 |
A Review of Geophysical Methods Used in Archaeology
by Jeffrey C. Wynn
ABSTRACT
Geophysical methods have been used
with increasing frequency in archaeology since 1946;
aerial photography has been used since 1919. The
geophysical methods that are most commonly used at
present are electrical resistivity, magnetics, and
ground-probing radar. Magnetic detectors, particularly
when used in a gradient mode or with a continuously
recording base station, are used at almost all sites
where any geophysical methods are used. Portable,
noncontacting electromagnetic soil-conductivity systems
are also being increasingly used because of their very
high rate of data acquisition. Less commonly used
methods include self-potential (sometimes called
spontaneous potential), microgravity, radiometric,
thermal infrared imagery, and sonic or seismic
techniques. Recent developments in image processing and
graphic representation have contributed substantially to
the archaeologist’s ability to do "rescue archaeology,"
that is, to carry out high-speed, nondestructive
reconnaissance surveys for ancient human cultural
evidence in advance of modern industrial development.
INTRODUCTION
Classical
archaeological methods, including trenching with trowels
and brushes, require an enormous expenditure of human
energy often provided by unpaid student labor.
Enthusiasm can provide payment only up to a point,
however. On some of the large contract archaeological
surveys, such as the Tombigbee Waterway, Dolores
Reservoir, and Black Mesa coal mining, archaeological
surveyers were paid to scan the surface for evidence of
human occupation (John Weymouth, oral communication,
1985). This is extremely inefficient because most buried
evidence will be missed, and the effort is not very
cost-effective. Several geophysical methods are now
available that are capable of mapping as much as a
quarter hectare of land (at a 1-m grid spacing) in a
day’s time. Some conductivity surveys can cover as much
as a hectare in a day under special circumstances. As a
result, the enormous expenditure of human hands
and-knees effort can now be used much more efficiently.
Archaeologists often call ground geophysical methods
"archaeological remote sensing," or "archaeogeophysics."
They now use these methods with ever-increasing
frequency as an important adjunct to several aspects of
their work.
Archaeologists
began using air photos immediately after World War I.
Since the mid 1940s, they have been using other
geophysical methods for prospection of archaeological
sites. (Prospection is a European term for
archaeogeophysics occasionally used by North Americans.)
Initially, their efforts focused on resistivity surveys,
followed soon after in the early ’50s by magnetic
surveys. High-resolution seismics, ground and spaceborne
radar, infrared imagery, and self-potential methods were
available to the archaeologist by the 1980s.
The reason for this
growth in activity is that geophysical methods can
provide an extremely rapid, three-diamensional
reconnaissance of a site. They can also provide a
synoptic view of the potential human cultural resources
of a target area. Surface geophysical methods currently
in use can usually detect soils disturbed by burials.
They can identify hollows and voids in structures such
as pyramids and ancient fortifications, and can map
buried stone foundations. All this can be done rapidly,
without ever disturbing the ground. This ability to
explore without damaging a site probably does not seem
important to a geologist. An archaeologist, however,
knows that excavation opens the way for the inevitable
destruction of preserved remains by weather and vandals.
Excavation, in fact, destruction. J.W. Weymouth (written
communication, 1985) remarks that "archaeology is the
one science that destroys its own lab-no repeated
experiments ... it better be done right the first time,
or not at all."
Modern examples of
the use of geophysics include searching for hidden
cavities in Chephren’s Pyramid at Giza in Egypt (Moussa
and Dolphin, 1977), searching for hidden caves in
Victorio Peak, New Mexico (Dolphin et al., 1978), and
locating buried Olmec basalt monuments in east-central
Mexico (Breiner and Coe, 1972). Lost 200year-old graves
in Maryland have been found using geophysical
techniques; in one gravesite an iron coffin from the
19th century was identified and dated, and at the same
time information was provided on how it was cast
(reversed magnetic polarity suggests that it was cast
upside down from its final resting configuration; J.
Wynn, unpublished data, 1985). Other modern uses of
geophysics include mapping of ancient Indian villages in
Washington State (Huggins, 1984) and - of submerged
paleoshorelines in the search for settlement sites in
the Gulf of Mexico (Stright, 1986). Finally, geophysical
methods are an integral part of the search for
5000-year-old shaft tombs in Jordan (Frohlich and
Lancaster, in press), and the search for Herod’s tomb in
Israel (L. Dolphin, oral communication, 1984).
TERMINOLOGY
Before proceeding
further, a digression into essential terminology is
necessary. "Archaeogeophysics" and "archaeological
prospection" are the terms most commonly applied to the
field described in this paper. This field includes
geophysical methods used in site prospection but not
isotopic provenance or archaeomagnetic dating.
Provenance, incidentally, (sometimes spelled
"provenience") is the study of the source of ancient
artifacts to document ancient trade and communication
patterns (Aitken, 1974). The latter technologies are
included along with prospection methods in the broader
term "archaeophysics." A survey of the broader field of
physics applied to archaeology is available in excellent
summaries by Aitken (1974), and Wolfman (1984).
