measurements were made us<strong>in</strong>g a self-record<strong>in</strong>g<br />

temperature probe that protruded <strong>in</strong>to thermally undis-<br />

turbed s<strong>of</strong>t sediment ahead <strong>of</strong> the core at selected<br />

depths. Although the technology has improved, modified<br />

versions <strong>of</strong> this approach still provide a reliable estimate<br />

<strong>of</strong> formation temperature <strong>and</strong> are superior to borehole<br />

fluid measurements. Self-record<strong>in</strong>g temperature logg<strong>in</strong>g<br />

tools <strong>and</strong> versions <strong>of</strong> the cor<strong>in</strong>g temperature probe are<br />

used today <strong>in</strong> rout<strong>in</strong>e ODP operations.<br />

Logg<strong>in</strong>g-while-drill<strong>in</strong>g tools. Over the last 5-10<br />

years, new technology has been developed to measure <strong>in</strong><br />

situ properties <strong>in</strong> oil <strong>in</strong>dustry drill holes that are drilled<br />

horizontally where conventional logg<strong>in</strong>g with a flexible<br />

wirel<strong>in</strong>e is not feasible. This <strong>in</strong>novative technology is<br />

called "logg<strong>in</strong>g while drill<strong>in</strong>g" (LWD) <strong>and</strong> uses sensors<br />

placed just above the drill bit, allow<strong>in</strong>g measurements <strong>of</strong><br />

porosity, resistivity, density, <strong>and</strong> natural gamma radia-<br />

Neutron<br />

measurement-to-bit<br />

13.28 m<br />

Density<br />

measurement-to-bit<br />

11.38 m<br />

Gamma-ray<br />

measurement-to-bit<br />

6.29 m<br />

Resistivity<br />

measurement-to-bit<br />

2.91 m<br />

Figure 5. Schematic diagram <strong>of</strong> logg<strong>in</strong>g-while-drill<strong>in</strong>g (LWD)<br />

tools located immediately above the drill bit that enable measurements<br />

with<strong>in</strong> m<strong>in</strong>utes after the hole is made [from Shipley<br />

et al., 1995].<br />

tion, among other properties discussed below, to be<br />

made m<strong>in</strong>utes after the drill bit cuts through the formanor<br />

sonic measurements made while drill<strong>in</strong>g has been<br />

used yet <strong>in</strong> deep mar<strong>in</strong>e environments.<br />

tion [e.g., Allen et al., 1989; Bonner et al., 1992; Murphy,<br />

<strong>Downhole</strong> <strong>Measurements</strong>: Long-Term Observatories<br />

1993]. A schematic diagram <strong>of</strong> LWD <strong>in</strong>struments is<br />

<strong>and</strong> Multiple Reentry Logg<strong>in</strong>g<br />

shown <strong>in</strong> Figure 5. <strong>The</strong>se sensors allow data to be<br />

In recent years, several <strong>in</strong>dependent approaches to<br />

recorded almost immediately after drill<strong>in</strong>g, so that<br />

long-term <strong>and</strong> multiple-reentry measurements <strong>in</strong> boreephemeral<br />

<strong>in</strong> situ physical properties can be measured.<br />

holes have been based on the emplacement <strong>of</strong> <strong>in</strong>stru-<br />

<strong>The</strong> primary advantage <strong>of</strong> LWD over wirel<strong>in</strong>e logg<strong>in</strong>g <strong>in</strong> ments after drill<strong>in</strong>g is completed. Such experiments are<br />

near-vertical oceanic holes is that data can be acquired designed to measure seismic, physical, <strong>and</strong> fluid properwithout<br />

gaps below the seafloor, at the bottom <strong>of</strong> the ties, pore pressure, <strong>and</strong> chemistry <strong>in</strong> reoccupied mar<strong>in</strong>e<br />

drill hole, or through <strong>in</strong>tervals that are difficult to drill boreholes that have recovered from drill<strong>in</strong>g-<strong>in</strong>duced dis<strong>and</strong><br />

<strong>of</strong>ten deteriorate after drill<strong>in</strong>g. However, differences turbance over periods <strong>of</strong> months to years. Long-term<br />

<strong>in</strong> measurement technologies must be taken <strong>in</strong>to consid- borehole observations may be used to monitor many<br />

eration when directly compar<strong>in</strong>g results from wirel<strong>in</strong>e geologic processes, <strong>in</strong>clud<strong>in</strong>g tectonic movements, heat<br />

<strong>and</strong> LWD tools [e.g., Evans, 1991].<br />

flow, hydrogeology, <strong>and</strong> earthquake activity.<br />

Instruments for mak<strong>in</strong>g measurements while drill<strong>in</strong>g One technology that has recently been developed<br />

have recently been developed for <strong>in</strong>vestigat<strong>in</strong>g a variety allows for seafloor reentry <strong>of</strong> preexist<strong>in</strong>g holes <strong>and</strong> their<br />

<strong>of</strong> other formation properties. When acoustic velocities hydraulic seal<strong>in</strong>g <strong>and</strong> <strong>in</strong>strumentation for long-term<br />

exceed approximately 2 km/s, they can be measured monitor<strong>in</strong>g <strong>of</strong> <strong>in</strong> situ temperature <strong>and</strong> pressure. To do<br />

dur<strong>in</strong>g drill<strong>in</strong>g [e.g.,Aron et al., 1994]. A resistivity device this, the exchange <strong>of</strong> seawater with formation fluids must<br />

can be used to generate high-resolution (approximately be sealed <strong>of</strong>f, the seafloor itself <strong>in</strong> the vic<strong>in</strong>ity <strong>of</strong> the<br />

5 cm), oriented images us<strong>in</strong>g sensor electrodes that borehole must be impermeable relative to deeper layers,<br />

rotate near the drill<strong>in</strong>g bit <strong>and</strong> produce a full 360 ø scan <strong>and</strong> time must elapse to allow the borehole pressure <strong>and</strong><br />

<strong>in</strong> medium- to high-resistivity formations, as shown <strong>in</strong> temperature to return to their ambient conditions before<br />

Figure 6 [Lovell et al., 1995]. <strong>The</strong> recorded images re- drill<strong>in</strong>g. Regions <strong>of</strong> permeable ocean crust overla<strong>in</strong> by<br />

semble FMI data with somewhat poorer resolution, but mar<strong>in</strong>e sediments, buried faults, <strong>and</strong> deep deformation<br />

unlike the FMI, wall coverage is complete <strong>and</strong> data are zones are good environments for such experiments.<br />

recorded before hole conditions can severely deterio- Fluid samplers <strong>and</strong> thermistor arrays can be emplaced <strong>in</strong><br />

rate. Us<strong>in</strong>g these image data, three-dimensional strati- sealed boreholes to monitor pressure, temperature, <strong>and</strong><br />

graphic <strong>and</strong> structural <strong>in</strong>terpretation is possible with less fluid changes under <strong>in</strong> situ conditions for up to 3 years<br />

than complete core recovery. Neither resistivity imag<strong>in</strong>g after deployment us<strong>in</strong>g current data-record<strong>in</strong>g capabili-<br />



RAB Image<br />


ß<br />

..i'it;::i:;:.;;:<br />

FMI Image<br />

BOTTOM<br />

..<br />

, .,, .<br />

..<br />

.<br />

ß<br />

..<br />

ß<br />

'.L ., i:, .... " ....<br />

TOP<br />

Figure 6. Comparison <strong>of</strong> (left) Resistivity-at-bit<br />

LWD tool <strong>and</strong> (right) FMI wirel<strong>in</strong>e tool electrical<br />

images <strong>of</strong> dense fractur<strong>in</strong>g <strong>in</strong> mar<strong>in</strong>e<br />

sediments [after Lovell et al., 1995]. Both images<br />

<strong>of</strong> the <strong>in</strong>terior <strong>of</strong> the borehole wall are<br />

oriented to top at 0 ø magnetic decl<strong>in</strong>ation. Although<br />

the LWD tool measures with >30 times<br />

poorer vertical resolution than the wirel<strong>in</strong>e<br />

tool, it <strong>of</strong>fers the advantage <strong>of</strong> data coverage<br />

around the entire circumference <strong>of</strong> the bore-<br />

hole <strong>and</strong> measurements with<strong>in</strong> m<strong>in</strong>utes after<br />

the hole is made. See Table 1 for list <strong>of</strong> abbre-<br />

viations.<br />

ties [Davis et al., 1992]. S<strong>in</strong>ce 1991, more than 10 <strong>in</strong>stru- frequency seismic noise as a step toward evaluat<strong>in</strong>g<br />

mented <strong>and</strong> sealed borehole observatories have been different approaches to acquir<strong>in</strong>g broadb<strong>and</strong> seismic<br />

established by ODP. <strong>The</strong> borehole seal <strong>and</strong> <strong>in</strong>strumen- data <strong>in</strong> the world's oceans. Stephen et al. [1994] <strong>in</strong>dicated<br />

tation package must be deployed us<strong>in</strong>g a drill ship; that seismic noise <strong>in</strong> the upper 100 m <strong>of</strong> sediments below<br />

subsequently, an observatory may be serviced <strong>and</strong> data the seafloor was reduced for a borehole geophone sysrecovered<br />

by submersible or by remote-operation vehi- tem deployed dur<strong>in</strong>g a wirel<strong>in</strong>e reentry operation <strong>in</strong> the<br />

cles. Davis <strong>and</strong> Becker [1994] describe the prelim<strong>in</strong>ary western Atlantic.<br />

results <strong>of</strong> recovered temperature data from the earliest Most logg<strong>in</strong>g tools could be deployed by wirel<strong>in</strong>e<br />

borehole observatories, which <strong>in</strong>dicate that high-temper- reentry. <strong>Downhole</strong> tools that were omitted dur<strong>in</strong>g drilature<br />

fluids may be expelled for long periods <strong>of</strong> time <strong>in</strong> l<strong>in</strong>g operations because <strong>of</strong> time or weather restrictions<br />

the crust. Long-term fluid samplers emplaced <strong>in</strong> bore- or tools that are too large to be lowered through the drill<br />

hole observatories have not yet been recovered. pipe could be used. Current ODP limitations restrict<br />

<strong>Downhole</strong> logs have been recorded by revisit<strong>in</strong>g <strong>and</strong> <strong>in</strong>struments to those that can be lowered through a<br />

reenter<strong>in</strong>g preexist<strong>in</strong>g holes after drill<strong>in</strong>g us<strong>in</strong>g st<strong>and</strong>ard 10-cm drill pipe, yet they must be able to make measureoceanographic<br />

ships <strong>and</strong> submersible-assisted systems ments <strong>in</strong> holes that are <strong>of</strong>ten as large as 30 cm <strong>in</strong><br />

[Langseth <strong>and</strong> Spiess, 1987; Gable <strong>and</strong> DIANA UT Ship- diameter. This gap seriously limits sensor design <strong>and</strong><br />

board Party, 1992; Spiess et al., 1992]. In these reentry restricts the use <strong>of</strong> many exist<strong>in</strong>g wirel<strong>in</strong>e devices. Boreexperiments<br />

a remote deep-sea vehicle suspended vari- hole gravimeters that measure bulk density, resistivity<br />

ous logg<strong>in</strong>g tools over preexist<strong>in</strong>g holes <strong>and</strong> lowered imag<strong>in</strong>g tools that extend for greater borehole coverage,<br />

them on a wirel<strong>in</strong>e. <strong>The</strong> DIANAUT program [Gable <strong>and</strong> nuclear magnetic resonance tools that estimate sediment<br />

DIANA UT Shipboard Party, 1992] used a submersible- permeability, <strong>and</strong> downhole samplers that extract pore<br />

assisted system to reenter three Atlantic holes. Wire l<strong>in</strong>e fluids could be deployed by wirel<strong>in</strong>e reentry [e.g.,<br />

operations at these sites <strong>in</strong>cluded temperature, caliper, Schultz, 1989; Colley et al., 1992; Black, 1992; Schlum<strong>and</strong><br />

borehole televiewer logs; clamped borehole seis- berger Educational Services, 1992; Motriss et al., 1993]. In<br />

mometers; <strong>and</strong> downhole fluid samplers [Floury <strong>and</strong> addition, holes drilled through unstable formations that<br />

Gable, 1992; Mor<strong>in</strong> et al., 1992; Gieskes <strong>and</strong> Magenheim, must be cased to prevent collapse can be logged with<br />

1992]. <strong>The</strong> postdrill<strong>in</strong>g thermal equilibration <strong>of</strong> these certa<strong>in</strong> tools by wirel<strong>in</strong>e reentry. Porosity <strong>and</strong> velocity<br />

holes was critical for accurate <strong>in</strong> situ temperature <strong>and</strong> have been measured successfully beh<strong>in</strong>d the cas<strong>in</strong>g us<strong>in</strong>g<br />

fluid sampl<strong>in</strong>g; <strong>in</strong> addition, the negligible effects <strong>of</strong> heave high-energy nuclear <strong>and</strong> low-frequency shear sonic tools<br />

for submersible-assisted reentry operations improved that penetrate deeply <strong>in</strong>to the borehole wall [e.g., Moos<br />

depth accuracy <strong>of</strong> the imag<strong>in</strong>g <strong>and</strong> high-resolution logs. et al., 1997]. When repeated logs are acquired by wire-<br />

