A Review of Remote Sensing Application in Archaeological Research
A Review of Remote Sensing Application in Archaeological Research
A Review of Remote Sensing Application in Archaeological Research
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A <strong>Review</strong> <strong>of</strong> <strong>Remote</strong> <strong>Sens<strong>in</strong>g</strong> <strong>Application</strong><br />
<strong>in</strong> <strong>Archaeological</strong> <strong>Research</strong><br />
Geography 795.28 (333)<br />
By: Aaron Osicki<br />
Pr<strong>of</strong>essor: Dr. Darren Sjogren<br />
Course: Geography 795.28 (333)<br />
Date: December 07, 2000
Introduction<br />
Archaeology has a long history <strong>of</strong> search<strong>in</strong>g the earth’s surface for m<strong>in</strong>ute<br />
traces <strong>of</strong> human impact created centuries to millennia prior, <strong>in</strong> hopes <strong>of</strong> piec<strong>in</strong>g<br />
together the history and development <strong>of</strong> our species. In terms <strong>of</strong> research and<br />
analysis, the remote sens<strong>in</strong>g application <strong>of</strong> aerial photography has long been a<br />
crucial element <strong>in</strong> the understand<strong>in</strong>g <strong>of</strong> archaeological rema<strong>in</strong>s. However,<br />
remote sens<strong>in</strong>g is a very large and dynamic field which <strong>in</strong>corporates much more<br />
than just aerial photography. In def<strong>in</strong><strong>in</strong>g remote sens<strong>in</strong>g, Jensen (2000), gives<br />
an all-encompass<strong>in</strong>g def<strong>in</strong>ition where “remote sens<strong>in</strong>g is the acquir<strong>in</strong>g <strong>of</strong> data<br />
about an object without touch<strong>in</strong>g it” (Jensen, 2000:4). Broad def<strong>in</strong>itions such as<br />
this <strong>in</strong>clude all forms <strong>of</strong> remote sens<strong>in</strong>g, from the human eye to hyper-spectral<br />
sensors mounted on satellite platforms orbit<strong>in</strong>g the Earth. For obvious reasons,<br />
this def<strong>in</strong>ition is far too general to be adequately covered <strong>in</strong> the scope <strong>of</strong> this<br />
review. Therefore, only the major forms <strong>of</strong> remote sens<strong>in</strong>g technology used <strong>in</strong><br />
archaeological <strong>in</strong>vestigation and analysis will be discussed.<br />
The use <strong>of</strong> remote sens<strong>in</strong>g, as with many other technologies used <strong>in</strong><br />
archaeology, has largely relied on the developments and discoveries <strong>of</strong> other<br />
discipl<strong>in</strong>es. Geography and geology have cont<strong>in</strong>ually been responsible for the<br />
primary push <strong>in</strong> the development and application potentials <strong>of</strong> remote sens<strong>in</strong>g, so<br />
as to fit their chang<strong>in</strong>g and <strong>in</strong>creas<strong>in</strong>gly demand<strong>in</strong>g needs for precision and<br />
accuracy <strong>in</strong> Earth observations. As a result, us<strong>in</strong>g sub-surface remote sens<strong>in</strong>g<br />
techniques, archaeologists now have the ability to look below the ground without<br />
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digg<strong>in</strong>g. Us<strong>in</strong>g multi-spectral imag<strong>in</strong>g, they can see aspects <strong>of</strong> features that the<br />
human eye is not capable <strong>of</strong> detect<strong>in</strong>g. As well, us<strong>in</strong>g satellite sensors and<br />
powerful image process<strong>in</strong>g and analysis s<strong>of</strong>tware, archaeologists can observe<br />
past human <strong>in</strong>fluence on local environments from space through the comfort <strong>of</strong><br />
their home or <strong>of</strong>fice computer. As a result <strong>of</strong> benefits like these, with the aid <strong>of</strong><br />
remote sens<strong>in</strong>g, archaeologists are able to extract <strong>in</strong>creas<strong>in</strong>gly more abundant<br />
and complex <strong>in</strong>formation from the rema<strong>in</strong>s <strong>of</strong> a cont<strong>in</strong>ually dim<strong>in</strong>ish<strong>in</strong>g cultural<br />
resource.<br />
The body <strong>of</strong> this paper covers two ma<strong>in</strong> discussions <strong>of</strong> remote sens<strong>in</strong>g <strong>in</strong><br />
archaeology. First, is the Historic Use <strong>of</strong> <strong>Remote</strong> <strong>Sens<strong>in</strong>g</strong> <strong>in</strong> Archaeology,<br />