Archaeologists
frequently use the term rescue archaeology." It refers
to emergency evaluation of an area for human cultural
resources. This normally is done, under pressure, in
advance of industrial development (the bulldozer used to
prepare a site for modern human cultural occupation). It
is nearly impossible to do this effectively without
assistance from geophysical methods. The term
"non-destructive archaeology" refers to using remote
sensing" methods to provide three-dimensional
information about a large tract of land. The key element
here is that the evaluation is done without disturbing
the land. Archaeologists commonly use the term "remote
sensing," incidentally, for more than just photo or
LANDSAT image analysis. For them, it includes the whole
range of surface geophysical and geochemical methods.
Within the general
area of physics used in archaeology, a distinction can
be made between prospection and nonprospection methods.
Geophysical methods used to prospect for sites not yet
found, or to, search for features within sites already
known (intra-site mapping) fit within the category of
prospection methods. Archaeomagnetism (for example, the
dating of kilns and hearths by analysis of the
orientation and intensity of their remanent magnetism)
is a nonprospecting method. Recent summaries of this
specialty are available in Wolfman (1984), and Tarling
et al. (1986). Isotopic analysis is another
nonprospecting method. It is used both for dating
purposes (for example, radiocarbon dating; see Olsson,
1970), as well as for provenance studies. Except for
providing the above references as starting points, I
will not discuss these other methods further.GEOPHYSICAL METHODS USED IN ARCHAEOLOGY
Airphotos and Modern Digital Imaging Systems
Probably the first
instrument-based prospection method applied to
archaeological sites, first used in England just after
World War 1, was aerial photography (Beazeley, 1919).
Archaeologists have frequently used it since then
because it gives a synoptic view, something especially
useful before a site is excavated (Aitken, 1974; Binford,
1964). Aerial photogrammetric methods are particularly
useful because they help identify soil marks and crop
marks. These subtle differences in soil color, moisture
content, and texture indicate buried structures such as
walls, or past human agricultural activity. A typical
application of this method is to photograph a site from
a low-flying airplane or a tethered balloon. Fine-scale
contour base maps are then constructed from photos taken
in stereo pairs. Photos are also used to look for any
features not readily visible on the ground that may give
the archaeologists some place from which to start their
excavation efforts.
Airborne and
space-borne digital imagery has been used experimentally
since the 1960s (Stringer and Cook, 1974; Lyons and
Avery, 1977; Ebert and Lyons, 1978) with only limited
success. The resolution of the imagery, especially
LANDSAT, is too low to allow effective use in intra-site
archaeological prospection. A good summary of the modern
state-of the-art is found in Ebert (1984), who points
out that such imagery can sometimes identify land forms
and vegetative types conducive to human occupation.
Berlin et al. (1977) have successfully used Landsat
imagery to map areas in northern Arizona that had been
cultivated and then abandoned almost 700 years ago.
The U.S. Geological
Survey and NASA have experimented with aircraft digital
imagery as a tool for mapping mineral resources. This
kind of imagery has considerably higher resolution than
LANDSAT data, and is limited only partly by the height
above ground. It also covers a much wider spectral range
than even the space-borne LANDSAT Thematic Mapper system
(in one case, 128 spectral channels). To the author’s
knowledge, this kind of imagery has not yet been used in
an archaeological application, but could prove extremely
useful. An extensive bibliography on air- and
space-borne remote sensing applications in archaeology
is available in Lyons et al. (1980).
Thermal methods,
both ground and airborne, have great potential for use
in the archaeological sciences. These methods include
temperature probes placed in the ground (Benner and
Brodkey, 1984), and thermal infrared imagery, used to
map subtle temperature differences in exposed surfaces
and soils. Thermal-inertia methods, using aircraft
thermal imaging systems flown at two different times,
are also included in this category; buried stone blocks,
for example, lose thermal energy at different rates than
the surrounding topsoil, resulting in anomalous day time
night time thermal ratios. Specific applications could
include mapping voids and other structures in ancient
fortifications such as those that exist in Peru, Egypt,
and Jerusalem. Perisset and Tabbagh (1981) have
demonstrated that digital imagery in the infrared can be
used in archaeological applications.Seismic Methods
Historically,
bosing has proven to be a useful but nonquantitative
exploration tool (Aitken, 1974). (Bosing refers to
thumping the ground with a heavy rammer- to detect
different sounds caused by resonant effects over
hollows, structures, and soils of different compaction.)
A sonic spectroscope (Carabelli, 1966) was used to test
stone and brick walls for voids and variations in
thickness. The method uses transmitted sonic energy in
the frequency range of about 20-3000 Hz. Resonance
effects are detected by means of a wide bandwidth
accelerometer or geophone attached to a wall. Typically,
the low natural resonant frequency (5-20 Hz) of a solid
wall will increase in the vicinity of a void.
Geophysicists have
experimented with refraction seismic methods in
archaeological applications with relatively little
success (Carson, 1962; Dolphin, oral communication,
1985; Aitken, 1974). Refraction methods work best in
mapping undisturbed layers that have velocities
increasing with depth. The method becomes less useful
and interpretation becomes very qualitative and
difficult when there are velocity inversions
representative of human cultural disturbance, or highly
three-dimensional objects such as burial sites or stone
foundations.
Seismic-reflection
methods (for example, sonar) are known to work well in
marine applications. Shipwrecks buried in sediments in
the Mediterranean stand out clearly by means of this
kind of technique (McGhee et al., 1968; Edgerton, 1972).
