World War One brought the discovery that photographs behind enemy lines
taken from airplanes could be of great value in warfare. Not longer after
this, observers taking random photographs from the air over rural England
noticed that traces of old Roman walls, forts and roads could be seen on
aerial photographs but otherwise went unnoticed under cornfields and pastures
when archaeologists wandered about the countryside on foot.
Terrain photos from captive balloons had been made even earlier (1860) but
it was only in the 1930's and 40's that archaeologists began to take advantage
of photos from the air over archaeological sites. Today, of course, stereo-pair
color and color infra-red film photographs (or even the newer multi-spectral
imaging methods) from the air, are the place to begin in mapping and understanding
an archaeologically interesting area.
Prior to the Second World War electronic methods began to be employed
in earnest in searching hr oil and large mineral deposits beneath the surface
of the earth. Because of the big economic payoff, successful discoveries
made possible by even primitive geophysical methods were high enough that
R&D budgets soon became generous. An explosion of knowledge in geology,
earth science, geophysical and remote sensing followed. After World War
2 all the sophistication brought by war time research then also became available
to private industry, producing a new, even bigger, boom in geophysical exploration.
Historically, the scale of exploration required for oil and mineral exploration
for most of these methods was very large (of the order of kilometers), while
in contrast the scale of interest to an archaeologist is only centimeters
or meters.
In addition to highly evolved aerial photography, airborne and satellite
multi- spectral imaging instruments, good ground based geophysical instruments
began to be commercially available in the 30's taking advantage of various
physical phenomena.
Some basic geophysical methods include the following: (1) Seismic Reflection
& Refraction, (2) Gravity, (3) Magnetics, (4) Electrical, and (5) Radioactivity.
Method (1) is commonly used in oil exploration, engineering geology, and
regional geology studies. The gravity method (2) is especially useful in
oil exploration. Methods (3) and (4) find common application in mineral
exploration, oil exploration, and regional geology studies. Finally radioactive
methods are used in exploration for radioactive minerals.
Common geophysical instruments and methods include:
As mentioned, the application of some of the above geophysical methods
to archaeology began in earnest after World War II, but in contrast to the
huge budgets available for petroleum and mineral exploration, archaeological
budgets have almost always been minuscule. Usually the chief archaeologist
at a site is a reputable and experienced professor whose modest salary is
paid by his school so that he can teach university classes and do some seasonal
field research on the side. The field work in archaeology has always depended
mostly on student volunteers and assistants. Small amounts of financing
are sometimes available from museums or grant institutions such as National
Geographic Society, the National Science Foundation or the Smithsonian Institution.
Usually digging at an archaeological site must be done by hand though occasionally
massive amounts of overburden must be removed, or trenching done, with the
help of a back-hoe or bulldozer. Cataloging, preserving artifacts (conservation),
and publication of scientific papers occupies the off-season, but often
funding levels for these important activities are also minimal.
But even with the limited budgets archaeologists have with for decades,
geophysical methods can be of great value to an archaeologist. Some of these
reasons include:
Ground Penetrating Radar (GPR) was invented in the 1970's, originally
for military purposes such as locating land-mines and underground military
tunnels. Soon public utility companies began to be keenly interested in
such radars in hopes they would provide a practical method for mapping pipes
and utility lines under city streets, and for locating cavities and voids.
Most recently radars of this type have been used from aircraft for mapping
the surface of the earth through jungle or forest cover. GPR technologies
have proven to be of great usefulness in archaeology, especially in Israel.
Radar from the air is seldom of use to the archaeologist these days except
for large sites covered by jungle such as are found in the Yucatan or Central
America. Foliage-penetrating radars are now used widely for topographic
mapping of the land surface beneath jungle canopy and forest cover.