followed by, the Current Use <strong>of</strong> <strong>Remote</strong> <strong>Sens<strong>in</strong>g</strong> <strong>in</strong> Archaeology. The first<br />
section will discuss past applications <strong>of</strong> <strong>in</strong>corporat<strong>in</strong>g remote sens<strong>in</strong>g <strong>in</strong>to<br />
archaeological research. This part will briefly look at <strong>in</strong>itial uses <strong>of</strong> remote<br />
sens<strong>in</strong>g <strong>in</strong> archaeological projects and some <strong>of</strong> its most common applications <strong>in</strong><br />
the past. In the next section, which is divided <strong>in</strong>to three ma<strong>in</strong> parts based on<br />
prom<strong>in</strong>ent types <strong>of</strong> remote sens<strong>in</strong>g <strong>in</strong> archaeology (Aerial Photography, Aerial<br />
and Satellite <strong>Remote</strong> <strong>Sens<strong>in</strong>g</strong>, and Sub-Surface <strong>Remote</strong> <strong>Sens<strong>in</strong>g</strong>), contemporary<br />
uses <strong>of</strong> remote sens<strong>in</strong>g will be discussed. Aerial photography, still be<strong>in</strong>g the<br />
most widely applied form <strong>of</strong> remote sens<strong>in</strong>g used <strong>in</strong> archaeology, will be<br />
discussed separately. The other two subsections will discuss a variety <strong>of</strong><br />
techniques and <strong>in</strong>struments used <strong>in</strong> archaeological work with respect to either<br />
surface or sub-surface remote sens<strong>in</strong>g.<br />
2
Historic Use <strong>of</strong> <strong>Remote</strong> <strong>Sens<strong>in</strong>g</strong> <strong>in</strong> Archaeology<br />
The archaeological community began employ<strong>in</strong>g remotely sensed data<br />
<strong>in</strong>to their research relatively early <strong>in</strong> comparison with other discipl<strong>in</strong>es (Sever,<br />
1995). Around the beg<strong>in</strong>n<strong>in</strong>g <strong>of</strong> the twentieth century, aerial photography was<br />
adopted by archaeologists primarily to view features on the Earth’s surface which<br />
were difficult, if not impossible, to visualize and, or conceptualize from ground<br />
level. In 1907 Stonehenge was photographed from a balloon, which provided a<br />
very different, quite unique and more regional view and understand<strong>in</strong>g <strong>of</strong> the<br />
archaeological feature (Capper, 1907). S<strong>in</strong>ce this time aerial photography has<br />
played a major role <strong>in</strong> record<strong>in</strong>g, describ<strong>in</strong>g, and study<strong>in</strong>g archaeological sites.<br />
<strong>Archaeological</strong> <strong>in</strong>vestigation dur<strong>in</strong>g the time <strong>of</strong> World War I further<br />
experimented with the use <strong>of</strong> black-and-white aerial photography <strong>in</strong> identify<strong>in</strong>g<br />
unrecorded sites and features. In so do<strong>in</strong>g, subsequent realizations <strong>of</strong> the<br />
benefits <strong>in</strong> remote sens<strong>in</strong>g techniques were identified. British archaeologists<br />
were able to identify long abandoned and destroyed features and structures <strong>in</strong><br />
the terra<strong>in</strong>, soil formations, and vegetative cover <strong>of</strong> the land, which were<br />
overlooked <strong>in</strong> previous traditional ground survey techniques (Sever, 1995). This<br />
realization <strong>of</strong> the potential <strong>in</strong> us<strong>in</strong>g aerial photography <strong>in</strong> archaeological<br />
survey<strong>in</strong>g lead to the discoveries <strong>of</strong> “ancient Roman villas and roadways <strong>in</strong><br />
Europe, sites <strong>in</strong> the Middle East, and earthworks <strong>in</strong> the Mississippi River Valley”<br />
(Sever, 1995:83), to name only a few.<br />
3
In 1931 the first use <strong>of</strong> a tethered photographic balloon was conducted to<br />
record ongo<strong>in</strong>g excavations at Megiddo (figure 1), site <strong>of</strong> biblical Armageddon<br />
(Meyers and Meyers, 1995).<br />
Figure 1. P.L.O. Guy us<strong>in</strong>g a tethered photographic balloon <strong>in</strong> 1931 to record the<br />
excavation <strong>of</strong> Megiddo (photo from Meyers and Meyers, 1995:85).<br />
This procedure allowed for high-quality, large scale, visual record<strong>in</strong>g <strong>of</strong> sites and<br />
features. The process was less expensive and resource <strong>in</strong>tensive compared to<br />