Seismic-reflection methods have also been used to detect
cavities in otherwise homogeneous rock masses or in
ancient stone structures. One particular example was the
detection of a cavity beneath the ancient
mosque/synagogue at Machpelah in modern Israel, where
tradition suggests that Abraham, the ancient Hebrew
prophet, was buried (Dolphin, oral communication, 1984).
Seismic-reflection methods were also used to map faults
and cavities in Chephren’s pyramid at Giza in Egypt
(Dolphin, 1981).
High-resolution
structure mapping technologies are the most recent
application of seismic reflection in archaeology. These
methods are the same as those used to investigate sites
for ocean-drilling platforms. Recently, they were used
to map sites of potential human occupation on ancient
shorelines beneath the sediments of the Gulf of Mexico (Stright,
1986). These surveys have resolution capabilities
sufficient to permit the identification of maddens
(trash heaps) and perhaps even dwellings. They hold out
the promise of making major contributions to the study
of the early human migration across the Bering Straits,
where most potential sites are now underwater (Kontrimavichus,
1984; Dikov, 1983; McManus et al. 1983).Magnetic Methods
Magnetic methods
were first used in the 1950s (Belshe, 1957; Aitken et
al. 1958), and have since become the backbone of
archaeological prospection. They are now used even more
frequently than electrical prospection methods.
Typically, soils that have had a campfire maintained
over them develop an increased magnetic susceptibility
resulting from the consequent reducing environment. This
reducing environment causes the formation of magnetite
if even moderate amounts of iron are present.
Magnetometers can easily detect variations of less than
0. 1 percent magnetite content in the soil.
Soils compacted by
human occupation or disturbed by a burial will also show
a variation from background values of magnetic
susceptibility. Burials frequently cause localized
oxidation, creating a void in magnetite content. These
phenomena have been exploited by geophysicists to
produce large-scale maps of aboriginal occupation sites
in the Great Plains of North America (Weymouth and
Huggins, 1985). They have also been used to map
Roman-age occupation sites in Europe (Scollar et al.,
1986). Archaeologists now even use susceptibility meters
to map soils modified by human activity (Colani and
Aitken, 1966).
Field procedures
for the magnetic prospection method have been refined
considerably in the last two decades. Some
archaeologists now use magnetic gradiometers and
microcomputer-controlled automatic data gathering
systems to make low-noise, high-productivity surveys. It
is not unusual for a field crew to acquire as much as a
third of a hectare of 1-m-grid data in a single day.
Some archaeologists have reintroduced fluxgate
magnetometers to speed up the data gathering process
even further (Clark, 1986). Experiments have also been
carried out to physically model the effects of campfires
and human occupation. This results in better
interpretation of field acquired data (Weymouth and
Huggins, 1985; Weymouth, 1986; Gibson, 1986). Figure 1,
provided by John Weymouth, shows the remarkable detail
and coverage now possible with magnetic methods.
Archeaomagnetism
Archaeomagnetism is
not a prospection method in itself, but is mentioned
here for completeness. Archaeologists and physicists
have used it to date ancient sites with identifiable
hearths and kilns in North America, Britain, and
southern Europe (Wolfman, 1984; Tarling et al., 1986).
Carefully oriented samples are first acquired from the
walls and floors of these kilns and hearths. The age of
last firing can then be inferred from the intensity and
orientation of the remanent magnetic field of the
sample. The process requires the calibration of the
declination, inclination, and intensity of the ancient
earth field. This process is still being refined by
means of dendrochronology (tree-ring analysis) and
radiocarbon-14 methods.Electrical Methods
Electrical methods
were first used in an archaeological application in the
1940s (Atkinson, 1952; Aitken, 1974). They have been
used extensively ever since, especially in Europe. These
methods are divided into the non-contacting
electromagnetic (sometimes called EM, or induction)
methods, and the soil contacting (sometimes called
galvanic or resistivity) methods.
Some of the initial
work with EM methods was carried out by Scollar (1962),
Foster (1968), Tite and Mullins (1969, 1970), and
Tabbagh (1974). Modern shallow-penetration,
high-resolution EM systems are now routinely used in
archaeology. They are ideally suited for site mapping
because of their resolution and speed. Frolich and
Lancaster (in press) have shown that data can be
acquired as fast as a person can walk. The method proved
invaluable in the mapping of 5000year-old shaft tombs in
Jordan and similar sites in Saudi Arabia, Bahrain,
Kuwait, and elsewhere (B. Frohlich, oral communication,
1984). Tabbagh (1986) has given a good summary of EM
work up to the 1980s, and introduces the concept of
using careful parameter control to derive magnetic
susceptibility information at the same time that
conductivity information is acquired. EM methods were
also used in archaeological prospection in the United
States with considerable success (Bevan, 1983; Wynn and
Sherwood, 1984).
Galvanic, or
soil-conduction electrical methods have been used since
the 1950s (Atkinson, 1952; Aitken, 1974). The best-known
method is resistivity profiling, and typically a Wenner
or pole-pole array is used. Sumner (1976) has provided a
good description of methods and arrays in common use
along with their relative advantages. Induced
polarization and self-potential methods are also
soil-contacting (Wynn and Sherwood, 1984).Resistivity methods are especially
helpful in detecting gross porosity changes caused by
buried stone structures. These changes are almost always
invisible to present-day surficial examination.
