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Gee-whiz geophysics: Archeologist's best friend?

POSTED 5 JUNE 2008
Grand entrance flanked by columns and ornate carvings set into red, dusty cliff faceGot a Jones for Indiana Jones?
No, we haven't seen the latest film in the franchise, either, but anything that puts archeology in the papers suits us just fine. Still, the picture of the suave, swashbuckling archeologist does not jibe with the reality of archeology, where it’s usually more critical to see through dirt than to fight off marauding locals.

You could say Harrison Ford’s all-purpose bull whip has been replaced, at least in the real world, by a bevy of sensitive instruments that can almost literally “see through the ground.”

The giant archeological site at Petra, Jordan, was home to the Nabateans and then the Romans. Although ground-penetrating radar has found buildings beneath a flat, featureless surface, no fancy gadgets were needed to find this building, which was carved from solid stone. Courtesy Larry Conyers

Sensors on satellites and airplanes can locate unknown sites that are hidden under jungle. But geophysical techniques, which apply physics to the study of the Earth, can detect and analyze archeological remains from the surface.

Although some of these geophysical instruments have been in use for decades, they are getting cheaper and more sensitive. Software advances have made the systems easier to use and archeologists have even borrowed medical-imagery techniques to produce 3-D images: An MRI of an archeological site; no shovels required.

Two men chisel and brush away debris around a square building base set in gray ash
In the summer of 2004, excavations at Tambora, a civilization that was obliterated by an 1815 eruption, recovered bronze bowls, ceramic pots, iron tools, and the remains of two adults. Ground-penetrating radar was one of the high-tech tools used at this dig. Photo: Haraldur Sigurdsson, University of Rhode Island

Geophysics: Getting technical
One early geophysical technique, electrical resistivity, measured electrical resistance between pairs of buried electrodes. Because water generally reduces resistance, materials that absorb different amounts of water can stand out in the results.

A magnetic susceptibility instrument measures how easily substances become magnetic, while a magnetometer measures how buried objects alter Earth's magnetic field. Because fires change the magnetic orientation of atoms, a magnetometer can detect firepits and other traces of the past, including the streets of an ancient Roman city.

Gray square, raised circle-like enclosure in upper right-hand corner of square
An exploration at the Pawnee Indian Museum in Kansas used magnetic sensors to find earth lodges and fortifications at a Pawnee village dating to about 1800. Courtesy Archeophysics. Click to see how different geophysical measurements were interpreted here.

Geophysical explorations first gained currency in Europe and the Middle East, where the early instruments were better suited to detecting large buildings. As the technology has gained sensitivity, geophysics has been applied to a much tougher job in North America: finding the hearths and post-holes that are the only legacy of many ancient peoples. In virtually all cases, computers process the data to remove noise and diagram the good stuff.

The menu of archeological geophysics has now mushroomed to include ground-penetrating radar and an underground version of sonar, which detects reflections of sound waves. As geophysical archeology advances, the urge to combine instruments grows irresistible.

Woman in blue jeans grabs metal rod sticking up from grass and twists with all of her might
Geophysical techniques can remotely measure electrical magnetic properties under ground. This hand corer is drilling a 1-inch hole for a magnetic sensor at an archeological site. Human activities like fire can change the magnetism enough to be detected from the surface. Working inside a small hole allows readings from deeper levels of the site. Courtesy Rinita A. Dalan, Minnesota State University Moorhead.

The obvious benefit of geophysical sensors is the opportunity to leave the shovels in the shed. But geophysics can guide future excavations, interpret adjacent areas that cannot be excavated, avoid futile excavations that find no features, and reduce damage to precious archeological sites. What's not to love?

Ground-penetrating radar: It's not about catching speeding moles…

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Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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