us<strong>in</strong>g manned air balloons. Also, the process was able to produce higher quality,<br />
more detailed, large-scale photographs, than could be produced by cameras<br />
mounted on aircraft. Further development <strong>in</strong> related technologies made this<br />
method <strong>of</strong> aerial photography even more efficient. Radio-controlled motor driven<br />
cameras allowed pictures to be taken <strong>in</strong> quick succession without hav<strong>in</strong>g to<br />
manually w<strong>in</strong>d the film forward, which greatly reduced the amount <strong>of</strong> time needed<br />
to take pictures <strong>of</strong> features and sites (Meyers and Meyers, 1995). Higher quality<br />
4
film provided more detailed and accurate images <strong>of</strong> sites and features (Meyers<br />
and Meyers, 1995). An improved aerodynamic blimp design produced a more<br />
stable platform on which to mount the camera thus, provid<strong>in</strong>g clearer pictures <strong>of</strong><br />
features and sites at w<strong>in</strong>dy locations (Meyers and Meyers, 1995) (figure 2).<br />
Figure 2. Current improvements on the tethered photographic balloon <strong>in</strong>clude an<br />
aerodynamic blimp designed for stability <strong>in</strong> w<strong>in</strong>d, and remote camera<br />
operation for quicker photograph production (photo from Meyers and Meyers,<br />
1995:86).<br />
Current Use <strong>of</strong> <strong>Remote</strong> <strong>Sens<strong>in</strong>g</strong> <strong>in</strong> Archaeology<br />
The evolution <strong>of</strong> archaeology and archaeological science has progressed<br />
along way from its beg<strong>in</strong>n<strong>in</strong>g. One <strong>of</strong> the most prom<strong>in</strong>ent changes <strong>in</strong> the way<br />
archaeologists conduct their research is through the use <strong>of</strong> multidiscipl<strong>in</strong>ary<br />
studies and <strong>in</strong>vestigation (Scarre, 1999; Sever, 1998b; Wiseman, 1998).<br />
Archaeologists are us<strong>in</strong>g discoveries, technologies, equipment, and resources<br />
from many other discipl<strong>in</strong>es when attempt<strong>in</strong>g to piece together human history.<br />
5
Advances <strong>in</strong> computers and computer s<strong>of</strong>tware have allowed archaeologists to<br />
more thoroughly analyze the data they produce. Medical and forensic<br />
improvements allow more <strong>in</strong>-depth understand<strong>in</strong>g <strong>of</strong> skeletal rema<strong>in</strong>s as they<br />
perta<strong>in</strong> to the person’s health, diet, behavior, and <strong>in</strong>teractions with other<br />
populations (Roberts and Manchester, 1997). Developments <strong>in</strong> nuclear physics<br />
have provided archaeologists with tools such as electron microscopes for microanalysis,<br />
particle accelerators for improved dat<strong>in</strong>g techniques, and laser<br />
technologies used <strong>in</strong> survey<strong>in</strong>g and mapp<strong>in</strong>g equipment (Scarre, 1999). In<br />
conjunction with this multidiscipl<strong>in</strong>ary approach is an <strong>in</strong>creas<strong>in</strong>g trend towards<br />
non-destructive techniques <strong>in</strong> archaeological <strong>in</strong>vestigation (Wiseman, 1998;<br />
Sheets and Sever, 1988), as well as the grow<strong>in</strong>g importance seen <strong>in</strong> view<strong>in</strong>g and<br />
<strong>in</strong>terpret<strong>in</strong>g archaeological f<strong>in</strong>ds at a regional scale (Wiseman, 1998) as opposed<br />
to isolated entities. This is where advances <strong>in</strong> remote sens<strong>in</strong>g are show<strong>in</strong>g<br />
considerable applicability and potential for the future. In us<strong>in</strong>g remotely sensed<br />
data there is no destruction <strong>of</strong> the archaeological site or feature be<strong>in</strong>g analyzed.<br />
As well, space and airborne sensors are ideally suited for observ<strong>in</strong>g regional<br />
trends with<strong>in</strong> the vic<strong>in</strong>ity <strong>of</strong> one or more sites. To add to these benefits, remote<br />
sens<strong>in</strong>g also has the capability to analyze parts <strong>of</strong> the electromagnetic spectrum<br />
that humans can not even visually detect. This allows archaeologists to identify<br />
subtle changes <strong>in</strong> the reflectance <strong>of</strong> various vegetation, soils, and other<br />
environmental entities, so as to uncover subsurface archaeological features. As<br />
remote sens<strong>in</strong>g technology and data improves, and it becomes more accessible<br />