Resistivity methods were extensively used for
archaeology in England (Aitken, 1974; Clark, 1986) and
in Italy (Carabelli, 1967; Linington, 1970a). They are
particularly useful where high water tables prevent
mapping of Roman ruins in England or Europe (Aitken,
.1974; Pattantyus-A, 1986). Automatic data-acquisition
systems using string-and-pulley field coordinate input
methods are now used to produce field maps such as
Figure 2 (from Wynn and Sherwood, 1984).
Figure 1.
Magnetic grid-point map of parts of four hectares of the
Big Hidatsa site (32ME12), Knife River Indian Villages
National Historic Site, near Bismarck, North Dakota.
Each print character is at a grid point, and each
increment in character density represents an increase of
two nanoteslas in total field strngth. From Weymouth and
Hugging (1985). Copyright Yale Univ. Press.
The induced-polarization (IP)
method has been used with moderate success since the
1960s (Aspinall and Lynam, 1968,1970). IP is useful
because it can provide information on the presence of
disturbed clay- or pyrite-rich horizons in an area where
there has been human occupation. Limited field
experience suggests that the IP method provides
information of greater clarity than resistivity methods
(Aitken, 1974: 191). The requirement of nonpolarizing
electrodes slows down the field work considerably,
however. The method is only rarely used now because of
this time constraint and the cost of necessary
sophisticated electronic equipment.
The self-potential
(SP) method has proven to be the least expensive
geophysical method used in archaeological applications.
The equipment consists of only a digital volt meter,
some wire, and several low noise, nonpolarizing
electrodes. Among other things, the method was used in a
marine application to map the presence of an ironclad
ship sunk in 1864 during the U.S. Civil War (Corwin,
1973). Wynn and Sherwood (1984) have shown that the
method often gives anomalous responses over
archaeological targets in areas where one or more other
geophysical methods have failed to indicate anything
unusual.
The cause of the SP
response is thought to be the streaming potential, or
selective stripping of ions from percolating rain water
by the soil. This effect is different for areas where
the soil homogeneity has been disturbed by burials or
soil compaction. Electrode-drift noise levels usually
encountered in mineral resource SP surveys are about 10
- 30 millivolts. The principle difficulty with the SP
method is that archaeological responses are in the range
of this mineral survey noise threshold. Stringent field
procedures and some filtering can lower this threshold
to manageable levels, however. Figure 3 (modified from
Wynn and Sherwood, 1984) shows the typical response
found over archaeological targets.A new method for archaeological
prospection became available in the mid-1970s. This was
ground-probing radar, originally developed for rapid
engineering reconnaissance of building sites (Moffat,
1974; Morey, 1974; Cook, 1974; Vickers and Dolphin,
1975; Dolphin et al., 1978; Ulriksen, 1982). The method
uses continuous (sometimes pulsed) radar transmission
from one or two towed antennas. These typically operate
in the 150-500 megahertz range, and give reflections
from conductivity contrasts caused by metallic objects
or disturbed soil horizons. Sites of houses long since
lost have been identified by mapping reflections over
soil horizons excavated for a basement. Burials and
ancient man-made structures often give similar
soil-layer-interruption signatures. Vaughan (1986)
mapped burials and whale vertebrae of a Basque whaling
station in Labrador using groundprobing radar. Figure 4
(Bevan, 1983), shows a typical record obtained by means
of a ground-probing radar system.
Figure 2. Conductivity map, made with an EM31 device of the Ft. Washington
(Warburton Manor) site, east side of the Potomac River, south of Washington,
D.C., in Maryland. The contour defining the conductivity low (saw-toothed
symbols) outlines a buried stone foundation. The arrows indicate the location of
the profile shown in Figure 3. From Wynn and Sherwood (1084).
The principal
weakness of the groundprobing radar method is that it
cannot normally penetrate below a clay horizon (though
it will detect gaps in one readily enough). The
equipment is also very expensive, and interpretation of
the data can be complex and difficult. Often, the use of
an incorrect antenna means that important features are
obscured or missed entirely. This happens because of
poor resolution (using an antenna of too low a
frequency) or excessive attenuation (using an antenna of
too high a frequency) for the feature of interest.
Efforts are only just beginning to apply moveout
corrections and signal processing technologies to radar
data (Gary Olhoeft, U.S. Geological Survey, oral
communication, 1985). These methods, of course, have
been routinely used with seismic data in the search for
hydrocarbons for more than two decades.
Space-borne
applications of radar to archaeological prospecting have
suddenly appeared in the literature, almost seemingly by
accident. Recently, dendritic features were observed in
Shuttle Imaging Radar (SIR-A) data acquired in southern
Egypt and northern Sudan. Closer examination in the
field has shown that these features were caused by
ancient drainage systems beneath the sand cover. Related
Stone Age occupation sites buried beneath at least five
m of this unusually dry sand in the eastern Sahara were
also found (McCauley et al., 1982). Under conditions
found in most of the rest of the world, radar energy
cannot penetrate more than a few centimeters because of
the presence of water in the soil. Penetration can be
improved only by coupling the antenna directly to the
ground surface.Less
Commonly Used Methods
Radiometric data
have been experimented with at least since 1967 (Peschel,
1967). Their use stems from the fact that soils normally
have greater uranium, thorium, and potassium content
than underlying sedimentary rock. Consequently one would
suspect that soil-filled pits would give anomalously
high radioactivity. Human remains and middens also have
high levels of phosphate in them along with attendant
radiogenic isotopes (Eldt, 1977). However, soils only a
few centimeters thick can mask any radioactivity
generated by sources buried beneath them. Measurement
times with a small portable crystal detector are also on
the order of a minute per reading. These limitations
will probably prevent the method from ever being widely
used.