Spiess et al. [1992] <strong>and</strong> Montagner et al. [1994] described l<strong>in</strong>e reentry, whether to compensate for time, technical,<br />

wirel<strong>in</strong>e reentry experiments for mar<strong>in</strong>e tests <strong>of</strong> low- or physical constra<strong>in</strong>ts dur<strong>in</strong>g drill<strong>in</strong>g operations, the


data can also be used to study temporal changes <strong>in</strong><br />

formation properties over periods <strong>of</strong> days to years.<br />

Over time, the physical properties <strong>of</strong> formations penetrated<br />

by a borehole, as well as the borehole itself,<br />

change as stresses are applied <strong>and</strong> fluids flow through<br />

them. Such temporal changes can be measured only over<br />

an elapsed period <strong>of</strong> time. <strong>Downhole</strong> measurements<br />

made days to years after a borehole is drilled allow<br />

properties such as temperature <strong>and</strong> pore fluid composition<br />

to approach their condition prior to drill<strong>in</strong>g. Repeated<br />

deployment <strong>of</strong> wirel<strong>in</strong>e logg<strong>in</strong>g tools provides<br />

logs that reflect temporal changes <strong>in</strong> the physical <strong>and</strong><br />

borehole properties. An example <strong>of</strong> temperature logs<br />

repeated dur<strong>in</strong>g four experimental episodes spann<strong>in</strong>g<br />

6.5 years is shown <strong>in</strong> Figure 7. <strong>The</strong>se temperature logs<br />

were recorded <strong>in</strong> ODP Hole 504B <strong>in</strong> the eastern equatorial<br />

Pacific as drill<strong>in</strong>g penetrated progressively deeper<br />

<strong>in</strong>to the oceanic crust <strong>and</strong> illustrate the change <strong>in</strong> the<br />

movement <strong>of</strong> fluids <strong>in</strong> the ocean crust over time [Langseth<br />

et al., 1983; Gable et al., 1989]. Alt et al. [1993]<br />

estimate the equilibrium bottom hole temperature to be<br />

about 195øC at a depth <strong>of</strong> 2111 m below the seafloor.<br />

Becker et al. [1983], among others, describe a decrease,<br />

followed by an <strong>in</strong>crease, <strong>in</strong> fluid flow with<strong>in</strong> the upper<br />

400 m <strong>of</strong> the borehole s<strong>in</strong>ce it was drilled, as monitored<br />

by these successive temperature logs. <strong>The</strong> decrease <strong>in</strong><br />

temperatures reflects the downhole flow <strong>of</strong> cold seawater<br />

through permeable cracks <strong>and</strong> faults <strong>in</strong> the oceanic<br />

crust, which results <strong>in</strong> chemical exchanges with the rock<br />

[Alt et al., 1986]. <strong>Downhole</strong> temperature measurements<br />

made over time, as well as long-term measurements<br />

made <strong>in</strong> sealed boreholes, allow for study <strong>of</strong> the longevity<br />

<strong>and</strong> distribution <strong>of</strong> fluid circulation <strong>in</strong> the crust.<br />

Experimental Strategy<br />

<strong>The</strong> best designed downhole experiments take <strong>in</strong>to<br />

account which <strong>in</strong>strument, or comb<strong>in</strong>ation <strong>of</strong> <strong>in</strong>struments,<br />

will provide the highest-quality data needed to<br />

address a scientific objective. Drill<strong>in</strong>g technology also<br />

dramatically affects the collection <strong>of</strong> high-quality downhole<br />

<strong>in</strong>formation. An appropriate strategy may <strong>in</strong>clude<br />

the drill<strong>in</strong>g <strong>of</strong> dedicated holes for wirel<strong>in</strong>e logg<strong>in</strong>g, use <strong>of</strong><br />

logg<strong>in</strong>g while drill<strong>in</strong>g, wirel<strong>in</strong>e reentry logg<strong>in</strong>g, repeat<strong>in</strong>g<br />

logs over time, or <strong>in</strong>terpretation <strong>of</strong> logs jo<strong>in</strong>tly with core<br />

analyses. Improved experimental strategies have contributed<br />

as much to the success <strong>of</strong> downhole measure-<br />

lOOO<br />

lfiOO<br />

Leg 148 (1993)<br />

......... Leg 111 (1986)<br />

Leg 140 (1991),<br />

..... Leg 137 (1991)<br />

2000<br />

0 50 100 150 200<br />

Temperature ("C)<br />

Figure 7. Temperature logs recorded <strong>in</strong> ODP Hole 504B<br />

successively dur<strong>in</strong>g four expeditions over 6.5 years [after Alt et<br />

al., 1993]. <strong>The</strong> variations <strong>in</strong> temperature over time illustrate<br />

the episodic downhole flow <strong>of</strong> cool ocean water <strong>in</strong>to the<br />

uppermost 300-400 m <strong>of</strong> porous formations below the seafloor.<br />

exam<strong>in</strong>ation. Without a riser, fluid circulates down the<br />

drill pipe but is not returned to the surface, <strong>and</strong> the hole<br />

conditions are less stable. In addition, the diameter <strong>of</strong><br />

the drill pipe is larger when a riser is used, allow<strong>in</strong>g more<br />

ments <strong>in</strong> mar<strong>in</strong>e geology <strong>and</strong> geophysics as the development<br />

<strong>of</strong> new <strong>in</strong>strument technology.<br />

<strong>The</strong> lack <strong>of</strong> a riser <strong>and</strong> drill<strong>in</strong>g mud cake <strong>in</strong> deep<br />

mar<strong>in</strong>e drill<strong>in</strong>g operations makes the use <strong>of</strong> wirel<strong>in</strong>e<br />

width for downhole <strong>in</strong>struments. Nevertheless, eng<strong>in</strong>eer<strong>in</strong>g<br />

advances have improved hole conditions for downhole<br />

measurements <strong>in</strong> lieu <strong>of</strong> a riser. Among the most<br />

prom<strong>in</strong>ent, a drill pipe is lowered to clear borehole<br />

logg<strong>in</strong>g technology more challeng<strong>in</strong>g than <strong>in</strong> cont<strong>in</strong>ental<br />

obstructions while a wirel<strong>in</strong>e tool is downhole. <strong>The</strong> drilor<br />

oil <strong>in</strong>dustry drill<strong>in</strong>g, where risers are commonplace. A l<strong>in</strong>g <strong>of</strong> dedicated holes without cor<strong>in</strong>g also improves hole<br />

riser system circulates drill<strong>in</strong>g fluid down the drill pipe, conditions for downhole measurements. In the future,<br />

through the bit, <strong>and</strong> around the exterior <strong>of</strong> the drill pipe with the development <strong>of</strong> the planned deep-sea riser<br />

back to the platform. <strong>The</strong> drill<strong>in</strong>g fluid, or "mud," usu- system <strong>and</strong> other eng<strong>in</strong>eer<strong>in</strong>g advances, drill<strong>in</strong>g <strong>and</strong> hole<br />

ally conta<strong>in</strong>s dense <strong>and</strong> viscous compounds that clean conditions are expected to improve well beyond current<br />

the borehole, build up a mud cake to stabilize it for capabilities <strong>and</strong> to cont<strong>in</strong>ue to add to the scientific<br />

logg<strong>in</strong>g, <strong>and</strong> return rock cutt<strong>in</strong>gs to the surface for success <strong>of</strong> these experiments.



[e.g., Hays et al., 1976; Shackelton et al., 1984]. Due to the<br />

gravitational pull <strong>of</strong> the Sun <strong>and</strong> Moon on the equatorial<br />

<strong>The</strong>re are few areas <strong>of</strong> mar<strong>in</strong>e geology <strong>and</strong> geophysics bulge <strong>of</strong> the Earth, our planet wobbles slightly as it sp<strong>in</strong>s,<br />

that cannot benefit from logg<strong>in</strong>g <strong>and</strong> downhole measure- def<strong>in</strong><strong>in</strong>g its precession. This wobble results <strong>in</strong> climate<br />

ments. Data are recorded <strong>in</strong> situ <strong>and</strong> usually <strong>in</strong> a con- cycles that last 19,000-23,000 years. <strong>The</strong> Earth's tilt, or<br />

t<strong>in</strong>uous pr<strong>of</strong>ile to the bottom <strong>of</strong> the borehole, provid<strong>in</strong>g obliquity, varies with a period <strong>of</strong> 41,000 years, <strong>and</strong> its<br />

researchers with an accurate proxy <strong>of</strong> the geology at orbit varies from nearly circular to 95 % eccentricity with<br />

various depths. Exploration <strong>of</strong> the structure, deforma- periods <strong>of</strong> 95,000 <strong>and</strong> 413,000 years, affect<strong>in</strong>g its distance<br />

tion, <strong>and</strong> stresses <strong>in</strong> the Earth's crust that are accessible from the Sun. <strong>The</strong>se periodicities have been observed <strong>in</strong><br />

by drill<strong>in</strong>g has <strong>of</strong>ten been supplemented by downhole a number <strong>of</strong> mar<strong>in</strong>e <strong>and</strong> terrestrial sedimentary environmeasurements<br />

<strong>and</strong> proven to be critical <strong>in</strong> underst<strong>and</strong>- ments. Pliocene <strong>and</strong> Pleistocene mar<strong>in</strong>e sediments have<br />

<strong>in</strong>g subsurface phenomena. <strong>The</strong> Earth's environment demonstrated the clearest correlation between sedimenlikewise<br />

has been <strong>in</strong>vestigated by scientific drill<strong>in</strong>g <strong>and</strong> tary layers <strong>and</strong> Milankovitch periodicities because <strong>of</strong> the<br />

downhole measurements to discern details <strong>of</strong> the sedi- high recovery <strong>of</strong> material that can be accurately dated<br />

ment record over time. In the follow<strong>in</strong>g sections, recent us<strong>in</strong>g mar<strong>in</strong>e core sections. Climate cycles are not limresearch<br />

studies that highlight the application <strong>of</strong> down- ited to the younger sediments, however; cor<strong>in</strong>g <strong>in</strong> both<br />

hole measurements to topics related to the Earth's en- Cretaceous <strong>and</strong> Triassic rocks shows evidence <strong>of</strong> Mivironment<br />

<strong>and</strong> Earth's crust are discussed.<br />

lankovitch cycles [Arthur et aL, 1984; Fischer, 1986; Olsen,<br />

1986].<br />

Paleoclimate<br />

Milankovitch orbital variations may alter the deposi-<br />

Because Earth's climate has large effects on the tional environment considerably. Spectral analysis <strong>of</strong> the<br />

ocean's chemistry, circulation, <strong>and</strong> biological productiv- cont<strong>in</strong>uous <strong>and</strong> regular sampl<strong>in</strong>g that log data provide<br />

ity, changes <strong>in</strong> the global environment leave a recogniz- can resolve Milankovitch periodicities <strong>and</strong> thus sedimenable<br />

signature <strong>in</strong> the deep-sea sedimentary record. How tation rates for a known depth <strong>in</strong>terval. In cores, sedithe<br />

global climate system has changed <strong>and</strong> the causes <strong>of</strong> mentation rates are typically determ<strong>in</strong>ed by estimat<strong>in</strong>g<br />

these changes therefore may be deduced from downhole the elapsed time between biostratigraphic, radioisotopic,<br />

data if these signatures can be measured. Short- <strong>and</strong> or magnetostratigraphic markers that can be identified.<br />

long-term changes <strong>in</strong> such variables as temperature, This requires an absolute timescale, which can be suphumidity,<br />

<strong>and</strong> w<strong>in</strong>d patterns, the <strong>in</strong>flux <strong>of</strong> energy from plied by correlat<strong>in</strong>g Milankovitch periodicities with the<br />

the Sun, sea level, water temperature, <strong>and</strong> oceanic up- cores. Us<strong>in</strong>g the cont<strong>in</strong>uous logs to identify cycles, a<br />

well<strong>in</strong>g can all affect the m<strong>in</strong>eralogy, porosity, <strong>and</strong> gra<strong>in</strong> timescale can be extended millions <strong>of</strong> years <strong>in</strong>to the past<br />

size <strong>of</strong> sediment layers deposited on the seafloor. Most with greater precision than can be achieved with the<br />

downhole log responses are affected by sediment poros- core-based methods alone. Log data may be used to<br />

ity, which is closely related to the m<strong>in</strong>eralogy <strong>and</strong> gra<strong>in</strong> illustrate major trends <strong>and</strong> rapid changes <strong>in</strong> the sedisize,<br />

both <strong>of</strong> which can vary dramatically as ocean up- mentation rate by shifts <strong>in</strong> the wavelength <strong>of</strong> their variwell<strong>in</strong>g<br />

patterns <strong>and</strong> mean water temperature alter bio- ation with depth <strong>in</strong> areas with variable or discont<strong>in</strong>uous<br />

logic productivity. <strong>The</strong> amount <strong>of</strong> clay <strong>in</strong>corporated <strong>in</strong> accumulation or poor age control from core data [Mothe<br />

sediment is also <strong>of</strong>ten determ<strong>in</strong>ed by climate <strong>and</strong> will l<strong>in</strong>ie et al., 1990]. Jarrard <strong>and</strong> Arthur [1989] describe a<br />

strongly affect the m<strong>in</strong>eralogy <strong>and</strong> porosity. <strong>The</strong> natural method to determ<strong>in</strong>e sedimentation rates us<strong>in</strong>g log data<br />

gamma ray log is particularly sensitive to these variations that requires neither precise biostratigraphy nor an ab<strong>in</strong><br />