Thermal-infrared imaging methods measure the surface temperature of the
earth to an accuracy of a fraction of one degree. The electronic scanning
equipment necessary for such measurements was originally available only
to the military and the instruments cost from $100,000 to $1,000,000. In
recent years portable instruments of great sensitivity have become commercially
available at greatly reduced prices. These instruments can be used from
a tripod on the ground, or from helicopter or airplane by viewing through
a hole in the fuselage.
Borehole Technology. Radars, seismic and resistivity other probes are often
lowered into holes drilled into an archaeological site, to permit geophysical
probing at depth. Core drill soil samples can be a big help in identifying
the various historic levels and strata at a layered archaeological site
such as a tell. When chambers or voids are encountered while drilling, these
can be explored (and video taped) using a down-hole television camera equipped
with lights. Holes drilled into an archaeological site are obviously much
less damaging than trenches or tunnels and they can either be filled or
capped after use.
Not all individuals or companies who offer geophysical assistance to the
archaeologist are reputable or professionally competent. Fraudulent self-made
experts---whose instruments may be little more than electronic water dowsing
rods-commonly offer services that are of little value. Some geophysical
instruments on the market may promise amazing results in identifying metals
at great depth by type and quantity but many of these operate by methods
unknown to reputable science. Geophysical records, even when made using
legitimate instruments, are also of little value unless the data is collected
and interpreted correctly. Archaeologists should not expect his geophysicist
to work wonders for him at all sites. In some cases a combination of instruments
may be appropriate, in other cases no known method may prove really very
useful or cost effective.
The following legitimate geophysical methods and instruments are in use
in the service of archaeology today:
A wide variety of "metal detectors" are commercially available
today; they have the advantage of being easy to use and most cost only a
few hundred dollars. The larger the search coil, the deeper the penetration;
however coins and small metal objects can be detected only a few inches
deep and very large metal objects only to depths of a few feet. Non-metal
objects are not detected. Some areas are too "noisy" for metal
detectors. "Noise" can originate from power lines, or from obscuring
signals caused by nearby parked cars, scattered nails, re-bar or metallic
litter at the site. Highly mineralized areas are difficult to work in, and
certain rocks such as iron-rich basalt can be troublesome for metal detector
work.
Metal detectors are "active" instruments. A battery-powered transmitter
in the unit radiates a relatively low-frequency alternating current signal
into the ground by means of a transmitting coil. If the signal from the
transmitter encounters any type of conducting metal or mineral in the ground
an induced current flows in the subsurface target. This induced current
then re-radiates a weak signal back to the surface. The latter signal is
out-of-phase with the transmitted signal and thus is easily detected by
a receiving coil. Modern metal detectors have circuitry for carefully balancing
out any direct signal leakage between transmitter and receiver coils and
for discriminating between large and small, shallow or deep, and ferrous
or non-ferrous metals.
The simpler instruments of this type are useful for "coin shooting"
at old ghost town sites, or archaeological sites (on land or under the sea),
and for locating gold or silver deposits within a quartz vein in a lode
mine. Small objects such as coins usually must lie within a few inches to
a foot of the surface to be detected by metal detectors.
The sensitivity of metal detectors is a steep function of the coil diameter,
however with large coils and ample transmitter power larger metal objects
can be located to depths of 10 or 15 feet using metal detectors. Claims
for detection at greater depths as well as identification of metals by type
are suspect.
The resistivity method of subsurface exploration is powerful but often
tedious to employ unless an automated instrument is available. The method
is simple: Current is introduced into the ground through one pair of electrodes.
Current flow between these electrodes fans out through the ground in a pattern
and intensity that depends on the conductivity of the ground and any stratification
or obstacles that lie in the vicinity of the electrodes. A second pair of
electrodes is then used to quantitatively measure the voltage pattern on
the surface resulting from the current flow pattern of the first set of
electrodes. A number of different electrode configurations are used in practice,
but in simplest form the operator takes measurements along a straight line
("traverse"), moving his electrodes in pairs. He then repeats
the measurements along a parallel line until the area of interest has been
covered with a rectangular grid of electrode positions. If multiple electrodes
are used and the results recorded automatically at the push of a button,
the area to be examined can be searched more efficiently, and also probed
at various depths at the same time. (As a rule of thumb, the depth of maximum
sensitivity for resistivity sounding is about 1.5 times the electrode spacing
in typical arrays). A crew of two can easily study an area of perhaps 1000
square meters in a day. Typical electrode spacings might be 0. 3 to 1.0
meters for shallow targets.