Neutron scattering
can detect moisture content in anthropogenic soils (Alldred
and Shepherd, 1963). It is theoretically possible to
detect stone structures beneath a porous soil, or voids
behind a rock or brick wall. Penetration limits of 10-20
cm, however, make it hardly worth serious consideration.
Aitken (1974) did state that a void behind one brick
thickness of a wall would be detectable.Gravity methods were tried in
archaeology several times since 1965 (Linington, 1966;
Kolendo et al., 1973; Fajklewicz et al., 1982).
Anthropogenic gravity signatures are typically extremely
small, however, and usually close to the instrumental
noise threshold.
Centimeter-precision accuracy in
elevation control is absolutely necessary when applying
the gravity method in archaeology. This accuracy
requirement, and similar precision requirements in
calculating terrain corrections (with the enormous
computational labor this requires), have significantly
dampened the interest of geophysicists who might have
considered applying this method.
Figure 3. East-west profiles, looking north, of self-potential, toopographic
relief (here exaggerated twice for clarity), conductivity, and total magnetic
field intensity from the Ft. Washington (Warburton Manor) site shown in Figure
2. From Wynn and Sherwood (1984)
DATA ACQUISITION, PROCESSING, AND
IMAGE ENHANCEMENT
In the last two
decades, geophysicists working in archaeological
applications have introduced many innovations to speed
up data acquisition and the presentation/interpretation
of the data. Many of these innovations have simply been
applied to archaeology as the technology became
available in other fields. The most important
developments in the field equipment include automated
microprocessor controlled field data and coordinate
acquisition systems (Weymouth and Huggins, 1985). Other
innovations, such as streamlined field procedures and
customized instrumentation, demonstrate the blossoming
of human genius when faced with a tedious and
time-consuming task.
Linington (1970b)
reported initial efforts to develop filtering procedures
for archaeogeophysical data. The desire to extract every
useful piece of information from field data has
preoccupied geophysicists working in archaeological
applications ever since. One of the more significant
breakthroughs in the last decade has been the
introduction of image processing display and filtering
technology. These technologies derive mainly from
electrical engineering and remote sensing
(geophysicists’ meaning of the word) research. Skilled
interpreters often miss subtle information existing in
the data when it is presented in the form of contoured
maps. Filtering methods and image-enhancing algorithms
of remarkable sophistication are now commonly applied to
archaeological geophysical data, which are then
displayed in the form of an image rather than a contour
map. This usually enhances subtle linear features
otherwise lost in the presence of strong anomalies of
geologic origin (Scollar et al., 1986).
In the Federal
Republic of Germany and elsewhere in Europe, image
enhancement methods are routinely applied by
archaeologists to generate land utilization maps (Scollar
et al., 1986). These maps show potential human
occupation sites that are ubiquitous on the continent.
The sites are not usually apparent to an archaeologist
from surface examination of a site, however. Maps of
this sort are now used to decide whether or not a new
land development will be allowed to proceed. There are
political consequences when large development projects
are held up by the process of archaeological
certification, of course. This has led to significant
governmental financial support for the development of
fast, efficient methods of data acquisition and image
presentation (Scollar et al., 1986).
Figure 4. Ground probing radar profile over the site of the Taylor house,
destroyed and location lost during the seige of Petersburg, Virginia, American
Civil War, 1864. The location of the cellar of the original structure can be
easily seen ("b" on the figure) because of the interruption of the clay layer
into which it was dug. The feature indicated by an "*" is modern metallic debris
or a pipe. Figure provided by Bruce Bevan. Figure reprinted by permission of the
Society for Historical Archaeology.
LIMITATIONS OF GEOPHYSICAL METHODS
Geophysical methods
used in archaeology are not an unqualified panacea for
the archaeologist. In fact, there are several reasons
why geophysical methods do not work, or are not
cost-effective, in archaeological applications. A
primary reason is that they are for the most part
instrumentation, computer, and interpretation intensive.
Use of a geophysical consultant can be prohibitively
expensive, and some ground-probing radar systems cost
upwards of $50,000. Data processing and image
enhancement methods are also expensive, usually
requiring custom application to each data set.
Archaeologists do not usually deal with the relatively
large costs that geoscientists are much more accustomed
to.
Nonanthropogenic
sources for geophysical anomalies are also a major
problem with geophysical measurements over
archaeological sites. Often, anomalies caused by ancient
human cultural activity lie beneath the noise threshold
of the surrounding geologic environment. A frustrating
example is that the normal variation of magnetite
content in soils and rocks underlying a site frequently
exceeds the anthropogenic anomalies by an order of
magnitude or more. This problem is not insurmountable,
however. The higher frequency content of shallow
archaeological sources means that filtering can remove
most of the unwanted noise.
