RESEARCH· ·1970·

~towever, many of the available samples are not representative
o:f: the total stratigraphy of an area, and in
such areas the real bulk density 1nn.y still be unknown.
Also, re.liable alluvium bulk densities are difficult to
obtain because laboratory ann.lysis of alluvium often
is not feasible.
The search for a fast n.nd reliable method of measuring,
in situ, the bulk density of rocks and alluvium has
been long ~tnd frequently frustrating (Grant and 'Vest,
1965, p. 196-200). The present paper describes a
method of computing the in situ bulk density of a
1 arge volume of rocks n.nd (or) allu vi mn from subsud:ace
gravity data by use of the IB1\1 System/360,
1\1odel ()5 computer. Ten sets of subsurface gravity
datn. were obtained: four with the U.S. Geologioo.l
Survey-La Coste and Romberg, Inc., borehole gravity
meter and six with stn.ndard '~r orden and La Coste­
Rom berg gravity meters. Of the six data sets, five
were obtn.ined in cased 48-inch-diameter drill holes
and one was obtained in a vertical shaft.
The borehole gravity meter was used to log four
small-diameter drill holes at the Nevada Test Site and
in I:Iot Creek Valley. These observations were made
by lowering the steel-housed instrument into drill
holes no smaller than 634, inches in diameter by means
of a rnulticonductor armored cable. The observations
were made n.t predetermined intervn.ls to coincide, as
nearly ns possible, w.itJh known lit;hologic changes. The
readings were recorded graphically at the surface. The
logged holes ranged in depth from 1,236 to 6,488 feet.
Each of the five 48-inch-dia:rt:J.eter holes had a small
chamber excn.vated n.t the detonation point near the
bottom. These chambers were located between 2,247
and 4,748 feet below the ground surface. The size of
these holes plus the size of the chamber at the bottom
mn.de it possible for a man to physically transport the
gravity meter to the bottom and obtain the gravity
ren.dings. Usually, three round trips from the surface
to the chamber were made to insure accurate observations.
Nine gravity observations were also made at approximately
100-foot intervals down a 11- by 15-foot
vertical slutft whose total depth was 825 feet.
Alluvium densities were determined from gammagamma
density logs taken in 10 drill holes in Yucca
Flat and I:Iot Creek Valley. These data, which sampled
18,748 feet of ttl.luvium, supplemented the gravitylneter
data and provided a better understanding of the
n.lluvium density.
The sensor o:f the U.S. Geological Survey-La Coste
and Romberg borehole gravity meter consists of a La
Coste and Romberg geodetic gravity meter (G-95)
HEALEY
B53
that has been modified and fitted into a waterproofed,
high-pressure steel housing. Details of construction,
electronics~ remote reading procedures, precision, and
support equipment requirements have been described
by l\1cCulloh ( 1967) , McCulloh, LaCoste, Schoellhamer,
and Pampeyan (1967), and McCulloh, SchoellhaJner,
Pampeyan, and Parks ( 1967).
This instrument evolved from the combined efforts
of many people to make precise subsurface gravity
observations a reality. Airy (1856) made the first underground
measurements of gravity by 1neans of pendulum
apparatus. Since then many papers that describe
the theory, practice, instrumentation, or results
of 1neasurements in vertical shafts, mines, tunnels, or
boreholes have been published. Some of the significant
papers are those regn.rding theory and principles of
subsurface gravimetry, by Lorenz (1938), Smith
( 1950), and Domzalski ( 1955) ; those regarding instrumentation
problems and considerations, by Smith
(1950), Gilbert (1952), Dolbear (1959), Lukavchenko
( 1962), and Goodell and Fay ( 1964) ; and those regarding
subsurface methods and observational data,
by I-Iammer ( 1950), Domzalski ( 1954), and 1\1cCulloh
( 1965). For a more complete bibliography the reader
is referred to 1\1cCulloh ( 1965, 1966). As a result of all
these efforts several workable borehole gravity meters
have been developed (Goodell and Fay, 1964; Howell
and others, 1965; l\1cCulloh, 1965).
Acknmoledgments.-J. E. Schoellhamer, L.A. Beyers,
E. A. Barker; and E. H. Pampeyan, of the U.S.
Geological Survey, operated the borehole gravity meter
and its supporting equipment while the four holes
were being logged; F. E. Currey made, or assisted
with, all other subsurface gravity observations and· the
free-air gradient measurements; and C. H. 1\1iller assisted
with the gravity observations in U19c. J. R.
I-Iearst, of the Lawrence Radiation Laboratory, Livermore,
Calif., provided his computer program, Code
1\1oria. R. R. Wahl, of the U.S. Geological Survey and
1\1etropolitan State College, Denver, Colo., converted
Code 1\1oria to the IBM System 360, 1\1odel 65 computer,
and added several subroutines to make the program
more adaptable to our requirements.
DATA REDUCTION
Subsurfa·ce gravity observations are influenced by the
in situ bulk densities of the adjacent rocks, the free-air
gradient, the borehole or shaft void, the excavated
chamber or other underground workings, and the adjacent
topography (terrain correction). The general form
of these relationships has been given by Hammer