clay content. If the temporal changes can be ade- solute timescale. <strong>The</strong>y constra<strong>in</strong> the sedimentation rate<br />

quately resolved, the geophysical <strong>and</strong> geochemical mea- to the Milankovitch orbital periods by comput<strong>in</strong>g spectra<br />

surements taken downhole can serve as proxies for cli- <strong>of</strong> log data <strong>and</strong> identify<strong>in</strong>g peaks <strong>in</strong> the cont<strong>in</strong>uous<br />

mate change <strong>in</strong> the sediment record. Us<strong>in</strong>g downhole spectra that have a constant spac<strong>in</strong>g ratio between them.<br />

logs <strong>in</strong> paleoclimate studies has the significant advantage <strong>The</strong> use <strong>of</strong> spectra for <strong>in</strong>terpret<strong>in</strong>g precise sedimenta<strong>of</strong><br />

measur<strong>in</strong>g several <strong>in</strong>dependent parameters that all tion rates between time markers identified <strong>in</strong> cores has<br />

reflect climatic changes throughout the stratigraphic se- proven to be a powerful technique <strong>in</strong> paleoclimate studies.<br />

quence. In addition, the cont<strong>in</strong>uous <strong>and</strong> uniform sam- In 1989, a site <strong>in</strong> the Sea <strong>of</strong> Japan was drilled through<br />

pl<strong>in</strong>g <strong>of</strong> downhole records allows for rigorous statistical 2.8 m.y. <strong>of</strong> f<strong>in</strong>e sediment layers to a depth <strong>of</strong> 459 m<br />

analyses that are not possible us<strong>in</strong>g short or <strong>in</strong>complete below the seafloor. Ingle et al. [1990] describe the recovdata<br />

from core sections.<br />

ered core as dark opal-rich layers with organic carbon<br />

Of the numerous <strong>in</strong>terrelated factors driv<strong>in</strong>g climatic <strong>in</strong>terlayered with clay-rich sediments that are derived<br />

changes, few are generally accepted to be more impor- from aeolian dust blown from loess deposits <strong>in</strong> Ch<strong>in</strong>a.<br />

tant than the variation <strong>in</strong> Earth's tilt <strong>and</strong> orbital cycles <strong>The</strong> layers alternate about every 4 m. Variation <strong>in</strong> nataround<br />

the Sun. Milankovitch [1941] was the first to ural gamma radioactivity versus age <strong>in</strong> this hole is shown<br />

recognize that periodicity <strong>in</strong> Earth's orbit <strong>and</strong> tilt alters <strong>in</strong> Figure 8, where the age-depth conversion has been<br />

the solar energy flux fall<strong>in</strong>g at any given latitude <strong>in</strong> cycles computed from a sedimentation rate that was conlast<strong>in</strong>g<br />

thous<strong>and</strong>s to hundreds <strong>of</strong> thous<strong>and</strong>s <strong>of</strong> years stra<strong>in</strong>ed by paleomagnetic reversals <strong>and</strong> biostratigraphy


recorded from the core. Increases <strong>in</strong> gamma radiation<br />

reflect high clay content <strong>of</strong> the cont<strong>in</strong>ental clay-rich<br />

layers, which have abundant K <strong>and</strong> A1. <strong>The</strong>se cyclical<br />

signatures are apparent <strong>in</strong> other downhole data; the<br />

density <strong>and</strong> resistivity logs reflect the lower porosity <strong>of</strong><br />

the cont<strong>in</strong>ental clays. DeMenocat et at. [1992] compare<br />

the gamma ray log <strong>and</strong> a mar<strong>in</strong>e 8SO record to illustrate<br />

the strong correlation with variations <strong>in</strong> northern hemisphere<br />

ice volume [e.g., Raytoo et at., 1989]. <strong>The</strong> concentration<br />

<strong>of</strong> 8SO has <strong>of</strong>ten been used as a proxy for<br />

paleoclimatic changes <strong>in</strong> ice volume, temperature, <strong>and</strong><br />

sea level because it has a lower concentration <strong>in</strong> fresh<br />

water from polar ice than <strong>in</strong> seawater. When polar<br />

temperatures decrease, ice volume <strong>in</strong>creases <strong>and</strong> sea<br />

level drops; <strong>in</strong> warmer periods, ice volume decreases <strong>and</strong><br />

sea level rises. <strong>The</strong> <strong>in</strong>creased gamma radioactivity <strong>in</strong> the<br />

clays <strong>in</strong> the Sea <strong>of</strong> Japan is associated with long periods<br />

Age<br />

(Ma)<br />

<strong>of</strong> drought when loess deposits erode <strong>and</strong> are transported<br />

eastward by prevail<strong>in</strong>g w<strong>in</strong>ds. When the Asian<br />

climate warms, the major deposition <strong>in</strong> the Sea <strong>of</strong> Japan<br />

is much lower <strong>in</strong> radioactive K as well as A1, <strong>and</strong> the<br />

deposits are <strong>in</strong>terpreted to be fluvial.<br />

<strong>The</strong> most prom<strong>in</strong>ent periodicity <strong>of</strong> the climatic variation<br />

<strong>in</strong> the Sea <strong>of</strong> Japan is 41,000 years, correspond<strong>in</strong>g<br />

2.0<br />

with the predicted Milankovitch cycle caused by changes<br />

<strong>in</strong> Earth's tilt. Figure 9 illustrates how the downhole logs<br />

from the Sea <strong>of</strong> Japan can be used to determ<strong>in</strong>e characteristic<br />

periodicities for a time <strong>in</strong>terval. Note that<br />

2.5<br />

different <strong>in</strong>struments vary <strong>in</strong> their time resolution. All<br />

i,-1,,, i,,, i,, ,i<br />

three resistivity logs show the characteristic 41,000-year<br />

100 60 20<br />

periodicity, as well as variability at shorter periods, us<strong>in</strong>g<br />

two <strong>of</strong> the three different tools, the shallow resistivity log<br />

798B Gamma Ray<br />

with 0.75-m resolution <strong>and</strong> the FMS log with 5-mm<br />

Incr. terrigenous content<br />

resolution (see Figure 4). Worth<strong>in</strong>gton [1990] described a<br />

relationship based on the Nyquist periodic sampl<strong>in</strong>g<br />

Figure 8. Correlation <strong>of</strong> the natural gamma radiation log <strong>in</strong><br />

ODP Hole 798A <strong>in</strong> the Sea <strong>of</strong> Japan with the mar<strong>in</strong>e sO<br />

condition between the <strong>in</strong>strument resolution h (<strong>in</strong><br />

record from the North Atlantic [from deMenocal et al., 1992].<br />

meters), the sedimentation rate s (<strong>in</strong> meters per million<br />

<strong>The</strong> depth scale <strong>of</strong> the log was converted to age us<strong>in</strong>g the<br />

years), <strong>and</strong> the cycle period -r (<strong>in</strong> millions <strong>of</strong> years) to be paleomagnetic reversal sequence shown. <strong>The</strong> f<strong>in</strong>e-scale correh<br />

= s-r/2. For a given sedimentation rate, climate cycles lation between the logs confirms that aridity dur<strong>in</strong>g glacial<br />

below this resolution limit <strong>of</strong> the <strong>in</strong>strument will be<br />

periods cause terrigeneous deposits <strong>in</strong> the Sea <strong>of</strong> Japan.<br />

aliased <strong>and</strong> appear at lower frequencies. <strong>The</strong> deep resistivity<br />

log has 2-m vertical resolution <strong>and</strong> requires a<br />

sedimentation rate <strong>of</strong> at least 20 m/m.y. to detect eccen- periods <strong>in</strong> the range <strong>of</strong> 2000-10,000 years are not well<br />

tricity cycles <strong>of</strong> 95,000-123,000 or 413,000 years <strong>and</strong> at understood, but they have been l<strong>in</strong>ked to cycles <strong>in</strong> ocean<br />

least 100 m/m.y. to detect precession cycles <strong>of</strong> 19,000- circulation, iceberg raft<strong>in</strong>g <strong>in</strong> surface waters, <strong>and</strong> solar<br />

23,000 years. With the FMS tool the m<strong>in</strong>imum sedimen- radiation [e.g., Dansgaard et al., 1982; Henrich, 1989].<br />

tation rate required for detection <strong>of</strong> 19,000- to 23,000- <strong>The</strong> ability to predict these short-period, non-Milankoyear<br />

precession cycles is 22 m/m.y., which is commonly vitch events that cause abrupt changes <strong>in</strong> climate may<br />

exceeded <strong>in</strong> many nonerosive mar<strong>in</strong>e environments. Us- benefit society significantly if potential hazards due to sea<br />

<strong>in</strong>g the FMS, cycles at even f<strong>in</strong>er scales can be detected. level rise, drought, or solar radiation can be prevented.<br />

For example, the 7-m <strong>in</strong>terval <strong>of</strong> the FMS log <strong>in</strong> Figure In summary, the wide variety <strong>of</strong> properties measured<br />

9 corresponds approximately with a 60,000-year <strong>in</strong>terval us<strong>in</strong>g logs can detect periodic changes due to climate or<br />

shown <strong>in</strong> the deep <strong>and</strong> shallow resistivity logs <strong>and</strong> re- other driv<strong>in</strong>g forces cont<strong>in</strong>uously <strong>and</strong> at extremely f<strong>in</strong>e<br />

solves cycles with suborbital periods as short as 6000 scale. Detection methods us<strong>in</strong>g downhole data are powyears.<br />

Peaks <strong>in</strong> the spectrum represent the 41,000-year erful, rapid, <strong>and</strong> complete compared with laboratory<br />

cycles as well as the lower-magnitude peaks at suborbital analyses on core samples <strong>and</strong> are <strong>in</strong>dependent <strong>of</strong> core<br />

periods <strong>of</strong> 6000, 8000, <strong>and</strong> 12,000 years [deMenocat <strong>and</strong> recovery, which is rarely cont<strong>in</strong>uous <strong>in</strong> the deeper bore-<br />

K<strong>in</strong>g, 1995]. <strong>The</strong> causes <strong>and</strong> consequences <strong>of</strong> suborbital holes.<br />

1.0<br />

1.5<br />

18<br />

Glacial Interglacial<br />

Jar<br />

5 4.5 4 3.5 3<br />

I''''1 .... I''''1''''1<br />

Old --<br />



1.15<br />

1.20<br />

1.25<br />

Age<br />

(Ma)<br />

1.30<br />

1.35<br />

1.45<br />

ß<br />

0.4<br />

Resistivity<br />

0.8<br />

FMS (16 button avg.)<br />

4 2 0 -2 -4 -6<br />

125 ,,,,,,,,,<br />

126<br />

127<br />

128<br />

129 %<br />

.....<br />

130<br />

10-<br />

10 -2<br />

10 -3<br />

10 -4<br />

10 -6<br />

Site 798B<br />

Sub-orbital variability<br />

41 kyr<br />

12-13 kyr<br />

8 kyr<br />

6 kyr<br />

0 0.05 0.1 0.15 0.2 0.25 0.3<br />

1.4o 41 kyr cycles freq (i/kyr)<br />

132 I<br />

Figure 9. Comparison <strong>of</strong> data from three wirel<strong>in</strong>e resistivity tools: shallow <strong>and</strong> deep resistivity <strong>and</strong> the<br />

averaged FMS record that show cycles at a 41,000-year orbital period <strong>and</strong> below <strong>in</strong> the comparative power<br />

spectra [after deMenocal <strong>and</strong> K<strong>in</strong>g, 1995]. <strong>The</strong> resistivity logs have been converted to age, <strong>and</strong> the averaged<br />

FMS data are plotted on relative scale over a 7-m <strong>in</strong>terval. <strong>The</strong> f<strong>in</strong>e vertical resolution <strong>of</strong> the FMS data allows<br />

observation <strong>of</strong> periods as short as 6000 years.<br />

Magnetostratigraphy<br />

<strong>and</strong> correlated both with core measurements <strong>and</strong> with<br />

<strong>The</strong> ability to obta<strong>in</strong> direct <strong>and</strong> closely spaced mea- the geomagnetic polarity timescale over several hundred<br />

surements <strong>of</strong> absolute age has been a long-term goal <strong>of</strong> meters depth [Thibal et al., 1995]. Figure 10 shows the<br />

mar<strong>in</strong>e sediment cor<strong>in</strong>g. Usually, biostratigraphic or downhole polarity sequence compared with the reversal<br />

magnetostratigraphic time markers are identified <strong>in</strong> a time scale. Although the downhole record agrees with<br />

core sample <strong>and</strong> their age is <strong>in</strong>terpreted from the tie to the polarity sequence derived from core data, it is more<br />

a magnetic reversal timescale or biostratigraphic stage. complete <strong>and</strong> allows for a confident comparison with a<br />

From the accumulation <strong>of</strong> sediment between age markers, geomagnetic timescale. In addition, the cont<strong>in</strong>uous<br />

the sedimentation rate can be determ<strong>in</strong>ed <strong>in</strong> recovered downhole geochronology provided an un<strong>in</strong>terrupted<br />

core sections where sedimentation has been un<strong>in</strong>ter- record <strong>of</strong> paleomagnetic <strong>in</strong>tensity. Thibal et al. [1995]<br />

rupted. With the development <strong>of</strong> downhole <strong>in</strong>struments compared the cont<strong>in</strong>uous change <strong>in</strong> relative magnetic<br />

that can measure high-resolution magnetic field <strong>in</strong>ten- <strong>in</strong>tensity over time at this high-latitude site with the<br />

sity, however, a direct <strong>and</strong> cont<strong>in</strong>uous <strong>in</strong> situ record <strong>of</strong> results from low-latitude core measurements made by<br />

the mar<strong>in</strong>e magnetic reversal history may be effectively Valet <strong>and</strong> Meynadier [1993] <strong>and</strong> confirmed that magnetic<br />

measured <strong>in</strong> a variety <strong>of</strong> sedimentary environments. <strong>in</strong>tensity decreases sharply at the onset <strong>of</strong> a reversal,<br />