Sometimes, ancient
anthropogenic anomalies are unobservable due to the
large variations caused by modern cultural interference
(powerlines, roads, and so on). This means that there
will be areas, especially where population density is
large, where geophysical prospecting for archaeological
sites can be carried out with only extreme difficulty.
It is also not
unusual for physical properties to vary little with the
human disturbance of a solid horizon. This problem
forces the archaeologist or geophysicist to search for
more than one physical property in which contrasts are
sufficient to be useful in mapping. Often the problem
can be mitigated with careful field procedures that
increase the signal-to-noise ratio in the data. This is
especially true of the self-potential method.
Resolution and
depth limitations are important restraints on the use of
geophysical methods in most applications. Because most
anthropogenic features are near the surface, this does
not usually cause problems for an archaeologist working
on the earth’s surface. A notable exception is work
conducted by Ain Shams University and Stanford Research
Institute at the pyramid of Chephren. Here radar echo
and seismic-reflection searches for hidden chambers were
frustrated by moisture and clay content, and the
presence of many joints in the limestone blocks. Signals
had to traverse distances as much as 100 m because of
the size of the pyramid (Moussa and Dolphin, 1977).FUTURE DEVELOPMENTS ON THE
HORIZON
The trend in
geophysical data gathering in archaeological prospecting
is toward acquiring and using very large data bases.
This is being done with microprocessor-controlled
instruments and innovative developments in field
procedures. Together these permit gathering of large
numbers of data points in a relatively short amount of
time. Large databases mean that the archaeologist can
cover larger areas with higher resolution than ever
before. The larger amounts of data and higher resolution
also mean we can expect increased performance from
digital image-enhancement algorithms; there is more data
to work with. Consequently, ground geophysical methods
can now provide the kind of synoptic site examination
formerly available only with aerial photography.
High resolution
airborne and spaceborne imagery systems, such as the
Large Format Camera on the U.S. Space Shuttle (Doyle,
1985), are now coming online. This means that
archaeologists can examine large areas with a resolution
good enough for archaeological mapping. Visible and
near-infrared, as well as thermal inertia systems also
hold considerable promise for the archaeologist.
Analytical combinations of several types of ground data
and imagery will probably become the norm in the next
several years, as it is in the geophysics profession in
general. Techniques now available to register and
overlay different gridded data sets will probably lead
to major breakthroughs in site identifications during
the remainder of the 20th century.
There has been
extensive use of laboratory measurements in provenance
studies, and elsewhere in archaeology and anthropology (Aitken,
1974). Little work has been done, however, in
characterization and identification of geophysical
signatures of anthropogenic materials. The complexity of
compaction and weathering processes, and variations in
environmental conditions (including moisture and
magnetite content) certainly make this a daunting
project. Until this work is undertaken, geophysicists
will be unable to carry out detailed quantitative
interpretations of archaeogeophysical data. Generic
studies may turn out not to be very useful; we won’t
know until they have been tried,, however.
John Weymouth and
Bruce Bevan have kindly provided Figures 1 and 4 of this
paper. The early and tireless work of Weymouth, Bevan,
A. Tabbagh, R.E. Linington, 1. Scollar, D. Tarling, A.
Clark, S. Breiner, and others contributed to the present
advanced state of geophysical experience in
archaeological applications.
REFERENCES
Aitken, M.J. (1974). Physics and
archaeology, 2nd edition. Oxford: Clarendon Press, 286
pp. Aitken, M.J., Webster, G., and Rees, A. (1958).
Magnetic prospecting: Antiquity 32, 270-271.
Alldred, J.C., and Shepherd, A.
(1963). Trial of neutron scattering for the detection of
buried walls and cavities. Archaeometry 6, 89-92.
Aspinall, A., and Lynam, J.T.
(1968). Induced polarizationasatechnique
forarchaeologicalsurveying.Prospezioni Archeologiche 3,
91-93.
Aspinall, A., and Lynam, J.T.
(1970). An induced polarization instrument for detection
of near surface features. Prospezioni Archeologiche 5,
67-76.
Atkinson,R.J.C.(1952).Methodeselectriquesdeprospection
en archeologie, pp. 59-70, in A. Laming, Ed., La
Decovert de Passe. Paris: Picard.
Beazeley, G.A. (1919). Air
photography in archaeology. Geographical Journal 53,
330-335.
BeIshe, J.C. (1957). Recent
magnetic investigations at Cambridge University.
Advances in Physics 6,192193.
Benner, S.M., and Brodkey, R.S.
(1984). Underground detection using differential heat
analysis. Archaeometry 26(l), 21-26.
Berlin, G.L., Ambler, J.R., Hevly,
R.H., and Schaber, G.G. (1977). Identification of
Sinagua agricultural fields by aerial thermography, soil
chemistry, pollen/ plant analysis and archaeology.
American Antiquity 42(4), 588-600.
Bevan, B. (1983). Electromagnetics
for mapping buried earth features. Jol;rnaL
ofFieldArchaeology 10, 4754.
Binford, L.R. (1964). A
consideration of archaeological research design.
American Antiquitv 29, 425-441. Breiner, S. and Coe,
M.D. (1972). Magnetic exploration of the Olmec
civilization. American Scientist 60, 566-575.
Carabelli, E. (1966). A new tool
for archaeological prospecting-the sonic spectroscope
for the detection of cavities. Prospezioni
Archeologicahe 1, 25-35.
