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In recent years, remote-sensing technologies have become as commonplace in archeological
fieldwork as khakis, spades, and brushes. Such tools for virtual excavation generate rapid
results and are non-destructive, highly accurate, and usually cost-effective. Here are ten
of the modern archeologist's most trusted remote-sensing tools, many of which routinely
prove invaluable in excavations such as that chronicled by NOVA at
Israel's Cave of Letters.
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Aerial Photography
The simplest of the remote-sensing techniques that
archeologists use, aerial photography allows experts to discern aspects of a
site that may be invisible from the ground, such as floral patterns, the layout
of large monuments, and traces of old walls and roads. The technique involves
taking photographs with conventional camera and film from an airplane, tethered
blimp, helicopter, hot-air balloon, or other airborne vehicle.
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Geographic Information Systems (GIS)
GIS has been described as a kind
of layer cake, the ingredients of which, for archeologists, include the
plethora of field data they typically collect in and around excavation sites.
While in the field, archeologists use GIS on their laptop computers to fashion
and manage detailed site maps, and they can combine the results of
remote-sensing tests with spatial maps of the region created with the aid of
Global Positioning System units. Resulting maps collate the most
archeologically promising areas and display these sites
three-dimensionally.
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Geophysical Diffraction Tomography (GDT)
A type of sonar used for
detecting subterranean objects, a GDT device shoots small shells into the soil.
The shells generate sound waves that bounce off underground features.
Archeologists drill small boreholes into the ground to eavesdrop on these sound
waves and measure their tonal shifts. As the sound waves ricochet off
objects, they can reveal the depth and shape of the features, which
archeologists can then map three-dimensionally on computers.
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Ground Penetrating Radar (GPR)
Ranging in size from small handheld
models that one places against the ground to larger ones that one drags across
a site, GPR devices use low-power radio waves to detect changes in density
underground. Unlike traditional radar, which broadcasts into the air and
uses a parabolic dish to focus the returned waves, GPR uses a small but
sensitive receiver placed directly against the ground. Depending on
their needs, archeologists can adjust radio frequencies upward for shallow
sites or downward for deeper areas, though GPR devices produce the greatest
definition when reading depths of three feet or less.
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Imaging radar
Using radar across a broad spectrum of frequencies,
imaging radar can see through the ground to depths of up to ten feet,
penetrating sand, dirt, and even heavy vegetation; a buried section of China's
Great Wall was discovered this way. Space shuttles or satellites outfitted with
this equipment can generate imaging radar maps by day or night and even in poor
weather conditions.
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Infrared aerial photography
Buried structures can disturb vegetation
above them by blocking plants' growth or their access to groundwater. While the
archeologist's naked eye cannot perceive these subtle abnormalities, infrared
film can. By recording the heat signature that plants give off, and by
detecting places where that signature has been interfered with, infrared
photographs can hint at promising areas for excavation. Experts take such
photographs from the air with a conventional camera using infrared film.
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Magnetometer
The handheld magnetometer, also referred to as a
gradiometer, proton magnetometer, or simply "mag," is loosely related to metal
detectors used to sweep beaches in search of lost coins and jewelry. As one
moves it over the ground, the mag generates a small electronic signal
that measures the intensity of the magnetic field below the surface. Where
there is a break in the bedrock—at the entrance of a rock-cut tomb, for
example—the magnetometer records a dip in the magnetic field. Archeologists
often use mags in conjunction with Global Positioning System receivers (which
use satellites to compute precise positions) to create detailed maps of the
subsurface.
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Seismic vertical profiling survey
A seismic vertical profiling survey
involves setting off explosive charges that send seismic waves reverberating
through the ground. Archeologists measure and analyze the acoustic waves
reflected from rock layers beneath the surface. These signals produce a
cross-section showing potential cavities where, for example, buried tombs might
lie.
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Soil resistivity mapping
A soil resistivity meter evaluates how well
the soil conducts electricity by measuring its moisture content. Heavily
compacted soil, such as a buried road or the floor of a building, holds less
moisture and is less conductive, while ground that has been tampered with, such
as trenches or ditches, have high moisture content and readily conduct
electricity. In either case, archeologists use soil resistivity mapping to
pinpoint disturbed areas beneath the surface.
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Thermographic Infrared Multispectral Scanner (TIMS)
Originally
designed for geological research, TIMS picks up visible, infrared,
microwave, and thermal data in a single shot and allows archeologists to
examine potential sites in spectral bands best suited to their particular
needs. TIMS units mounted onto satellites and aircraft and can detect and
image patterns of disturbed soil at high resolution up to 30 feet below ground.
This remote-sensing tool is particularly useful for showing buried geologic
features, such as ancient river beds, along which people may have settled.
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This overview originally appeared, in a slightly different form, on NOVA's Lost Roman Treasure Web site.
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