Precession magnetometers <strong>and</strong> <strong>in</strong>duction susceptibil- followed by an <strong>in</strong>crease <strong>in</strong> field strength. Each period <strong>of</strong><br />

ity tools have been used <strong>in</strong> comb<strong>in</strong>ation to determ<strong>in</strong>e the constant polarity then decreases <strong>in</strong> <strong>in</strong>tensity. This <strong>in</strong>ter<strong>in</strong><br />

situ magnetostratigraphy <strong>in</strong> weakly magnetized sedi- pretation <strong>of</strong> the downhole data suggests that the duraments<br />

[Desvignes et al., 1992; Etchecopar et al., 1993; tion <strong>of</strong> the Earth's polarity field is <strong>in</strong>versely proportional<br />

Pozzi et al., 1993]. <strong>The</strong>se tools are used <strong>in</strong> comb<strong>in</strong>ation <strong>in</strong> to the mean rate <strong>of</strong> decrease <strong>of</strong> magnetic <strong>in</strong>tensity, that is,<br />

order to subtract the <strong>in</strong>duced magnetization <strong>of</strong> the sed- its length is determ<strong>in</strong>ed by the <strong>in</strong>tensity <strong>of</strong> the reversal.<br />

iments from the total measured magnetic field <strong>and</strong> de- In addition, downhole magnetic susceptibility logs<br />

term<strong>in</strong>e the polarity <strong>of</strong> the remanence. In recent studies have been correlated to core susceptibility measure<strong>in</strong><br />

Pliocene sediments from the northern Pacific, rema- ments at several sites, provid<strong>in</strong>g excellent stratigraphic<br />

nent magnetostratigraphy has been measured downhole control [Pariso <strong>and</strong> Johnson, 1991; Rob<strong>in</strong>son, 1992; Du-


buisson et al., 1995]. In recent drill<strong>in</strong>g along the east<br />

flank <strong>of</strong> the Reykjanes Ridge <strong>in</strong> the North Atlantic,<br />

identify<strong>in</strong>g changes <strong>in</strong> the Pliocene-Pleistocene climate<br />

was improved by a high-resolution correlation <strong>of</strong> core<br />

<strong>and</strong> log measurements <strong>of</strong> magnetic susceptibility [Higg<strong>in</strong>s<br />

et al., 1997]. In Figure 11, core <strong>and</strong> log magnetic<br />

susceptibility pr<strong>of</strong>iles at a site drilled through sediments<br />

accumulated over the past 3-3.5 m.y. at a high sedimentation<br />

rate (120-170 m/m.y.) were correlated peak by<br />

peak. <strong>The</strong> log data provide stronger susceptibility signals<br />

<strong>and</strong> more cont<strong>in</strong>uous records than core measurements,<br />

fill<strong>in</strong>g gaps limited by the core recovery <strong>in</strong> the deeper<br />

<strong>in</strong>terval <strong>of</strong> the hole. Both data sets show Milankovitch<br />

periodicities extend<strong>in</strong>g to the bottom <strong>of</strong> the hole <strong>and</strong> can<br />

be used to improve biostratigraphic <strong>and</strong> magnetostratigraphic<br />

age dat<strong>in</strong>g determ<strong>in</strong>ed from cores. <strong>The</strong> dat<strong>in</strong>g <strong>of</strong><br />

sedimentary sequences us<strong>in</strong>g cont<strong>in</strong>uous magnetic logs<br />

or by correlation with Milankovitch cycles observed <strong>in</strong><br />

other logs provides a critical l<strong>in</strong>k between depth <strong>and</strong> age<br />

<strong>in</strong> mar<strong>in</strong>e bas<strong>in</strong>s.<br />

0<br />

0<br />

50-<br />

100.<br />

150i<br />

200-<br />

Z(z)<br />

(10 -6 SI)<br />

2000<br />

B'(z) Geomagnetic<br />

(nT) polarity time<br />

- 100 0 100<br />

..... I ..... ! .....<br />

scale<br />

polarity <strong>Downhole</strong><br />

: sequenc<br />

I<br />

_ -m :... :i: ....<br />

................... g :':!½i-.½..'.-: --_ ....<br />

,<br />

,<br />

,<br />

Methane Gas Hydrates<br />

<strong>Downhole</strong> measurements <strong>in</strong> sedimentary bas<strong>in</strong>s reveal<br />

<strong>in</strong> situ properties that are hard to measure from<br />

core samples, such as high-pressure <strong>and</strong> gas-rich zones.<br />

250.<br />

,<br />

Methane gas hydrates are formed from methane <strong>in</strong> the<br />

buried sediments on cont<strong>in</strong>ental marg<strong>in</strong>s when the<br />

methane-water mixture freezes at cold temperatures <strong>and</strong><br />

300. .<br />

high pressures <strong>in</strong> a thermodynamic stability zone. Often<br />

s<strong>and</strong>, clay, <strong>and</strong> mixed s<strong>and</strong>-clay sequences conta<strong>in</strong> pres-<br />

-'::: ........ . l<br />

surized, methane hydrates that can be detected by large<br />

seismic reflections that run parallel to <strong>and</strong> below the<br />

seafloor <strong>and</strong> usually <strong>in</strong>dicate their spatial distribution<br />

[e.g., Stoll, 1971; Kvenvolden, 1993; Wood et al., 1994].<br />

350.<br />

<strong>Downhole</strong> logs have recorded data <strong>of</strong> the ephemeral<br />

400<br />

properties <strong>of</strong> methane hydrates where they have been<br />

<strong>in</strong>dicated on seismic surveys but have dissolved or been<br />

severely altered by attempts to drill <strong>and</strong> core them .: ... .::...... .... . ':..<br />

[Collett, 1993; Prensky, 1995]. <strong>The</strong> downhole velocity<br />

data have proven critical <strong>in</strong> quantify<strong>in</strong>g the nature <strong>of</strong> this<br />

Figure 10. Comparison <strong>of</strong> downhole magnetic susceptibility<br />

X(z) <strong>and</strong> remanent field B' (z) results <strong>in</strong> ODP Hole 884E <strong>in</strong><br />

seismic reflection; velocity decreases ow<strong>in</strong>g to free gas<br />

the northern Pacific with the geomagnetic polarity timescale<br />

below the hydrate layer <strong>and</strong> generates a large seismic<br />

[from Thibal et al., 1995]. <strong>The</strong> <strong>in</strong>terpretation <strong>of</strong> the downhole<br />

reflection [MacKay et al., 1994; ODP Leg 164 Shipboard polarity sequence from the logs enables sedimentation rate<br />

Scientific Party, 1996]. Because the occurrence <strong>of</strong> meth- <strong>and</strong> magnetic reversal strength to be determ<strong>in</strong>ed where core<br />

ane hydrates <strong>and</strong> free gas may be predicted from seismic data are not available.<br />

data <strong>and</strong> detailed downhole velocity data, more reliable<br />

estimates <strong>of</strong> the volume <strong>of</strong> gas <strong>and</strong> methane hydrate can<br />

be made to <strong>in</strong>dicate the extent <strong>of</strong> this vast untapped ties are essential. On the Guatemala marg<strong>in</strong>, evidence <strong>of</strong><br />

natural resource.<br />

high pressure was found <strong>in</strong> both recovered core <strong>and</strong> <strong>in</strong><br />

Ocean drill<strong>in</strong>g has acquired logs <strong>in</strong> methane hydrate logs <strong>of</strong> a 10-m-thick methane hydrate about 250 m below<br />

at several sites, most recently on the Blake Ridge <strong>of</strong>f <strong>of</strong> the seafloor [Mathews <strong>and</strong> yon Huene, 1985]. <strong>The</strong> data<br />

the U.S. eastern seaboard. Other locations <strong>of</strong> mar<strong>in</strong>e <strong>in</strong>dicated low gamma ray <strong>and</strong> density, high electrical<br />

hydrates that have been drilled <strong>in</strong>clude the Guatemala, resistivity <strong>and</strong> acoustic velocity, <strong>and</strong> generally consistent<br />

Peru, <strong>and</strong> Cascadia coastal sediments [Mathews <strong>and</strong> yon <strong>and</strong> uniform clay composition through the drilled <strong>in</strong>ter-<br />

Huene, 1985; Miller et al., 1991; MacKay et al., 1994]. val. From these downhole measurements, Mathews<br />

Because the porosity <strong>and</strong> permeability <strong>of</strong> sediments play [1986] assessed the concentration <strong>of</strong> methane hydrate.<br />

a critical role <strong>in</strong> gas migration <strong>and</strong> stability <strong>of</strong> methane Collett [1993] applied a similar approach us<strong>in</strong>g downhole<br />

hydrates, <strong>in</strong> situ measurements <strong>of</strong> their physical proper- logs on the cont<strong>in</strong>ental slope <strong>of</strong> Alaska. An <strong>in</strong>crease <strong>in</strong><br />

..:........:..: ..................


20<br />

40<br />

6O<br />

1 oo<br />

120<br />

140<br />

160<br />

Core Mag. Suscep. (SI)<br />

0 500 1060 0 500 1000 0<br />

core<br />

i<br />

log<br />

core<br />

500 1000<br />

I I I I<br />

500 1000 1500 500 1000 1500 500 1000 1500 2000<br />

Log Mag. Suscep. (SI)<br />

Figure 11. Comparison <strong>of</strong> downhole magnetic susceptibility log <strong>and</strong> shipboard measurements on core sections<br />

recovered from Hole 984D <strong>in</strong> the North Atlantic [after Higg<strong>in</strong>s et al., 1997]. <strong>The</strong> excellent correlation allows<br />

gaps <strong>in</strong> core data to be <strong>in</strong>ferred from the log results to improve the age dat<strong>in</strong>g determ<strong>in</strong>ed from cores.<br />

electrical resistivity <strong>and</strong> acoustic velocity was associated the dat<strong>in</strong>g <strong>of</strong> large eruptions. In mar<strong>in</strong>e sediments, volwith<br />

the hydrate occurrence because the frozen hydrate canic ash layers are <strong>of</strong>ten found with mar<strong>in</strong>e carbonates<br />

reduced porosity <strong>and</strong> stiflened the sediment beyond <strong>and</strong> nonvolcanic clays. Ashes are <strong>of</strong>ten observed <strong>in</strong> core<br />

what might be expected from sediment compaction. On samples <strong>and</strong> can be dist<strong>in</strong>guished by their relatively high<br />

the Blake Ridge, recent logg<strong>in</strong>g reveals <strong>in</strong>creases <strong>in</strong> ratio <strong>of</strong> Th/U concentration [e.g., Schlich et al., 1989].<br />

velocity <strong>and</strong> resistivity with <strong>in</strong>creas<strong>in</strong>g hydrate occur- When abrupt boundaries between carbonates, clay, or<br />

rence to a depth where the hydrates become unstable;<br />

ash layers occur, downhole log images are particularly<br />

then decreases <strong>in</strong> velocity <strong>and</strong> resistivity below it are<br />

useful for identify<strong>in</strong>g the location <strong>and</strong> attitude <strong>of</strong> these<br />

associated with the presence <strong>of</strong> free gas [ODP Leg 164<br />

unconformities. Figure 12 shows a FMS resistivity log<br />

Shipboard Scientific Party, 1996]. <strong>The</strong> use <strong>of</strong> LWD techfrom<br />

the Sea <strong>of</strong> Japan that illustrates a 1-cm-thick,<br />

nology would <strong>in</strong>sure the measurement <strong>of</strong> <strong>in</strong> situ properties<br />

with a m<strong>in</strong>imum <strong>of</strong> drill<strong>in</strong>g <strong>and</strong> cor<strong>in</strong>g disturbance <strong>of</strong><br />

horizontal ash layer at 174.7 m depth with<strong>in</strong> porous<br />

methane hydrate <strong>and</strong> gas-bear<strong>in</strong>g sediments.<br />

carbonate sediments that occurred dur<strong>in</strong>g a short,<br />

nearby volcanic eruption at approximately 1.5 Ma. <strong>The</strong><br />

high-resistivity (white) ash layer marks a def<strong>in</strong>itive po<strong>in</strong>t<br />

Volcanics <strong>and</strong> the Cretaceous-Tertiary Boundary that was used to shift the expansion-corrected core by<br />

VolCanic eruptions <strong>and</strong> the traces they leave <strong>in</strong> the +0.70 m to match the downhole log. Ash layer correlamar<strong>in</strong>e<br />

sedimentary record are not well understood, but tion from hole to hole is particularly <strong>in</strong>formative where<br />

ash layers recovered <strong>in</strong> core sections or observed <strong>in</strong> core recovery is low or when pulses <strong>of</strong> volcanic activity<br />

downhole measurements provide dist<strong>in</strong>ct markers for can be identified <strong>and</strong> dated. In the Mediterranean, for