Carab,elli, E. (1967). Ricerca
sperimentale dei dispositivi piu adatti alla prospezione
electrica di cavita sotteranee. Prospezioizi
Archeologiche 2, 9-21.
Carson, H.H. (1962). A seismic
survey at Harpers Ferry. Archaeonze" 5, 119- 122.
Clark, A.J. (1986). Review of
archaeological geophysics in Britain. Geoph.vsics 51,
(in press).
Colani, C., and Aitken, M.J.
(1966). Utilization of magnetic viscosity effects in
soils for archaeological prospection. Nature 212, 1446.
Cook, J.C. (1974). Status of
ground-probing radar and some recent experience, in
Proceedings of an Engineering Foundation Conference on
Subsurface Exploration for Underground Excavation and
Heavy Construction, held at New England College,
Hennikei-, New Hampshire, August 11-16, 1974: American
Society of Civil Engineers, New York.
Corwin, R.F. (1973). Offshore
application of selfpotential prospecting. PhD
Dissertation, University of California, Bei-keley.
Dikov, N.N. (1983). The stages and
routes of human occupation of the Beringian land bridge
based on archaeological data, 365-388, in P.M. Masters
and N.C. Flemming,Eds.,Quaternary coastlines and marine
archaeology: New York: Academic Press.
Dolphin, L.T. (1981). Geophysical
methods for archaeological surveys in Israel: Stanford
Research Intemational, Menlo Park, Calif., December
1981, 7 pp.
Dolphin, L.T., Tanzi, J.D., and
Beatty, W.B. (1978). Radar probing of Victorio Peak, New
Mexico: Geophysics 43, 1441-1448.
Doyle, F.J. (1985). The large
format camera on Shuttle Mission 41G: Photogrammetric
Engineering and Remote Sensing 51, 200.
Ebert, J.1. (1984) Remote sensing
applications in archaeology, in M. Schiffer, Ed.,
Advances in Archaeological Method and Theory 7, 293-362.
Ebert, J.1., and Lyons, T.T.
(1978). Prehistoric irrigation canals identified from
Skylab III and Lansat Imagery in Phoenix, Arizona, in
T.R. Lyons, assembler, Survey and Analysis: U.S.
National Park Service and University of New Mexico,
Reports of the Chaco Center, 5-9.
Edgerton, H.E. (1972). Underwater
archaeology—Sonar surveys in Greece: MASCA (Museum
Applied Science Center for Archaeology, University of
Pennsylvania, Philadepphia) Newsletter 9(l), 3.
Eidt, R.C. (1977). Detection and
examination of anthrosols by phosphate analysis. Science
197, 1327-1333.
Fajklewicz, A., Glinski, A., and
Sliz, J. (1982). Some applications of the underground
tower gravity vertical gradient- Geophysics 47,
1688-1692.
Foster, E.J. (1968). Fur-ther
developments of the pulsed induction metal detector.
Prospezioni Archeologiche 3, 95-99.
Frohlich, B., and Lancaster, W.J.
(1986). Electromagnetic surveying in current middle
eastern archaeology-Application and evaluation.
Geophysics 51, (in press).
Gibson, T.H. (1986). Magnetic
prospection on prehistoric sites in western Canada.
Geophysics 51, 553560.
Huggins, R. (1984). Some design
considerations for under-taking a magnetic survey for
archaeological resources. Proceedings of the Society of
Exploration Geophysicists Annual Meeting, Atlanta
Georgia, December 1984, 209-212.
Kolendo, J., Przenioslo, J., Iciek,
A., Jagodzinski, A., Taluc, S., and Porzezynski, S.
(1973). Geophysical prospecting for the historic remains
of Carthage, Tunisia (abs.]: Proceedings of the Society
of Exploration Geophysicists 43rd annual international
meeting Mexico City, October 1973, 30.
Kontrimavichus, V.L., Ed. (1984).
Beringia in the Cenozoic era: Amerind Publishing Co.,
New Delhi. (A translation from the Russian Beringia v
Kainozoe, originally published in Moscow in 1976.)
Linington, R.E. (1966). Test use of
a gravimeter on Etruscan chamber tombs at Cerveteri.
Prospezioni Archeologiche 1, 37-41.
Linington, R.E. (1970a). Techniques
used in archaeological field surveys: Royal Society of
London, Philosophical Transactions A, 269, 89 - 108.
Linington, R.E. (1970b). A first
use of linear filtering techniques on archaeological
prospecting results. Prospezioni Ar-cheologiche 5,
43-54.
Lyons, T.R., and Avery, T.E.
(1977). Remote sensing: A handbook for archaeologists
and cultural resource managers. U.S. Government Printing
Office, Washington, D.C.
Lyons, T.R., and Ebert, J.1., Eds.
(1978). Remote sensing and non-destructive archaeology:
Remote Sensing Division, Southwest Cultural Resources
Center, National Park Service, Publication No. 36, 72
pp.
Lyons, T.R., Hitchcock, R.K., and
Wills, W.H. (1980). Remote sensing, aerial
anthropological perspectives-A bibliography of remote
sensing in cultural resource studies. Supplement No. 3
to Remote Sensing, a handbook for archaeologists and
cultural resource managers, National Park Service, 25
pp.