and less expensive, its application <strong>in</strong> archaeology will only <strong>in</strong>crease.<br />
6
Aerial Photography<br />
The majority <strong>of</strong> remotely sensed data <strong>in</strong> archaeology relies on the basic<br />
notion that most <strong>of</strong> human history can be studied by identify<strong>in</strong>g and analyz<strong>in</strong>g the<br />
impacts <strong>of</strong> human behavior upon their geographic surround<strong>in</strong>gs (Sever, 1998a).<br />
As a result, aerial photographs have been used to “record and <strong>in</strong>terpret<br />
archaeological and environmental data as tonal and textural differences <strong>in</strong><br />
grow<strong>in</strong>g crops, tonal differences <strong>in</strong> bare soil and as shadows cast by relief<br />
features” (Cox, 1992:251) (figure 3).<br />
A<br />
B<br />
C<br />
Figure 3. Various environmental traits which allow archaeologist to identify sites or<br />
features us<strong>in</strong>g aerial photography. A) Iron age enclosure, visible by the sett<strong>in</strong>g<br />
sun, B) Belgic settlement, visible through crop retardation, and C) Roman<br />
villa, visible through chalky soil from foundation wall (photos from Schollar,<br />
1990).<br />
7
In western Canada, aerial photography has greatly facilitated <strong>in</strong> the record<strong>in</strong>g,<br />
understand<strong>in</strong>g, and visual perception <strong>of</strong> aborig<strong>in</strong>al features such as large effigies,<br />
medic<strong>in</strong>e wheels, bison kill sites and drive lanes, as well as camp sites and<br />
teepee r<strong>in</strong>gs (Bryan, 1991).<br />
A big benefit <strong>in</strong> aerial photography has always been the accurate and<br />
rapid assessment <strong>of</strong> regional, large-scale areas. Large sections <strong>of</strong> land can be<br />
surveyed for prom<strong>in</strong>ent archaeological sites much quicker us<strong>in</strong>g aerial<br />
photography techniques as opposed to ground survey techniques. The accuracy<br />
<strong>in</strong> detect<strong>in</strong>g large features comes from the ability, with aerial photographs, to be<br />
able to visually <strong>in</strong>spect large areas and recognize tonal, textural, or topographic<br />
differences with<strong>in</strong> associated targets signify<strong>in</strong>g past human <strong>in</strong>fluence. These<br />
techniques for identify<strong>in</strong>g sites are still be<strong>in</strong>g widely used today <strong>in</strong> reveal<strong>in</strong>g new<br />
archaeological f<strong>in</strong>ds. In lowland England, ridges and furrows from the late Saxon<br />
period are be<strong>in</strong>g detected and mapped us<strong>in</strong>g vertical aerial photographs through<br />
the analysis <strong>of</strong> their high-pr<strong>of</strong>ile shadows created dur<strong>in</strong>g sunlit w<strong>in</strong>ter conditions<br />
(Hall and Palmer, 2000). Romano-British settlements dat<strong>in</strong>g to around AD 48 are<br />
be<strong>in</strong>g discovered <strong>in</strong> England through aerial photograph analysis <strong>of</strong> textural and<br />
tonal differences <strong>in</strong> farmers’ fields (Potter and Rob<strong>in</strong>son, 2000) (figure 4).<br />
Increas<strong>in</strong>gly however, aerial photography is be<strong>in</strong>g used <strong>in</strong> conjunction and, or<br />
comparison with other airborne remote sensors to more fully analyze regional<br />
landscapes for archaeological resources.<br />
8
Figure 4. Romano-British settlement dat<strong>in</strong>g to around AD 48 at Grandford,<br />
Cambridgeshire, England (photo from Potter and Rob<strong>in</strong>son, 2000:31).<br />
Aerial and Satellite <strong>Remote</strong> <strong>Sens<strong>in</strong>g</strong><br />
Satellite remote sens<strong>in</strong>g, as a tool for analyz<strong>in</strong>g environmental variables<br />
with regards to archaeological prospect<strong>in</strong>g, has been grow<strong>in</strong>g ever s<strong>in</strong>ce the first<br />
launch <strong>of</strong> the Landsat satellite series <strong>in</strong> 1972, and the later addition <strong>of</strong> the French<br />
SPOT satellite series started <strong>in</strong> 1986 (Fowler, 1996). However, only recently,<br />
with the substantial improvements <strong>in</strong> personal comput<strong>in</strong>g power and<br />
development <strong>of</strong> less expensive, more powerful and readily understandable<br />
s<strong>of</strong>tware, has satellite remote sens<strong>in</strong>g become accessible to the wider<br />