I,I<br />

175.3 -<br />

175.4 -<br />

175.5 -<br />

CORE<br />


FMS<br />


0 60 120 180 240 300 360<br />

- 174<br />

- 175<br />

- 176<br />

Figure 12. Example <strong>of</strong> the correlation between (left) a core photograph <strong>and</strong> (right) an FMS image <strong>of</strong> a<br />

l-cm-thick ash layer <strong>in</strong> ODP Hole 798B <strong>in</strong> the Sea <strong>of</strong> Japan [from deMenocal e! al., 1992]. <strong>The</strong> FMS images<br />

<strong>of</strong> four orthogonal traces are oriented to north at 0 ø magnetic decl<strong>in</strong>ation <strong>and</strong> represent the ash as a<br />

high-resistivity (white) fiat layer at 174.7 m bsf. <strong>The</strong> expansion-corrected core depth was shifted by +0.70 m<br />

to match the downhole log. .<br />

example, downhole logs were used to determ<strong>in</strong>e zones <strong>of</strong> logic changes observed from core sections. At most <strong>of</strong><br />

abundant ash separated by 150,000- to 200,000-year pe- these locations, however, <strong>in</strong>complete core recovery has<br />

riods <strong>of</strong> mar<strong>in</strong>e deposition [Kastens et al., 1987]. made the exact location <strong>and</strong> thickness <strong>of</strong> the boundary<br />

Us<strong>in</strong>g downhole logs, the boundary between the Cre- difficult to determ<strong>in</strong>e. Although the downhole signatures<br />

taceous <strong>and</strong> Tertiary periods has been studied at eight <strong>of</strong> the Cretaceous-Tertiary (K-T) boundary are not alsites,<br />

<strong>in</strong> the south Atlantic, Caribbean, <strong>and</strong> eastern In- ways dist<strong>in</strong>ctive, these data can be critical <strong>in</strong> determ<strong>in</strong><strong>in</strong>g<br />

dian Oceans. At each site the ext<strong>in</strong>ction boundary was the contact <strong>and</strong> thickness <strong>of</strong> sedimentary deposits near<br />

determ<strong>in</strong>ed us<strong>in</strong>g biostratigraphic markers <strong>and</strong> litho- the K-T event. <strong>The</strong> boundary is <strong>of</strong>ten marked by abrupt<br />



250<br />

50O<br />

550<br />

4OO<br />

25o<br />

=5<br />

volcanic cycle T 640 k.y.<br />

T<br />

2 m.y.<br />

volcanic cycle<br />

Figure 13. Variations <strong>of</strong> potassium, thorium, <strong>and</strong> uranium<br />

versus depth recorded <strong>in</strong> two ODP holes <strong>in</strong> the central Indian<br />

Ocean [from Pratson et al., 1990]. Potassium is enriched at the<br />

Cretaceous/Tertiary (K-T) boundary due to nondeposition <strong>of</strong><br />

carbonates, although the thorium/uranium ratio does not dist<strong>in</strong>guish<br />

the volcanic cycles above or below this boundary.<br />

<strong>in</strong>creases <strong>in</strong> natural gamma radiation, <strong>in</strong>dicat<strong>in</strong>g high<br />

concentrations <strong>of</strong> K-rich clay as well as volcanics with<br />

high Th/U ratios. Anomalies <strong>in</strong> resistivity <strong>and</strong> velocity<br />

logs are also <strong>of</strong>ten observed at the K-T boundary. In<br />

Figure 13 the K-T boundary at two sites on the Kerguelen<br />

Plateau <strong>in</strong> the Indian Ocean is illustrated by<br />

natural gamma ray logs as a th<strong>in</strong> zone enriched <strong>in</strong> K. At<br />

the K-T event, the nondeposition <strong>of</strong> carbonates causes<br />

clay to concentrate, but the clays are not dist<strong>in</strong>ctive <strong>in</strong> Th<br />

T<br />

sections (H. Sigurdsson, personal communication, 1996).<br />

<strong>The</strong> log data made it feasible to determ<strong>in</strong>e the thickness<br />

<strong>of</strong> the poorly consolidated boundary clay deposits, which<br />

were only partially recovered by cor<strong>in</strong>g. From the downhole<br />

data <strong>in</strong> these studies, it is clear that <strong>in</strong>creased<br />

nondeposition <strong>of</strong> carbonate marked by K-rich clay deposits<br />

is typical across the K-T boundary. However,<br />

regional volcanism, rather than a dist<strong>in</strong>ctive worldwide<br />

event, may be superimposed on these downhole signa-<br />

tures.<br />

Earth's Crustal Structure<br />

Oceanic crust underlies 60% <strong>of</strong> the Earth's surface. It<br />

is formed at mid-ocean ridges by a complex comb<strong>in</strong>ation<br />

<strong>of</strong> processes, <strong>in</strong>clud<strong>in</strong>g extrusion <strong>of</strong> lava, <strong>in</strong>trusion <strong>of</strong><br />

basalt dikes, <strong>and</strong> underplat<strong>in</strong>g <strong>of</strong> magma, which plays an<br />

important role <strong>in</strong> the mass balance on Earth. As the<br />

crust ages over several hundred thous<strong>and</strong> to a few million<br />

years, it thickens from a few hundred meters at the<br />

ridge to several kilometers <strong>in</strong> the deep ocean. Models <strong>of</strong><br />

the formation <strong>and</strong> layer<strong>in</strong>g <strong>of</strong> the oceanic crust have<br />

relied almost exclusively on the results <strong>of</strong> seismic refraction<br />

experiments [Raitt, 1963; Talwani et al., 1971; Houtz<br />

<strong>and</strong> Ew<strong>in</strong>g, 1976; Ew<strong>in</strong>g <strong>and</strong> Purdy, 1982; l/era et al.,<br />

1990]. If drill<strong>in</strong>g through the sequence could be<br />

achieved, it could verify which model <strong>of</strong> the structure<br />

<strong>and</strong> evolution <strong>of</strong> the oceanic crust is correct. Unfortu-<br />

nately, penetrat<strong>in</strong>g deep enough to core crustal sections<br />

has proven to be a difficult task throughout the history <strong>of</strong><br />

ocean drill<strong>in</strong>g, <strong>and</strong> a complete crustal section has not<br />

been achieved to date. A variety <strong>of</strong> oceanic crustal<br />

environments have been drilled, however, from young<br />

midplate basalt to flood basalt prov<strong>in</strong>ces to highly de-<br />

formed gabbro, but <strong>in</strong> many cases, hole <strong>in</strong>stability has<br />

been the major reason for the somewhat limited recov-<br />

<strong>and</strong> U [Pratson et al., 1992]. <strong>The</strong> Th/U ratio shows that<br />

ery <strong>of</strong> core <strong>and</strong> downhole data [e.g., Gillis et al., 1993;<br />

Cannat et al., 1995]. One <strong>of</strong> the most successful DSDP/<br />

ODP efforts devoted four legs to drill<strong>in</strong>g <strong>and</strong> logg<strong>in</strong>g at<br />

Sites 417 <strong>and</strong> 418 <strong>in</strong> Cretaceous age crust <strong>in</strong> the Atlantic<br />

[Hyndman et al., 1984; Salisbury et al., 1988]. <strong>The</strong>se<br />

volcanic ash layers are found both above <strong>and</strong> below the expeditions acquired a comprehensive suite <strong>of</strong> downhole<br />

K-T boundary <strong>and</strong> are related to the same volcanic measurements that significantly augmented the coresource,<br />

but they do not correlate with the ext<strong>in</strong>ction based lithology <strong>and</strong> underst<strong>and</strong><strong>in</strong>g <strong>of</strong> the seismic <strong>and</strong><br />

event.<br />

porosity structure <strong>of</strong> the upper oceanic crust. Few sites<br />

At other Indian Ocean sites on the Exmouth Plateau, have been as successful, <strong>and</strong> only one hole has been<br />

the K-T boundary is <strong>in</strong>dicated by an <strong>in</strong>crease <strong>in</strong> clay drilled to a greater depth <strong>in</strong>to the crust.<br />

content as well as by decreases <strong>in</strong> velocity <strong>and</strong> resistivity Drilled dur<strong>in</strong>g seven DSDP <strong>and</strong> ODP expeditions to<br />

[Haq et al., 1990]. Resistivity logs on the Exmouth Pla- over 2.1 km below the seafloor, Hole 504B <strong>in</strong> the eastern<br />

teau also <strong>in</strong>dicated a reduction <strong>in</strong> permeability above the equatorial Pacific is the world's deepest DSDP/ODP<br />

K-T boundary [Borehole Research Group, 1990]. In the penetration <strong>in</strong> the oceans. Hole 504B has been the most<br />

southern Atlantic Ocean, however, drill<strong>in</strong>g across the <strong>in</strong>tensely studied site us<strong>in</strong>g core <strong>and</strong> downhole data, yet<br />

K-T boundary does not reveal volcanic ash layers like it still penetrates through only 30% <strong>of</strong> the oceanic crust.<br />

those observed on the Kerguelen Plateau but reveals an Table 2 summarizes four drill<strong>in</strong>g <strong>and</strong> logg<strong>in</strong>g expeditions<br />

<strong>in</strong>crease <strong>in</strong> K-rich clay below it [Ciesielski et al., 1988]. <strong>in</strong> the eastern Pacific <strong>and</strong> other crustal site locations<br />

Recently, downhole density, natural gamma ray, resis- s<strong>in</strong>ce 1993. It is clear that cor<strong>in</strong>g, <strong>and</strong> especially logg<strong>in</strong>g,<br />

tivity logs, <strong>and</strong> FMS images at two sites <strong>in</strong> the central <strong>in</strong> the ocean crust is not <strong>of</strong>ten as successful as it has been<br />

Caribbean have detected the thickness <strong>of</strong> th<strong>in</strong> clay layers <strong>in</strong> Hole 504B. With typical core recovery well below<br />

bound<strong>in</strong>g the K-T ext<strong>in</strong>ction event observed <strong>in</strong> core 50%, the downhole log record<strong>in</strong>gs <strong>of</strong> porosity, perme-


TABLE 2. Recent Drill<strong>in</strong>g <strong>and</strong> Logg<strong>in</strong>g at Crustal Sites<br />

Holes Holes<br />

Drilled Logged<br />

North Atlantic 17 0<br />

Mid-Atlantic 15 0<br />

East Pacific 2 2<br />

Central Pacific 13 1<br />

Total Depth Maximum Average Core<br />

Logged, m Penetration, mbsf Recovery, %<br />

0.0 125.7 12.7<br />

0.0 200.8 32.1<br />

2078.8 2111.0 17.8<br />

84.0 154.5 22.5<br />

ability, <strong>and</strong> velocity pr<strong>of</strong>iles have been essential for eval- the presence <strong>of</strong> secondary voids, such as fractures, <strong>and</strong><br />

uat<strong>in</strong>g the layered stratigraphy <strong>of</strong> the oceanic crust <strong>and</strong> alteration effects [e.g., Purdy, 1987; yon Herzen et al.,<br />

l<strong>in</strong>k<strong>in</strong>g rock properties, such as porosity <strong>and</strong> permeabil- 1992]. Sonic logs <strong>and</strong> borehole seismic experiments<br />

ity, to seismic data.<br />

agree well with seismic refraction results because they<br />

Porosity variation is <strong>of</strong>ten used to expla<strong>in</strong> changes <strong>in</strong> span larger volumes that <strong>in</strong>clude the secondary voids<br />

both log <strong>and</strong> seismic velocities with depth <strong>in</strong> the crust <strong>and</strong> alteration effects. Figure 14 illustrates the downhole<br />

[e.g., Mathews et al., 1984; Moos et al., 1986; Detrick et al., changes <strong>in</strong> velocity <strong>and</strong> resistivity (which is <strong>in</strong>versely<br />

1994], although nearly all logs respond <strong>in</strong> different ways related to porosity). Detrick et al. [1994] show a layered<br />

to porosity over different scales. Which type <strong>of</strong> log yields velocity model derived from seismic refraction data that<br />

the best estimate <strong>of</strong> porosity <strong>in</strong> a particular environment <strong>in</strong>dicates a gradational change <strong>in</strong> the velocity structure at<br />

is not clear, but resistivity measurements have been used approximately 1.2 km with<strong>in</strong> the sheeted dikes <strong>in</strong> Hole<br />

most <strong>of</strong>ten. Francis [1981] used a large-scale experiment 504B. In typical layered models <strong>of</strong> oceanic crust, how<strong>in</strong><br />

sediments to measure resistivity over <strong>in</strong>tervals <strong>of</strong> ever, this change has been associated with the boundary<br />

10-50 m; <strong>in</strong> Hole 504B a similar method was used to between sheeted dikes <strong>and</strong> gabbro; its location higher <strong>in</strong><br />

measure resistivities <strong>in</strong> the ocean crust [Becker, 1985]. the sequence may be due to an <strong>in</strong>crease <strong>in</strong> fractur<strong>in</strong>g <strong>and</strong><br />

Both large-scale experiments generally agree with lateral porosity at a shallower depth.<br />

resistivity logs. <strong>The</strong> porosity logs computed us<strong>in</strong>g Arch- In addition to velocity, seismic attenuation can play a<br />

ie's [1942] empirical relationship from resistivity data <strong>in</strong> major role <strong>in</strong> underst<strong>and</strong><strong>in</strong>g the alteration, fluid <strong>in</strong>cluthe<br />

crust reflect pore spaces filled both with seawater sion, texture, microstructure, <strong>and</strong> heterogeneity <strong>of</strong> the<br />

