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Remote Excavation
by Lexi Krock


Ancient Refuge homepage

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.


Aerial photo
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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.


   

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.


   

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.


   

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.


Imaging radar
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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.


Infrared aerial
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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.


   

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.


   

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.


Resistivity
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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.


Thermographic
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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.



This overview originally appeared, in a slightly different form, on NOVA's Lost Roman Treasure Web site.

Remote-sensing technologies

Aerial photography

Geographic Information Systems (GIS)

Geophysical Diffraction Tomography (GDT)

Ground Penetrating Radar (GPR)

Imaging radar

Infrared aerial photography

Magnetometer

Seismic vertical profiling survey

Soil resistivity mapping

Thermographic Infrared Multispectral Scanner (TIMS)




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Lexi Krock is associate editor of NOVA online.



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