McCauley, J.F., Schaber, G.G.,
Breed, C.S., Grolier, M.J., Haynes, C.V., Issawi, B.,
Elachi, C., and Blom, R. (1982). Subsurface valleys and
geoarchaeology of the eastern Sahara revealed by Shuttle
Radar. Science 218, 1004-1020.
McGhee, M.S., Luyendyk, B.P., and
Boegemen, D.E. (1968). Location of an ancient Roman
shipwreck by modern acoustic techniques-A critical look
at marine technology. Proceedings of the Marine
Technology Society, 4th Annual Conference, Washington,
D.C., July 1968.
McManus, D.A., Creager, J.S.,
Echols, R.J., and Holmes, M.L. (1983). The Holocene
transgression on the Arctic flank of Beringia—Chukchi
Valley to Chukchi Estuary to Chukchi Sea. 365-388, in
P.M. Masters and N.C. Flemming, Eds., Quaternary
coastlines and marine archaeology. New York: Academic
Press.
Moffatt, D.L. (1974). Subsurface
video pulse radar, in Proceedings of an Engineering
Foundation Conference on Subsurface Exploration for
Underground Excavation and Heavy Construction, held at
New England College, Henniker, New Hampshire, Aug-ust
11-16, American Soci’ety of Civil Engineers, New York.
Morey, R.M. (1974). Continuous
subsurface profiling by impulse radar, in Proceedings of
an Engineering Foundation Conference on Subsurface
Exploration for Underground Excavation and Heavy
Construction, held at New England College. Henniker, New
Harhpshire, August 11-16. American Society of Civil
Engineers, New York, 213-232.
Moussa, A.H., and Dolphin, L.T.,
(1977). Applications of modern sensing techniques to
Egyptology: Report of the 1977 field experiments, NSF
Grant No. INT76-00414, SRI International, Menlo Park,
California, Sept. 1977 (unclassified).
Olsson, I.V., Ed. (1970).
Radiocarbon variations and absolute chronology:
Stockholm: Almqvist and Wiksell. Pattantyus-A, M.
(1986). Geophysical results in archaeology in Hungary.
Geophysics 51, 561-567.
Perisset, M.C., and Tabbagh, A.
(1981). Interpretation of thermal prospection on bare
soils. Archaeometry 23(2), 169-189.
Peschel, G. (1967). Radiometrische
Messungen zum Nachweis ver-deckter archaologisher
Objecte.Ausgrabungen and Funde 12, 287-297.
Scollar, 1. (1962). Electromagnetic
prospecting methods in archaeology. Archaeometry 5,
146-153.
Scollar, I., Weidner, B., and
Segeth, K. (1986). Display of archaeological magnetic
data. Geophysics 51, 623633.
Stright, M.J. (1986). Evaluation of
archaeological site potential on the outer continental
shelf using highresolution seismic data. Geophysics 51,
605-622. Stringer, W.J., and Cook, J.P. (1974).
Feasibility study for locating archaeological village
sites by satellite remote sensing techniques: Report to
the U.S. National Aeronautics and Space Administration
for Contract No. NAS5-21833, Task 110-N, Washington,
D.C.
Sumner, J.S. (1976). Principles of
induced polarization for geophysical exploration. New
York: Elsevier, 277 pp-
Tabbagh, A. (1974). Methodes de
prospection electromagnetique applicables aux problemes
archeologiques. Archaeophysika 5, 350-437.
Tabbagh, A. (1986). Applications
and advantages of the slingram EM method for
archaeological prospecting. Geophysics 51, 576-584.
Tarling, D.H., Hammo, N.B., and
Downey, W.S. (1986). The scatter of magnetic directions
in archaeomagnetic studies. Geophysics 51, 634-639.
Tite, M.S. and Mullins, C. (1969).
Electromagnetic prospecting-A preliminary investigation.
Prospezioni Archeologiche 4, 95-102.
Tite, M.S., and Mullins, C. (1970).
Electromagnetic prospecting on archaeological sites
using a soil conductivity meter. Archaeometry 12,
97-104.
Ulriksen, C.P.F. (1982).
Application of the impulse radar to civil engineering.
PhD Dissertation, Lund University of Technology,
CODEN:LUTVDG, Lund, Swedeni 179 pp.
Vaughan, C.J. (1986). Ground
penetrating radar surveys used in archaeological
investigations. Geophysics 51, 595-604.
Vickers, R-, and Dolphin, LT.
(1975). A communication about an archaeological radar
experiment at Chaco Canyon: MASCA (Museum Applied
Science Center for Archaeology, University of
Pennsylvania, Philadelphia) Newsletter 11(l).
Weymouth., J.W. (1986)
Archaeological site surveying program at the University
of Nebraska. Geophysics 51, 538-552.
Weymouth, J.W., and Huggins, R.
(1985). Geophysical surveying of archaeo logical sites,
191-235, in G. Rapp and d.A. Gifford,.Eds.,
Archaeological geology. New Haven: Yale University
Press.
Wolfman, D. (1984) Geomagnetic
dating methods in archaeology, in M. Schiffer, ed.,
Advances in Archaeological Method and Theory 7, 363-458.
Wynn, J.C., and Sherwood, S.I.
(1984). The selfpotential (SP) method-an inexpensive
reconnaissance and archaeological mapping tool. Journal
of Field Archaeology 11(2), 195-204.
|
|