<strong>and</strong> with residual alteration m<strong>in</strong>erals that are deposited oceanic crust [Jacobson <strong>and</strong> Lewis, 1990; Wepfer <strong>and</strong><br />

by hydrothermal fluids. To <strong>in</strong>terpret these results, alter- Christensen, 1990; Jacobson, 1992]. Figure 15 illustrates<br />

ation effects <strong>in</strong> the apparent porosity estimated from the magnitude <strong>of</strong> attenuation due to velocity heterogedownhole<br />

resistivity data must be accounted for <strong>and</strong> neity measured us<strong>in</strong>g different downhole <strong>and</strong> core data;<br />

corrected [Pezard, 1990].<br />

measurements at the scale <strong>of</strong> log data agree with direct<br />

Hole 504B comprises over 1.7 km <strong>of</strong> pillow lava <strong>and</strong> attenuation estimates made us<strong>in</strong>g downhole seismic data<br />

sheeted dikes with permeability as low as 10 microdarcy [Swift <strong>and</strong> Stephen, 1992; Goldberg <strong>and</strong> Y<strong>in</strong>, 1994]. Us<strong>in</strong>g<br />

<strong>and</strong> corrected porosity less than 0.5%, effectively seal<strong>in</strong>g the velocity log <strong>in</strong> Hole 504B, Goldberg <strong>and</strong> Sun [1997]<br />

all but the uppermost 300-400 m <strong>of</strong> the hole to hydro- correlated porosity <strong>and</strong> velocity changes with seismic<br />

thermal fluid flow [Becker et al., 1989; Altet al., 1993]. attenuation due to scatter<strong>in</strong>g from heterogeneity that is<br />

Measured downhole permeabilities decrease with poros- largely due to changes <strong>in</strong> crustal morphology. <strong>The</strong>y<br />

ity <strong>and</strong> with depth <strong>and</strong> correspond to zones <strong>of</strong> dist<strong>in</strong>ctive found that seismic attenuation varies exponentially with<br />

crustal structure [Anderson et al., 1982]. Anderson et al. porosity <strong>and</strong> therefore decreases as the structure <strong>of</strong> the<br />

[1982], Becker et al. [1989], <strong>and</strong>Alt et al. [1993] conclude ocean crust changes with depth. <strong>Downhole</strong> measurethat<br />

fractur<strong>in</strong>g contributes most to the open porosity <strong>and</strong> ments <strong>of</strong> velocity, porosity, <strong>and</strong> attenuation are effective<br />

permeability <strong>and</strong> extends only to a shallow depth <strong>in</strong> Hole predictors <strong>of</strong> changes <strong>in</strong> structure, particularly <strong>in</strong> the<br />

504B. Pezard [1990] has measured the amount <strong>of</strong> frac- absence <strong>of</strong> core <strong>in</strong> crustal drill holes, <strong>and</strong> are critical for<br />

tur<strong>in</strong>g us<strong>in</strong>g the resistivity log <strong>and</strong> FMS imag<strong>in</strong>g. Little comparison with seismic refraction data <strong>in</strong> the <strong>in</strong>vesti<strong>and</strong><br />

Stephen [1985], Moos et al. [1986, 1990], <strong>and</strong> Wilkens gation <strong>of</strong> the layered structure <strong>of</strong> the Earth's crust.<br />

et al. [1991], among others, correlate the uppermost<br />

fractured <strong>and</strong> permeable layers <strong>in</strong> Hole 504B to changes Geochemistry <strong>of</strong> the Oceanic Crust<br />

<strong>in</strong> crustal structure that <strong>in</strong>crease velocity with depth <strong>in</strong> Fluid flow through permeable layers <strong>in</strong> the oceanic<br />

the hole.<br />

crust is important <strong>in</strong> controll<strong>in</strong>g the chemistry <strong>of</strong> the<br />

Numerous experiments verify a strong correlation <strong>of</strong> rock as well as the seawater. Our knowledge <strong>of</strong> the<br />

velocity with porosity <strong>in</strong> crustal rocks [e.g., Fox et al., composition <strong>and</strong> chemistry <strong>of</strong> the oceanic crust has been<br />

1973; Christensen <strong>and</strong> Salisbury, 1975; Wilkens et al., greatly enhanced by downhole measurements. In partic-<br />

1983; Christensen et al., 1989]. Laboratory-derived veloc- ular, nuclear logs which are sensitive to elemental chemities<br />

on ocean crustal samples, however, are difficult to istry <strong>of</strong> crustal materials can read changes <strong>in</strong> bulk chemcorrelate<br />

with seismic refraction velocities because <strong>of</strong> ical composition with depth [Herron <strong>and</strong> Herron, 1990;


0.0<br />

'o 0.8<br />

1.2<br />

( 1.6<br />

2.0<br />

Resistivity (-m)<br />

I 10 100 1000<br />

2<br />

Vs<br />

Velocity (km/s)<br />

I<br />

%<br />

i<br />

I<br />

I<br />

%<br />

I<br />

I<br />

4 5<br />

sonic log (P-wave)<br />

borehole seismic<br />

sonobuoy<br />

Layer 2<br />

er3<br />

Hole 504B<br />

? Gabbro ?<br />

Volcanics<br />

Transition<br />

Sheeted<br />

Dikes<br />

_. Base <strong>of</strong><br />

Hole 504B<br />

Figure 14. Summary <strong>of</strong> crustal velocity <strong>and</strong> resistivity logs <strong>in</strong> ODP Hole 504B <strong>in</strong> the eastern equatorial Pacific<br />

with the crustal velocity structure estimated from seismic surveys <strong>and</strong> lithostratigraphy <strong>in</strong>ferred from core<br />

[from Dettck et al., 1994]. <strong>The</strong> data <strong>in</strong>dicate a change <strong>in</strong> vertical velocity gradient at approximately 1.2 km<br />

subbasement that is associated with a physical change with<strong>in</strong> the sheeted dikes (layer 2-layer 3).<br />

Herron et al., 1993]. Thompson [1973] describes the hy- <strong>The</strong>ir f<strong>in</strong>d<strong>in</strong>gs suggest that hydrothermal fluids have<br />

drothermal alteration that subsequently affects the fluid mobilized the low-temperature alteration products Ti<br />

<strong>and</strong> rock chemistry <strong>and</strong> that may also be observed us<strong>in</strong>g <strong>and</strong> K through conf<strong>in</strong>ed high-porosity <strong>and</strong> permeable<br />

<strong>in</strong>formation from geochemical logs [e.g., Anderson et al., zones that later become sealed at shallow depths by<br />

1989, 1990]. In fresh basalt subjected to low-temperature m<strong>in</strong>eral deposition along th<strong>in</strong> horizons.<br />

alteration, K <strong>and</strong> Mg from seawater typically replace Ca, In a similar example <strong>in</strong> the southwest Indian Ocean,<br />

Fe, <strong>and</strong> Si [Altet al., 1986]. Us<strong>in</strong>g downhole nuclear logs downhole nuclear logs were acquired through 500 m <strong>of</strong><br />

<strong>in</strong> Hole 504B, Anderson et al. [1989] measured the bulk gabbro <strong>in</strong> a fracture zone [Rob<strong>in</strong>son et al., 1989]. This<br />

elemental abundance <strong>in</strong> basalt <strong>and</strong> computed the rela- section <strong>of</strong> lower crustal material penetrated deformed<br />

tive chemical change that the rocks had undergone. <strong>The</strong>y <strong>and</strong> altered rocks that showed no evidence <strong>of</strong> the systhen<br />

compared the results with fresh basalt from newly tematic layer<strong>in</strong>g expected from similar sections on l<strong>and</strong>.<br />

formed oceanic ridges <strong>and</strong> found from this comparison Figure 16 shows highly enriched concentrations <strong>of</strong> Fe<br />

that Ti is absent <strong>and</strong> that K is enriched by 20% <strong>in</strong> the <strong>and</strong> Ti measured by the nuclear activation log <strong>in</strong> a 40- to<br />

upper crust <strong>and</strong> depleted by 10% <strong>in</strong> the lower crust. 50-m-thick structure near 250 m depth. This enrichment<br />

I/Q<br />

0.4<br />

0 2<br />

0 !<br />

LAB<br />

o<br />

LOG<br />

VSP<br />

-3 -2 -1 0 1 2 3 4<br />

(m)<br />

Figure 15. Comparison <strong>of</strong> computed attenuation<br />

spectra (curves) with average direct measurements<br />

from laboratory, sonic waveform, <strong>and</strong> VSP data<br />

(bars) from ODP Hole 735B <strong>in</strong> the southwest Indian<br />

Ocean [from Goldberg <strong>and</strong> Y<strong>in</strong>, 1994]. Major<br />

peaks <strong>in</strong> the computed spectra are attributed to<br />

formation heterogeneities that scatter acoustic energy.


220<br />

240<br />

260<br />

280<br />

0 %<br />

Iron<br />

Titanium<br />

% 31o<br />

Gadol<strong>in</strong>ium<br />

ppm Bulk<br />

density [<br />

30 2.0 g/cm3 3.5<br />

Figure 16. Nuclear log data from ODP Hole 735B <strong>in</strong> the southwest Indian Ocean illustrat<strong>in</strong>g the layered<br />

composition <strong>of</strong> gabbro <strong>and</strong> a thick titanium-rich (>5% weight percent) layer between 230 <strong>and</strong> 280 m bsf [after<br />

Worth<strong>in</strong>gton et al., 1991].<br />

is also responsible for other physical anomalies <strong>in</strong> this ages may aid by orient<strong>in</strong>g structures partially observed <strong>in</strong><br />

zone that are observed <strong>in</strong> downhole velocity, density core sections. One example <strong>of</strong> a borehole image through<br />

(shown), <strong>and</strong> magnetic susceptibility pr<strong>of</strong>iles [yon Herzen a basalt sequence <strong>in</strong> the Sea <strong>of</strong> Japan is shown <strong>in</strong> Figure<br />

et al., 1992; GoMberg et al., 1992]. Dick et al. [1991] 17. <strong>The</strong> FMS tool was passed two times over this <strong>in</strong>terval<br />

<strong>in</strong>terpret the m<strong>in</strong>eralization <strong>of</strong> this layer as the result <strong>of</strong> with a rotation <strong>of</strong> approximately 45 ø between passes; this<br />

vertical fault<strong>in</strong>g <strong>and</strong> extensive high-temperature alter- resulted <strong>in</strong> eight separate traces from the four orthogoation<br />

<strong>in</strong> the fracture zone. <strong>Downhole</strong> geochemical data nal sensors on the tool. <strong>The</strong> layered structure <strong>of</strong> basalt<br />

are critical <strong>in</strong> determ<strong>in</strong><strong>in</strong>g bulk elemental composition pillows over this 2.5-m <strong>in</strong>terval is apparent, as well as the<br />

<strong>and</strong> the chemistry <strong>of</strong> fracture-fill<strong>in</strong>g m<strong>in</strong>erals <strong>in</strong> the electrically conductive (dark) fluid-filled contacts <strong>and</strong><br />

crust, particularly s<strong>in</strong>ce core recovery <strong>of</strong> fractured <strong>and</strong> fractures with<strong>in</strong> them, which appear to consist <strong>of</strong> sealtered<br />

crustal rocks is typically poor.<br />

quences <strong>of</strong> 20- to 30-cm-thick, west dipp<strong>in</strong>g lava flows.<br />

<strong>The</strong> orientation <strong>and</strong> dip <strong>of</strong> each flow can be determ<strong>in</strong>ed<br />

State <strong>of</strong> Stress<br />

<strong>in</strong> the FMS image from the amplitude <strong>and</strong> orientation <strong>of</strong><br />

Stress plays a critical role <strong>in</strong> determ<strong>in</strong><strong>in</strong>g local frac- its s<strong>in</strong>usoidal shape [e.g., Paillet et al., 1990]. Where<br />

tur<strong>in</strong>g, porosity, <strong>and</strong> tectonic processes <strong>in</strong> the Earth's cont<strong>in</strong>uous downhole images have been acquired, they<br />

crust. <strong>The</strong>se properties <strong>of</strong> the crust depend on its have almost always aided significantly <strong>in</strong> differentiat<strong>in</strong>g<br />

strength <strong>in</strong> relation to the size <strong>of</strong> the tectonic forces lithologies <strong>and</strong> structural features, such as <strong>in</strong> these basalt<br />

act<strong>in</strong>g upon it. <strong>The</strong> <strong>in</strong> situ stress is critical to underst<strong>and</strong>- flows, <strong>and</strong> have <strong>of</strong>ten enabled study <strong>of</strong> the state <strong>of</strong> stress<br />

<strong>in</strong>g these tectonic processes. From cores, <strong>in</strong> situ stresses by orientation <strong>of</strong> the recovered core.<br />

can be estimated us<strong>in</strong>g laboratory techniques to measure Images that are recorded over the same depths <strong>in</strong><br />

micr<strong>of</strong>racture displacements or shear failure strength; core <strong>and</strong> logs can be aligned us<strong>in</strong>g dist<strong>in</strong>ct features that<br />

however, for the measurement to be mean<strong>in</strong>gful, the<br />

orientation <strong>of</strong> the stresses <strong>and</strong> the cores must be deterare<br />

jo<strong>in</strong>tly tied to the downhole magnetic reference<br />

po<strong>in</strong>t. Us<strong>in</strong>g core <strong>and</strong> log data together<br />

m<strong>in</strong>ed. Because <strong>of</strong> the typically sparse recovery <strong>of</strong> core MacLeod et al. [1992, 1996] was able to orient the fracmaterial<br />