archaeological community.<br />
All satellite borne sensors are also used and, or were tested on airplanes<br />
before be<strong>in</strong>g sent <strong>in</strong>to orbit. Therefore, the sensors that will be discussed do not<br />
9
only perta<strong>in</strong> to satellites. Many times, especially <strong>in</strong> the case <strong>of</strong> archaeological<br />
survey<strong>in</strong>g, it is more beneficial to have the sensor mounted on an aircraft as<br />
opposed to a satellite because the spatial resolution is much better result<strong>in</strong>g from<br />
the sensor proximity to its target. As with all remote sens<strong>in</strong>g equipment, the type<br />
<strong>of</strong> sensor used depends on what target(s) are go<strong>in</strong>g to be looked at and what<br />
<strong>in</strong>formation one wants to obta<strong>in</strong> from the analysis. There are many sensors,<br />
with quite different functional abilities, that are <strong>in</strong>dividually ideally suited to<br />
various specific archaeological needs. The most prom<strong>in</strong>ent sensors are<br />
discussed below.<br />
Thermal Infrared Multispectral Scanner (TIMS)<br />
This scanner is composed <strong>of</strong> six channels, which measure the amount <strong>of</strong><br />
heat given <strong>of</strong>f by ground targets with an accuracy <strong>of</strong> 0.1 degree centigrade<br />
(Sever, 1998b; Sheets and Sever, 1988). The resolution <strong>of</strong> this scanner, as with<br />
most, is directly proportional to its distance from its target (Sever, 1998b; Sheets<br />
and Sever, 1988). This translates to a spatial resolution <strong>of</strong> 30m 2 pixels with the<br />
scanners on the Landsat satellites (Sever, 1998b), which would have limited<br />
applicability <strong>in</strong> archaeological prospect<strong>in</strong>g. However, when the TIMS scanner is<br />
used on an aircraft its resolution can be with<strong>in</strong> a meter squared (Sever, 1998b;<br />
Sheets and Sever, 1988).<br />
This sensor was used <strong>in</strong> 1985 to identify and analyze footpaths dat<strong>in</strong>g to<br />
around 2500 years B.P., which lead to and from a cemetery <strong>in</strong> the Arenal Region<br />
<strong>of</strong> Costa Rica (Sever, 1998b; Sever, 1998c; Sheets and Sever, 1988). Orig<strong>in</strong>ally<br />
10
the paths were identified through the use <strong>of</strong> colour <strong>in</strong>frared photographs (figure<br />
5).<br />
Figure 5. Colour <strong>in</strong>frared photo (converted to grayscale <strong>in</strong> this paper) show<strong>in</strong>g<br />
prehistoric footpath (photo from Sheets and Sever, 1988)<br />
However, with the additional use <strong>of</strong> TIMS, the archaeologists were able to trace<br />
the path network beneath the thick forest canopy <strong>of</strong> the area (Sever, 1998b;<br />
Sever, 1998c; Sheets and Sever, 1988). Selected portions <strong>of</strong> the paths were<br />
later excavated, us<strong>in</strong>g traditional archaeological techniques, to verify the paths’<br />
presence and to determ<strong>in</strong>e their antiquity (Sever, 1998b; Sever, 1998c; Sheets<br />
and Sever, 1988). A similar use <strong>of</strong> TIMS was conducted <strong>in</strong> Chaco Canyon, New<br />
Mexico, where prehistoric roads, walls, build<strong>in</strong>gs, and agricultural fields were<br />
identified, spann<strong>in</strong>g 200 miles and dat<strong>in</strong>g to approximately 900 AD (Sever,<br />
1998d) (figure 6). In this case, previous use <strong>of</strong> ground survey, aerial<br />
photography, and colour <strong>in</strong>frared photography revealed noth<strong>in</strong>g, and only through<br />
the use <strong>of</strong> the TIMS sensor were these features found (Sever, 1998d).<br />
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Figure 6. TIMS image show<strong>in</strong>g prehistoric roads at Chaco Canyon New Mexico. The<br />
roads have been highlighted by project analysts for easier recognition (Sever,<br />
1998d).<br />
Airborne Oceanographic Lidar (ADI)<br />
This is an active sensor that uses a laser device to record the topography<br />
<strong>of</strong> the earth’s surface (Sever, 1998b; Sheets and Sever, 1988). The laser is shot<br />
at the surface <strong>of</strong> the earth multiple times a second, tak<strong>in</strong>g measurements <strong>of</strong> the<br />
height <strong>of</strong> the various targets it encounters. The Lidar sensor is capable <strong>of</strong><br />
obta<strong>in</strong><strong>in</strong>g <strong>in</strong>formation on various features <strong>in</strong> a study area such as tree height,<br />