Once the resistivity data has been collected, a simple computer program
quickly generates a three-dimensional map of ground electrical resistivity
or conductivity. Targets most easily seen on resistivity surveys are cavities
or voids, but buried walls and filled trenches can often be mapped. The
target depth divided by the diameter of the target should be less than 3
or 4 for best sensitivity, though some experts claim to be able to detect
targets with a depth to diameter ratio of 9 or more. Boulders, geological
stratifications and water-table depth can also be successfully located by
the use of resistivity by selecting appropriate electrode spacing to allow
the probing current to enter the ground to the appropriate depth. Resistivity
meters employed in oil prospecting are often powered by large generators
using very high voltages and electrodes spaced perhaps hundreds of meters
or kilometers apart, but instruments suitable for archaeological use are
battery powered, easy to use, and usually priced under $1500. Resistivity
instruments no different than those used by professional geophysicists,
but with fancy labels attached, are often found advertised for five times
the price of standard instruments. Let the buyer beware!
Radars designed for probing into the earth typically operate from 30
to 300 MHz-the frequency being determined by the length of the dipole antennas
used. It is necessary to use relatively low frequencies because the earth
almost always is a good absorber of radar waves. Unfortunately, low frequencies
imply long probing wavelengths and long wavelengths imply low resolution.
A very short pulse is used allowing accurate measurement of depth to the
target, however the antenna beam is very broad (90-120 degrees usually)
and can not easily be narrowed because the antennas become too big and bulky.
Very often GPRs are mounted on a small wheeled cart which is hand towed
across the area of interest, that is, if the search area is reasonably flat
and relatively free of brush and boulders. The echoes are displayed in a
continuous strip oscilloscope false color record for ease of interpreting
results. In recent years the state of the art in GPR technology has been
greatly improved by computer signal processing methods, since the performance
of these radars is almost always "clutter limited." Clutter signals
are unwanted reflections, off-axis echoes, and multiple scattering echoes.
These signals obscure the target of interest under bands of signals but
in many cases digital processing improves radar performance by many orders
of magnitude.
When cart-mounted radar can be used, an experienced operator can often traversing
large areas of surface at a site in a single day. The radar output can be
recorded on a standard home video tape for archiving and detailed study,
and also printed out on strip-chart paper for immediate on site analysis.
GPRs are usually limited not only by clutter but also by attenuation of
the radar signal in the soil. This is most severe in clay soils and damp
soils where the salt content is high. The depth of penetration at some sites
may be less than 1 foot, or under favorable conditions, many tens of feet
or even hundreds of feet. Commercial cart GPRs are priced from about $18,000
to $40,000 and operator training and experience is necessary to interpret
the records.
Very often cart radars can not be used because of rugged surface terrain.
Or perhaps the area to be explored is underground---inside a tunnel or cistern
or along a confined area such as a hillside. Portable individual transmitting
and receiving dipoles are useful in such cases. But the data must now be
recorded point by point, usually by taking Polaroid photos. Targets of interest
can be triangulated and mapped if these targets can be viewed from various
aspect angles. Portable GPRs are well suited for discovering cavities and
voids, and when soil attenuation values are low they can detect caves, tombs,
or chambers one hundred feet or more in depth. Interpretation of GPR records
of all types is unusually difficult requiring operator skill and experience
for satisfactory results.