<strong>in</strong> the oceanic crust, core orientation is difficult tures <strong>in</strong> complex tectonic sett<strong>in</strong>gs <strong>in</strong> the Pacific. In these<br />

<strong>and</strong> rare. Log data processed to generate borehole im- studies, fractures identified <strong>in</strong> core <strong>and</strong> <strong>in</strong> FMS images<br />

<strong>in</strong> this manner,


584 -<br />

I<br />

586<br />

Orientation<br />

0 180 360<br />

JllllllllllllllllllllJllllllllllJlllllrlll11111111111,l,111<br />

Figure 17. Example <strong>of</strong> an FMS image through a basalt sequence<br />

<strong>in</strong> ODP Hole 798B <strong>in</strong> the Sea <strong>of</strong> Japan. <strong>The</strong> FMS tool<br />

was run two times over this <strong>in</strong>terval with a rotation <strong>of</strong> approximately<br />

45 ø between passes, result<strong>in</strong>g <strong>in</strong> eight separate traces<br />

from the four orthogonal sensors. <strong>The</strong> layered structure <strong>of</strong><br />

basalt pillows over this 3-m <strong>in</strong>terval is apparent, as are the<br />

electrically conductive (dark) fluid-filled contacts <strong>and</strong> fractures<br />

with<strong>in</strong> them, which appear to consist <strong>of</strong> a sequence <strong>of</strong> 20- to<br />

30-cm-thick west dipp<strong>in</strong>g lava flows.<br />

over the same <strong>in</strong>terval <strong>of</strong> deformed <strong>and</strong> altered rock<br />

were statistically compiled, then compared <strong>and</strong> aligned<br />

to magnetic north. By their orientation, MacLeod con-<br />

cluded that the rocks reflected tectonic stresses <strong>and</strong><br />

high-temperature deformation from early <strong>in</strong> the region's<br />

history. Mathis et al. [1995] also describe a precision<br />

visual method to align <strong>in</strong>dividual features <strong>in</strong> core <strong>and</strong><br />

downhole images, rather than by a statistical compilation<br />

<strong>of</strong> the two data sets. This method may be used for<br />

detailed correlation <strong>and</strong> alignment with a magnetic reference<br />

as long as both the core <strong>and</strong> downhole images<br />

overlap.<br />

Fractur<strong>in</strong>g <strong>and</strong> borehole breakouts may be the direct<br />

result <strong>of</strong> stresses <strong>and</strong> can also <strong>in</strong>dicate the regional stress<br />

fields [e.g., Bell <strong>and</strong> Gough, 1979; Paillet et al., 1990;<br />

Moos <strong>and</strong> Zoback, 1990]. Breakouts form around a borehole<br />

when regional stresses cause shear failure <strong>and</strong><br />

pieces <strong>of</strong> the wall fall out. <strong>Downhole</strong> imag<strong>in</strong>g tools such<br />

as the acoustic televiewer or FMS can identify <strong>and</strong> orient<br />

borehole breakouts <strong>and</strong> fractures that may be used to<br />

<strong>in</strong>fer regional stress fields [Zemanek et al., 1970; Plumb<br />

<strong>and</strong> Hickman, 1985]. In Hole 504B, Newmark et al.<br />

[1984], Zoback et al. [1985], <strong>and</strong> Mor<strong>in</strong> et al. [1990]<br />

measured breakout orientation <strong>and</strong> suggested that a<br />

strike-slip stress regime exists to 1.3 km depth below<br />

seafloor, a f<strong>in</strong>d<strong>in</strong>g consistent with regional earthquake<br />

focal mechanisms. Pezard et al. [1997] have suggested a<br />

stress change <strong>in</strong> Hole 504B at this depth <strong>and</strong> strike-slip<br />

deformation below. In general, the stress regimes <strong>in</strong> the<br />

ocean crust are not completely understood; many factors<br />

affect the orientation <strong>and</strong> magnitude <strong>of</strong> stress-<strong>in</strong>duced<br />

fractures, drill<strong>in</strong>g-<strong>in</strong>duced thermal fractures, <strong>and</strong> break-<br />

outs <strong>in</strong> a borehole. Direct downhole measurements that<br />

generate images provide the only means <strong>of</strong> observ<strong>in</strong>g<br />

these features.<br />

An alternate, but untried, approach to estimat<strong>in</strong>g<br />

stress magnitude <strong>and</strong> direction <strong>in</strong> the ocean crust could<br />

use logg<strong>in</strong>g <strong>in</strong>struments that directly measure shear wave<br />

velocity <strong>and</strong> anisotropy [e.g., Zemanek et al., 1991]. Both<br />

laboratory <strong>and</strong> downhole data <strong>in</strong>dicate that velocity <strong>and</strong><br />

amplitude changes occur <strong>in</strong> formations that are seismically<br />

anisotropic as a result <strong>of</strong> fractur<strong>in</strong>g <strong>and</strong> stress fields<br />

not associated with disturbances <strong>in</strong> the immediate vic<strong>in</strong>-<br />

ity <strong>of</strong> a borehole [Emersoy et al., 1994]. With this approach,<br />

the relationship between downhole velocity <strong>and</strong><br />

the state <strong>of</strong> stress <strong>in</strong> the oceanic crust could also be<br />

<strong>in</strong>vestigated.<br />

Passive<br />

Marg<strong>in</strong>s<br />

Many <strong>of</strong> the large bas<strong>in</strong>s <strong>of</strong> the world's oceans are<br />

partially filled with sediments transported from l<strong>and</strong><br />

dur<strong>in</strong>g the cyclical oscillations <strong>of</strong> global sea level. Major<br />

rivers build deltas <strong>and</strong> together with mar<strong>in</strong>e sediment<br />

sources can generate a complex stratigraphic history.<br />

Identify<strong>in</strong>g boundaries between layers is critical because<br />

the accumulation <strong>of</strong> sediment records a history <strong>of</strong> the<br />

mar<strong>in</strong>e environment <strong>in</strong> the bas<strong>in</strong>. Sedimentary layers<br />

with<strong>in</strong> a bas<strong>in</strong> are <strong>of</strong>ten <strong>in</strong>terpreted from regional seis-


mic surveys rather than by cor<strong>in</strong>g, but by accurately<br />

Sulfur/calcium<br />

Sal<strong>in</strong>ity-<strong>in</strong>dicator<br />

match<strong>in</strong>g the seismic boundaries with the correspond<strong>in</strong>g<br />

layers <strong>in</strong> the core, the exact depositional environment <strong>of</strong><br />

each seismic layer can be determ<strong>in</strong>ed. <strong>Downhole</strong> experlOO<br />

0<br />

ratio<br />

1 2<br />

ratio (CI/H)<br />

1 2 3<br />

iments can l<strong>in</strong>k these data by spann<strong>in</strong>g the scale <strong>of</strong><br />

centimeters <strong>in</strong> core samples to the scale <strong>of</strong> a few hundred<br />

or thous<strong>and</strong> meters typical <strong>of</strong> mar<strong>in</strong>e seismic surveys.<br />

<strong>Downhole</strong> measurements provide cont<strong>in</strong>uous<br />

records <strong>of</strong> sediment density, porosity, <strong>and</strong> acoustic ve-<br />

,.120<br />

locity, parameters that can locate unconformities <strong>and</strong><br />

identify stratigraphic boundaries <strong>in</strong> bas<strong>in</strong>s that fill episodically<br />

as sea level rises <strong>and</strong> falls. <strong>The</strong> stratigraphic<br />

140<br />

sequence <strong>in</strong>ferred from complete downhole pr<strong>of</strong>iles is<br />

Figure 18. Example <strong>of</strong> a 4-m-thick layer with high sulfur/<br />

therefore essential to "ground truth" the <strong>in</strong>terpretation<br />

calcium <strong>and</strong> low chlor<strong>in</strong>e/hydrogen ratios from the geochemi<strong>of</strong><br />

seismic reflections. By comb<strong>in</strong><strong>in</strong>g density <strong>and</strong> acoustic cal log, identify<strong>in</strong>g it primarily as gypsum <strong>in</strong> ODP Hole 967C <strong>in</strong><br />

velocity logs to produce a series <strong>of</strong> reflection coefficients the eastern Mediterranean Sea [from Emeis et al., 1996]. <strong>The</strong><br />

versus depth, a synthetic seismogram can then be gen- logs provided critical data <strong>in</strong> identify<strong>in</strong>g this early bas<strong>in</strong> deposit<br />

erated. <strong>The</strong> details for develop<strong>in</strong>g synthetic seismograms that was not recovered dur<strong>in</strong>g cor<strong>in</strong>g operations.<br />

from log data are well described <strong>in</strong> basic geophysical<br />

texts [e.g., Doveton, 1986]. <strong>The</strong> direct correlation <strong>of</strong> a<br />

synthetic seismogram with a seismic trace enables loca- ian desiccation event uniquely us<strong>in</strong>g the logg<strong>in</strong>g data at<br />

tion with p<strong>in</strong>po<strong>in</strong>t accuracy <strong>of</strong> the changes <strong>in</strong> physical this site.<br />

properties at specific depths that may then be associated<br />

with a particular seismic reflection.<br />

Active Marg<strong>in</strong>s<br />

Tectonic extension on passive cont<strong>in</strong>ental marg<strong>in</strong>s Tectonic processes deform the Earth's crust, particu<strong>and</strong><br />

<strong>in</strong> new ocean bas<strong>in</strong>s leads to subsidence. A critical larly along the marg<strong>in</strong>s <strong>of</strong> cont<strong>in</strong>ental <strong>and</strong> oceanic plates,<br />

element <strong>in</strong> decipher<strong>in</strong>g the tectonic <strong>and</strong> subsidence his- where massive shear zones or active subduction <strong>of</strong> one<br />

tory <strong>of</strong> a sedimentary bas<strong>in</strong> is a reliable measurement <strong>of</strong> plate beneath another may occur. At convergent plate<br />

<strong>in</strong> situ porosity. As a bas<strong>in</strong> fills through geologic time, boundaries a wedge <strong>of</strong> deformed rocks known as an<br />

porosity decreases as the sedimentary load <strong>in</strong>creases. accretionary prism is scraped <strong>of</strong>f the descend<strong>in</strong>g plate. In<br />

Accurate porosity data can reveal sediment layers that 1993 <strong>and</strong> 1994, drill<strong>in</strong>g <strong>and</strong> logg<strong>in</strong>g results <strong>in</strong> the Cascahave<br />

been progressively compacted as the underly<strong>in</strong>g dia <strong>and</strong> Barbados accretionary prisms produced new<br />

basement has th<strong>in</strong>ned, stretched, cooled, <strong>and</strong> subsided underst<strong>and</strong><strong>in</strong>g <strong>of</strong> these active tectonic regimes, their<br />

over time [e.g., Watts <strong>and</strong> Steckler, 1979]. Off the eastern physical properties, <strong>and</strong> the migration <strong>of</strong> pore fluids.<br />

U.S. coast, porosity logs from several holes <strong>in</strong>dicated <strong>Downhole</strong> data were used to address critical questions<br />

that lithology controls the compaction <strong>of</strong> sediments as such as the physical properties at the plate boundary<br />

well as their depth <strong>of</strong> burial, particularly when clay is fault, its role as conduit for fluid movement, <strong>and</strong> the<br />

present [Goldberg et al., 1987; Guer<strong>in</strong> <strong>and</strong> Goldberg, deformation <strong>of</strong> sediment accreted <strong>in</strong> the prism [e.g.,<br />

1996]. <strong>The</strong> estimation <strong>of</strong> porosity us<strong>in</strong>g downhole mea- Hyndman et al., 1993; Moore et al., 1995]. Log data are<br />

surements rema<strong>in</strong>s one <strong>of</strong> the most important applica- particularly useful <strong>in</strong> that they can quantify these proptions<br />

<strong>in</strong> decipher<strong>in</strong>g the stratigraphic history <strong>of</strong> a bas<strong>in</strong> erties <strong>in</strong> situ with m<strong>in</strong>imal effects from cor<strong>in</strong>g. Data on<br />

for both oil exploration <strong>and</strong> academic research. such transient features as faults or high-pressure zones<br />

Early rift<strong>in</strong>g <strong>and</strong> subsidence <strong>of</strong> the Mediterranean must be recorded immediately before they become alfollowed<br />

by multiple tectonic events has generated a tered by the sampl<strong>in</strong>g process.<br />

complex stratigraphy through its history. <strong>Downhole</strong> geo- <strong>Downhole</strong> data <strong>in</strong> ocean drill holes have been acchemical<br />

measurements through Miocene carbonate se- quired us<strong>in</strong>g LWD <strong>in</strong> one region (two sites) on the<br />

quences <strong>of</strong>f the coast <strong>of</strong> Cyprus provided critical data for Barbados accretionary prism <strong>in</strong> 1994. High-quality physidentify<strong>in</strong>g<br />

early bas<strong>in</strong> deposits that were not recovered ical property measurements were made <strong>in</strong> this environdur<strong>in</strong>g<br />

cor<strong>in</strong>g operations. Figure 18 shows a dist<strong>in</strong>ct ment where wirel<strong>in</strong>e logg<strong>in</strong>g had previously been unsuc-<br />