depression depth, elevation, slope, and aspect (Sever, 1998b). This sensor was<br />
used <strong>in</strong> conjunction with the TIMS sensor and <strong>in</strong>frared photographs <strong>in</strong> the<br />
analysis <strong>of</strong> the footpath network <strong>in</strong> Arenal, Costa Rica (Sever, 1998c; Sheets and<br />
Sever, 1988).<br />
12
Synthetic Aperture Radar (SAR)<br />
Like ADI this is another active sensor. The SAR <strong>in</strong>strument sends out a<br />
pulse <strong>of</strong> electromagnetic energy and records what comes back. In this case, a<br />
wave <strong>of</strong> energy is sent out to the earth’s surface and the sensor records the<br />
amount <strong>of</strong> energy that returns, and the time it took to do so. Radar is sensitive to<br />
l<strong>in</strong>ear and geometric features, as well as different wavelengths <strong>of</strong> energy are<br />
sensitive to different vegetation or other environmental elements (Sever, 1998b).<br />
Such traits are ideally suited for the identification <strong>of</strong> archaeological features, such<br />
as temples, build<strong>in</strong>gs, and road networks. Another beneficial feature <strong>of</strong> radar is<br />
its ability to penetrate dry ground surfaces, which has allowed archaeologists to<br />
identify topographic and archaeological features buried by sand (Wiseman,<br />
1998). In 1982 radar images from the space shuttle identified past watercourses<br />
<strong>in</strong> the Sudanese desert, which have long s<strong>in</strong>ce been dried-up (Sever, 1998b;<br />
Sheets and Sever, 1988). Radar us<strong>in</strong>g long wavelengths can also penetrate<br />
canopy cover, which has been used <strong>in</strong> detect<strong>in</strong>g Mayan field systems <strong>in</strong><br />
Guatemala, locat<strong>in</strong>g and mapp<strong>in</strong>g sites around Angkor <strong>in</strong> Cambodia, and<br />
provid<strong>in</strong>g further <strong>in</strong>sight <strong>in</strong>to early Khmer state development (Scarre, 1999). SAR<br />
was also used <strong>in</strong> the analysis <strong>of</strong> the footpaths <strong>in</strong> Costa Rica (Sever, 1998b;<br />
Sever, 1998c; Sheets and Sever, 1988).<br />
Satellite High-Resolution Imag<strong>in</strong>g<br />
This is an area <strong>of</strong> satellite remote sens<strong>in</strong>g that could prove to be extremely<br />
useful <strong>in</strong> archaeological prospect<strong>in</strong>g <strong>in</strong> the future. Until recently the most publicly<br />
13
accessible satellite imagery came from the Landsat satellites, which had/have a<br />
spatial resolution rang<strong>in</strong>g from 80m 2 to 30m 2 pixel size <strong>in</strong> their Multispectral<br />
Scanners (MSS)/ Thematic Mapper (TM) scanners (Jensen, 2000; Sheffner and<br />
Stoney, 1999). Such resolution has limited applicability when deal<strong>in</strong>g with<br />
archaeological data because features and, or sites need much f<strong>in</strong>er resolution to<br />
be detected. In 1986, with the <strong>in</strong>troduction <strong>of</strong> the French SPOT satellites, spatial<br />
resolution improved to 10m 2 pixels with the High Resolution Visible (HRV)<br />
Panchromatic images (Jensen, 2000). Even with this improvement <strong>in</strong> the<br />
resolution, only large archaeological structures, such as the Pyramids at Giza,<br />
could be seen, and only relatively limited use <strong>of</strong> the multi-spectral capabilities<br />
was conducted for the same reason.<br />
One archaeological study done by M. Fowler (1996) utilized a Russian<br />
satellite, KVR-1000, to document and analyze Stonehenge. The purpose <strong>of</strong> the<br />
analysis was to determ<strong>in</strong>e the feasibility and practicality <strong>of</strong> high-resolution<br />
satellite imagery <strong>in</strong> archaeological work. The KVR-1000 satellite had a<br />
panchromatic camera record<strong>in</strong>g an area <strong>of</strong> 40 x 300 km, at a spatial resolution <strong>of</strong><br />
3-4m 2 . The images produced by the panchromatic sensor on the satellite<br />
provided enough detail so that all the prom<strong>in</strong>ent features associated with the<br />
Stonehenge monument could be identified. Other ground features were also<br />
identifiable such as 18 th century earthworks <strong>of</strong> an unf<strong>in</strong>ished road, circular cropmarks<br />
and structural enclosures, as well as Celtic field systems. The satellite<br />