Sound waves are not easily coupled into soils, except at very low frequencies (a few Hertz, or cycles per second) but at higher frequencies sound waves can be used in rock or solid walls as a helpful diagnostic tool. Frequencies used for probing in bedrock or stone are generally 1000 to 30,000 Hertz (cycles/second). A coupling gel, or mud layer, is necessary to couple the seismic signal into and out of the transmitting and receiving transducers and this makes field measurements somewhat time consuming unless only a few locations are to be surveyed. High-frequency sounding is especially useful for finding tombs and voids in areas of high radar signal absorption. For example, the Valley of the Kings in Egypt has very high radar attenuation, but the same limestone can be probed with high-frequency sound waves for distances well beyond 100 feet. Measuring the thickness of a wall or pillar is readily done with this method. High-frequency seismic sounding instruments are not presently commercially available, but can be custom built for about $10,000.
The earth's magnetic field is slightly disturbed by some kinds of archaeological anomalies such as fired clay pottery. The magnetic signals associated with archaeological features are very small and easily obscured by trash metals, power lines, nearby automobiles, and the like. Magnetometers are most suited for remote, isolated sites away from modern buildings and debris. Magnetometers cost from about $1500 to $10,000 and can be used in pairs (a "differential magnetometer" to subtract out all but the wanted signals. Modern magnetometers are sensitive to field changes of about 1 gamma-the earth's weak magnetic field intensity is of the order of 50,000 gammas. Magnetometry has been successfully used to located imported stone at some well-known archaeological sites. Fired mud brick has a reasonably high magnetic anomaly and of course ferrous materials such as one might expect at an Iron-Age or later site, give rise to very large magnetic anomalies.
Gravity is one of the weakest of all forces found in nature. Yet, the earth's gravity field is very slightly altered by such features as subsurface voids or caves. Suitable gravity meters, known as "microgravimeters" cost of the order of $50,000 and require a very experienced trained operator. Point by point measurements must be made, which may be time consuming. The data must be carefully corrected for such things as surface topography and diurnally varying "earth tides." For these reasons gravity surveys have been little used in archaeology to date.
Conventional aerial (stereo-pair) photos of a site are very useful, as has been suggested, since outlines and features not visible from the ground frequently show up in aerial photos. Thermal infra-red (IR) imagery requires a scanner, usually cooled by liquid nitrogen, (instrument cost $15,000 to 50,000), but surface temperature differences of a small fraction of one degree can be measured. At night radiation cooling of the ground is not uniform if there are subsurface features that impede or enhance heat flow. In additional to diurnal heating and cooling, seasonal heat flow temperature changes can often be detected providing information on deeper archaeological anomalies. Heat flow through rock and soil is very slow---rock is an excellent heat insulator---so infra- red measurements give information about temperatures near the surface, not about temperatures deep within the earth. In spite of the limitations, false-color images showing temperature contours can thus provide interesting clues for the archaeologist at some sites, especially if such measurements can be made carefully at periodic intervals through an entire year. The Temple Mount in Jerusalem is an ideal site for on-going thermal infra-red imaging studies and Tuvia Sagiv, an architect from Tel Aviv, has already obtained some fascinating thermal IR images of the Temple Mount area. These can all be done from a distance or from the air.
If an archaeological site is complex and important, likely to be excavated
for many field seasons, geophysical methods can be most useful since they
are non-destructive and rapid. The archaeologist can hope to chose digging
priorities based on survey findings. Some sites (monuments or parks) contain
sites or buildings that can not be disturbed at all, so geophysical sensing
may provide the only means of studying the site. Advice from a geologist
who is familiar with an area can be helpful also. A combination of geophysical
methods can be helpful as each method has its strengths and limitations.
Archaeology is a time-honored exacting scientific discipline which provides
us with some of our best information on human history and the past. It is
to be hoped that more opportunities and sources of funding will develop
so that modern geophysical methods can assist the archaeologist even more
frequently than has been possible in recent years.
2/8/11
Lambert Dolphin
lambert@ldolphin.org
Library
I am now retired from Geophysical Work. Contact International Radar Consulnatnts, Inc. for expert geophysical help.