4-m-thick <strong>in</strong>terval that has high S/Ca <strong>and</strong> low C1/H cessful ow<strong>in</strong>g to poor hole conditions [ODP Leg 156<br />

ratios, identify<strong>in</strong>g it primarily as a gypsum layer [Emeis et Shipboard Science Party, 1995]. Figure 19 illustrates a<br />

al., 1996]. From its position <strong>in</strong> relation to the recovered seismic depth transect across the <strong>in</strong>terplate fault zone<br />

core sections, this evaporite deposit corresponded to an (high-amplitude reflectors at approximately 5.5 km<br />

age <strong>of</strong> approximately 6 Ma. Similar geochemical obser- depth) with two density pr<strong>of</strong>iles <strong>in</strong> superposition acvations<br />

from cores played a large role <strong>in</strong> the discovery <strong>of</strong> quired us<strong>in</strong>g LWD. Porosity has been computed from<br />

the Mess<strong>in</strong>ian desiccation event, a period <strong>of</strong> global cool- the density log, <strong>and</strong> the details <strong>in</strong> one hole show that<br />

<strong>in</strong>g <strong>and</strong> sea level lower<strong>in</strong>g at 6-7 Ma when the Mediter- porosity <strong>and</strong> resistivity <strong>in</strong>formation vary <strong>in</strong>versely with<br />

ranean dried up. Emeis et al. [1996] observe the Mess<strong>in</strong>- depth (Figure 20). It is easy to identify two th<strong>in</strong> 0.5- to


' :':-::"' ...... 947 LWD O48-LWD<br />

. .... ...... _.' ..... CORK 949<br />

7 :-:::.?':.- :. --'J .... :- -'::.:" -?;:..:? -"' ' '-" -- ' _<br />

Kilometers<br />

0 1 2 $ 4 6<br />

Figure 19. Seismic cross section <strong>and</strong> logg<strong>in</strong>g-while-drill<strong>in</strong>g density logs <strong>in</strong> superposition at two sites on the<br />

Barbados accretionary prism [after ODP Leg 156 Shipboard Science Party, 1995]. <strong>The</strong> LWD tools <strong>in</strong> ODP Hole<br />

948A were successful <strong>in</strong> penetrat<strong>in</strong>g the plate boundmy fault that can be identified by the large reflection near<br />

the bottom <strong>of</strong> the holes where wirel<strong>in</strong>e tools had previously failed.<br />

1.5-m zones <strong>of</strong> high porosity <strong>and</strong> low resistivity with<strong>in</strong> <strong>The</strong> Future <strong>of</strong> <strong>Downhole</strong> <strong>Measurements</strong><br />

the fault zone us<strong>in</strong>g the LWD data, which are too th<strong>in</strong> to In the past, drill<strong>in</strong>g legs that have been devoted to<br />

be resolved seismically. Moore et al. [1995] suggest that downhole experiments have been highly successful bethese<br />

zones are tectonically significant because the pore cause <strong>of</strong> their uniquely focused objective. With ODP's<br />

pressure <strong>in</strong>ferred from the porosity surpasses 90% <strong>of</strong> the future commitment to drill<strong>in</strong>g deeper <strong>and</strong> perform<strong>in</strong>g<br />

overburden pressure, dilat<strong>in</strong>g <strong>and</strong> fractur<strong>in</strong>g the forma- more experiments <strong>in</strong> ocean boreholes, dedicated legs for<br />

tion. <strong>The</strong>ir <strong>in</strong>terpretation <strong>of</strong> the <strong>in</strong> situ porosity implies<br />

that a lateral <strong>in</strong>flux <strong>of</strong> fluid occurs <strong>in</strong> the Barbados<br />

downhole measurements <strong>and</strong> multiple expeditions to the<br />

same site will be essential. When operations occur over<br />

accretionary prism, caus<strong>in</strong>g large seismic reflection amplitudes<br />

<strong>and</strong> high-pressure zones that are localized by<br />

the fault structure. <strong>The</strong> ability to acquire <strong>and</strong> resolve <strong>in</strong><br />

situ data <strong>of</strong> ephemeral properties, such as these th<strong>in</strong><br />

high-porosity layers, is critical because they change immediately<br />

after the hole is drilled.<br />

Us<strong>in</strong>g wirel<strong>in</strong>e logg<strong>in</strong>g technology, faults with<strong>in</strong> an<br />

a long time at a particular site, rapid ship-to-shore data<br />

communication can be used to evaluate <strong>and</strong> control data<br />

acquisition <strong>and</strong> to provide scientific expertise beyond<br />

that available at sea. Improved communications will<br />

permit a cont<strong>in</strong>ued <strong>in</strong>crease <strong>in</strong> scientific observations<br />

us<strong>in</strong>g both short- <strong>and</strong> long-term downhole experiments<br />

accretionary prism may be visualized <strong>and</strong> their properties<br />

measured when hole conditions permit logg<strong>in</strong>g. In<br />

the Pacific, near the New Hebrides isl<strong>and</strong>s, both down-<br />

Porosity Resistivity<br />

hole image logs from the borehole televiewer <strong>and</strong> FMS<br />

are able to resolve a th<strong>in</strong>, 80-cm-thick fault zone with<strong>in</strong><br />

500<br />

the prism (dar k zones), <strong>in</strong>dicat<strong>in</strong>g a high water content<br />

over this <strong>in</strong>terval (Figure 21). In other accretionary<br />

ß . 510<br />

prisms on the Cascadia <strong>and</strong> Nankai marg<strong>in</strong>s, wirel<strong>in</strong>e<br />

porosity logs show similar zones <strong>of</strong> high water content<br />

associated with fault<strong>in</strong>g, although sediment composition<br />

<strong>and</strong> the degree <strong>of</strong> cementation differ between these<br />

' 520<br />

locations [Hyndman et al., 1993; Jarrard, 1997]. It is clear<br />

that downhole measurements both us<strong>in</strong>g wirel<strong>in</strong>e <strong>and</strong><br />

540 I<br />

0. 4 oi "' o, ......... --.7 o.<br />

LWD technologies have identified porous fault zones<br />

Figure 20. Details <strong>of</strong> porosity <strong>and</strong> resistivity logs through the<br />

with<strong>in</strong> accretionary prisms, although LWD measure- plate boundary fault <strong>in</strong> ODP Hole 948A that <strong>in</strong>dicate high<br />

ment <strong>of</strong> physical pr<strong>of</strong>iles are clearly more robust than porosity <strong>and</strong> low resistivity <strong>in</strong> 0.5- to 1.5-m zones that are too<br />

st<strong>and</strong>ard wirel<strong>in</strong>e techniques.<br />

th<strong>in</strong> to be resolved seismically [from Moore et al., 1995].<br />

6 Km<br />

6 Km


i-<br />

iJJ<br />

258 -<br />

260 -<br />

261 -<br />


N E S W N<br />

I :: ...... ,- -" '. '";",.i I ..'i'.. '.':


ploy<strong>in</strong>g a new strategy that uses LWD to drill closely<br />

spaced holes could be used to produce three-dimensional<br />

maps <strong>of</strong> the <strong>in</strong> situ properties where holes are<br />

typically unstable, such as <strong>in</strong> the shallowest subseafloor<br />

section, through slide failures <strong>and</strong> fault zones, or <strong>in</strong><br />

young oceanic crust. As drill<strong>in</strong>g technologies cont<strong>in</strong>ue to<br />

advance, logg<strong>in</strong>g <strong>in</strong> highly deviated holes <strong>in</strong> conjunction<br />

with a riser, someth<strong>in</strong>g that has never been attempted<br />

for scientific drill<strong>in</strong>g objectives or <strong>in</strong> the deep ocean, will<br />

be possible. With a riser, ship motions can also be<br />

compensated for more effectively, improv<strong>in</strong>g the quality<br />

<strong>of</strong> the downhole data <strong>and</strong> the reconciliation <strong>of</strong> depth<br />

with core samples.<br />

All the downhole data discussed <strong>in</strong> this review <strong>and</strong><br />

those still to be recorded <strong>in</strong> the future will require<br />

archiv<strong>in</strong>g <strong>and</strong> retrieval systems that are consistent with<br />

core sample <strong>and</strong> surface data <strong>and</strong> that enable simple<br />

access for all ocean drill sites. <strong>The</strong> development <strong>of</strong> such<br />

a database is currently <strong>in</strong> progress for exist<strong>in</strong>g core <strong>and</strong><br />

downhole data through the ODP but will require advancement<br />

as new data cont<strong>in</strong>ue to be acquired.<br />

to def<strong>in</strong>e the formation permeability <strong>and</strong> the rate <strong>and</strong><br />

volume <strong>of</strong> fluid exchange below the seafloor.<br />

us<strong>in</strong>g logg<strong>in</strong>g-while-drill<strong>in</strong>g technology, <strong>in</strong> situ properties,<br />

such as porosity, have been measured <strong>in</strong> fault<br />

zones that act as fluid conduits <strong>in</strong> accretionary prisms.<br />

This technology will become <strong>in</strong>creas<strong>in</strong>gly important as<br />

core recovery becomes more difficult, such as when deep<br />

holes are drilled through accretionary prisms at several<br />

locations to study the lateral distribution <strong>of</strong> porosity <strong>and</strong><br />

tectonic deformation. <strong>Downhole</strong> data are most representative<br />

<strong>of</strong> <strong>in</strong> situ conditions when they are acquired<br />

imme. diately after drill<strong>in</strong>g or days to years later after<br />

conditions have returned to their predrill<strong>in</strong>g state. Us<strong>in</strong>g<br />

these strategies, high-resolution formation properties<br />

<strong>and</strong> transient <strong>and</strong> long-term temporal fluid variations<br />

will be possible to observe. Dur<strong>in</strong>g the next decade <strong>of</strong><br />

research <strong>in</strong> mar<strong>in</strong>e geology <strong>and</strong> geophysics, such downhole<br />

measurements are poised to exp<strong>and</strong> the scientific<br />

opportunities that will come from programs <strong>in</strong> new frontier<br />

environments planned for ocean drill<strong>in</strong>g.<br />


ACKNOWLEDGMENTS. Iwould like to acknowledge the<br />

shipboard scientists <strong>and</strong> crews <strong>of</strong> the GLOMAR Challenger <strong>and</strong><br />

the JOIDES Resolution, who have cont<strong>in</strong>uously improved the<br />

collection <strong>and</strong> use <strong>of</strong> downhole data <strong>in</strong> holes drilled through-<br />

In the last decade, measurements made with downhole<br />

technologies have provided crucial data that have<br />

led to the resolution <strong>of</strong> a variety <strong>of</strong> scientific problems <strong>in</strong><br />

out the world's oceans. It was not possible to cite or mention<br />

many key contributors to these efforts <strong>in</strong> this broad review. <strong>The</strong><br />

efforts <strong>of</strong> the editors, R. Jarrard <strong>and</strong> an anonymous reviewer,<br />

R. Anderson, <strong>and</strong> J. Totton greatly improved <strong>and</strong> eased the<br />

mar<strong>in</strong>e geology <strong>and</strong> geophysics. <strong>Downhole</strong> measure- preparation <strong>of</strong> this manuscript. C. Broglia <strong>and</strong> E. Pratson<br />

ments provided data for magnetostratigraphy <strong>and</strong> the provided critical assistance with log analysis. This review was<br />

chronology <strong>of</strong> Milankovitch cycles <strong>in</strong> sedimentary se- supported by the National Science Foundation grant JOI-94C.<br />

quences <strong>and</strong> their correlation with paleoclimatic chang- Lamont-Doherty Earth Observatory contribution 5624.<br />

es; these will be important when drill<strong>in</strong>g <strong>in</strong> new geologic Alan Chave was the editor responsible for this paper. He<br />

environments, such as <strong>in</strong> the Arctic <strong>and</strong> Antarctic, where<br />

complete core recovery is not certa<strong>in</strong>. Cont<strong>in</strong>ued improvement<br />

<strong>in</strong> the resolution <strong>of</strong> orbital <strong>and</strong> suborbital<br />

cycles by logg<strong>in</strong>g with the FMS <strong>and</strong> other downhole tools<br />

may reveal climate <strong>and</strong> sea level cycles as short as<br />

would like to thank Richard Jarrard <strong>and</strong> an anonymous person<br />

for their technical reviews. He would also like to thank Bonnie<br />

Buratti for her cross-discipl<strong>in</strong>ary review.<br />

2000-3000 years <strong>in</strong> areas with rapid sedimentation. Logs<br />

have contributed significantly to studies <strong>of</strong> the upper<br />


oceanic crust <strong>and</strong> its chemical composition, hydrother- Allen, D., T. Barber, C. Flaum, J. Hem<strong>in</strong>gway, B. Anderson,<br />

mal alteration, <strong>and</strong> seismic stratigraphy. Us<strong>in</strong>g wirel<strong>in</strong>e S. des Ligneris, B. Everett, <strong>and</strong> C. Morriss, Advances <strong>in</strong><br />

tools <strong>in</strong> previously undrilled sections <strong>of</strong> the lower ocean<br />

crust <strong>and</strong> deep <strong>in</strong>to cont<strong>in</strong>ental marg<strong>in</strong>s will undoubtedly<br />

provide new data to test models <strong>of</strong> the deformation<br />

at the boundary between the cont<strong>in</strong>ents <strong>and</strong> oceans, the<br />

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