images were identified as be<strong>in</strong>g comparable to medium-scale vertical aerial<br />
photographs. The big difference between the satellite and aerial photography is<br />
14
that due to the satellite’s coverage <strong>of</strong> a 40 x 300 km area, a s<strong>in</strong>gle image could<br />
cover the entire study area. Thus, Fowler concluded that based on a price and<br />
time comparison, satellites could be seen as a “cost-effective means <strong>of</strong> carry<strong>in</strong>g<br />
out a first pass <strong>in</strong>vestigation <strong>of</strong> a new study area” (Fowler, 1996:670).<br />
S<strong>in</strong>ce Fowler’s study (1996), satellite-imag<strong>in</strong>g resolution has cont<strong>in</strong>ued to<br />
improve. In September <strong>of</strong> 1999, Space Imag<strong>in</strong>g Inc., launched the IKONOS<br />
satellite, which has a panchromatic spatial resolution <strong>of</strong> 1m 2 pixels (Jensen,<br />
2000; Sheffner and Stoney, 1999). This resolution would prove even more useful<br />
to archaeological <strong>in</strong>vestigation because <strong>of</strong> its ability to detect smaller and more<br />
subtle irregularities <strong>in</strong> the Earth’s surface. However, as <strong>of</strong> yet, no archaeological<br />
research has been published which has used resolutions <strong>of</strong> this quality. With the<br />
cont<strong>in</strong>ual advances <strong>in</strong> the range and sensitivity <strong>of</strong> satellite sensors and the<br />
further addition <strong>of</strong> various satellite platforms <strong>in</strong> space, this technology is dest<strong>in</strong>ed<br />
to play a major role <strong>in</strong> future archaeological endeavors.<br />
Sub-Surface <strong>Remote</strong> <strong>Sens<strong>in</strong>g</strong><br />
Although aerial photography rema<strong>in</strong>s the dom<strong>in</strong>ant form <strong>of</strong> remotely<br />
sensed data used by archaeologists, geophysical prospect<strong>in</strong>g is be<strong>in</strong>g applied <strong>in</strong><br />
many archaeological projects as a technique <strong>of</strong> identify<strong>in</strong>g “hot spots” <strong>in</strong><br />
archaeological sites (White and Broadbent, 1993). These hot spots denote areas<br />
where there is an accumulation <strong>of</strong> subsurface material without hav<strong>in</strong>g to<br />
physically dig for it (White and Broadbent, 1993). Sub-surface remote sens<strong>in</strong>g is<br />
15
done at a much larger scale than aerial or satellite remote sens<strong>in</strong>g; commonly<br />
be<strong>in</strong>g localized to <strong>in</strong>dividual site locations. This type <strong>of</strong> remote sens<strong>in</strong>g helps<br />
archaeologists determ<strong>in</strong>e where good places to start digg<strong>in</strong>g are with<strong>in</strong> large<br />
sites. For example, a University <strong>of</strong> Calgary team, led by Earth Science program<br />
coord<strong>in</strong>ator Brian Moorman and directed by archaeologist Dr. Bill Glanzman,<br />
used ground penetrat<strong>in</strong>g radar (GPR) to identify major features with<strong>in</strong> the<br />
Mahram Bilqis site <strong>in</strong> Yemen (Wark, 2000) (figure 7).<br />
Figure 7. The use <strong>of</strong> GPR at the Mahram Bilqis site <strong>in</strong> Yemen ( photo from Wark,<br />
2000).<br />
Because <strong>of</strong> the sites size and complexity, it was imperative that an excavation<br />
strategy be developed before any large-scale digg<strong>in</strong>g took place. Not only are<br />
geophysical remote sens<strong>in</strong>g techniques useful <strong>in</strong> develop<strong>in</strong>g and manag<strong>in</strong>g<br />
excavation strategies, but they also apply to the multidiscipl<strong>in</strong>ary and nondestructive<br />
concerns <strong>of</strong> archaeologists discussed earlier.<br />
As with airborne remote sensors, use <strong>of</strong> the various sub-surface remote<br />
sens<strong>in</strong>g techniques and sensors are dictated by the environment <strong>in</strong> which the<br />
sensor is to be used and for what purpose the sensor is to be used for. Some <strong>of</strong><br />
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the major sensors and, or sens<strong>in</strong>g methods used <strong>in</strong> archaeological studies are<br />
mentioned below.<br />
The Magnetometer<br />
This <strong>in</strong>strument measures small changes <strong>in</strong> the earth’s magnetic field<br />
(Bevan, 1995). When systematically applied to an archaeological site one can<br />
create a magnetic map <strong>of</strong> the site, which shows where there are significant<br />
alterations <strong>in</strong> the magnetic field. For obvious reasons, the magnetometer is<br />
ideally suited for identify<strong>in</strong>g where there are iron artifacts however, it can also be<br />
used <strong>in</strong> the identification <strong>of</strong> fired earth, such as brick, ro<strong>of</strong>tiles, kilns, or furnaces<br />
(Bevan, 1995). As a result <strong>of</strong> the <strong>in</strong>strument’s sensitivity, it is less effective near<br />
or <strong>in</strong> urban areas or regions with igneous rock (Bevan, 1995).<br />
Resistivity Survey<br />
Resistivity <strong>in</strong>struments measure the ease at which electrical current<br />
passes through the ground between buried electrodes connected by wires to<br />
resistivity meters (Bevan, 1995). Archaeologists use this technology to map the<br />
distribution and vertical dimensions and characteristics <strong>of</strong> features with<strong>in</strong> a site<br />
(Griffiths and Barker, 1994). Although relatively easy to carry out, resistivity<br />
survey<strong>in</strong>g becomes <strong>in</strong>creas<strong>in</strong>gly difficult <strong>in</strong> rocky areas or areas <strong>of</strong> dense<br />
vegetation (Bevan, 1995).<br />
17
Conductivity Survey<br />
Conductivity <strong>in</strong>struments can be likened to very sensitive metal detectors,<br />
which locate similar features to those that would be identified <strong>in</strong> resistivity<br />
surveys (Bevan, 1995). Conductivity survey techniques do not suffer from the<br />
same problems as resistivity surveys do <strong>in</strong> vegetative or rocky areas however,<br />
they do not perform well near cities or areas with metal contam<strong>in</strong>ation (Bevan,<br />
1995).<br />
Ground Penetrat<strong>in</strong>g Radar (GPR)<br />
This <strong>in</strong>strument emits an electromagnetic pulse which bounces <strong>of</strong>f buried<br />
objects and, or features. The pulse is then recorded when it is returned to the<br />
sensor. The sensor records both the strength <strong>of</strong> the returned signal and the time<br />
it took the signal to come back <strong>in</strong> the same manner as satellite and aerial radar<br />
sensors perform. This <strong>in</strong>strument is beneficial <strong>in</strong> that it will detect a wider<br />
assortment <strong>of</strong> targets than the other sub-surface sensors mentioned (Bevan,<br />
1995). GPR technique is <strong>of</strong> limited use however, <strong>in</strong> regions where there are<br />
large boulders, or soils which are sal<strong>in</strong>e or clayey (Bevan, 1995).<br />
Conclusion<br />
<strong>Remote</strong> sens<strong>in</strong>g technologies have provided archaeologists a powerful<br />
tool set with which to further analyze past human lifeways. Satellite and aerial<br />
sensors, <strong>in</strong>dividually and, or <strong>in</strong> conjunction with each other have allowed for the<br />
18
apid, accurate, and non-destructive ability to prospect for unidentified sites, as<br />
well as, to identify and analyze characteristics with<strong>in</strong> and around sites. Through<br />
the use <strong>of</strong> sub-surface remote sens<strong>in</strong>g, archaeologists have been able to identify<br />
features and clusters <strong>of</strong> artifacts without exhum<strong>in</strong>g them, which destroys the<br />
context <strong>in</strong> which they were buried, elim<strong>in</strong>at<strong>in</strong>g the possibility for future analysis.<br />
Where excavation is needed, archaeologists have been able to use sub-surface<br />
remote sensors <strong>in</strong> develop<strong>in</strong>g excavation strategies, which allow for more<br />
effective use <strong>of</strong> the limited time and funds always plagu<strong>in</strong>g archaeological<br />
projects.<br />
Although remote sens<strong>in</strong>g techniques and technologies do not elim<strong>in</strong>ate<br />
the need for ground pro<strong>of</strong><strong>in</strong>g or the physical excavation <strong>of</strong> sites, they have, and<br />
will cont<strong>in</strong>ue to, allow archaeologists to better develop strategies for deal<strong>in</strong>g with<br />
the management <strong>of</strong> cultural resources.<br />
[R]emote sens<strong>in</strong>g may be as revolutionary <strong>in</strong> archaeology today as<br />
was the <strong>in</strong>troduction <strong>of</strong> radiocarbon dat<strong>in</strong>g <strong>in</strong> the 1950s (Sheets and<br />
Sever, 1988:35).<br />
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