Zapping the Ground Brings New Discoveries at Old Sites

TO a Salvadoran farmer who might have been watching, it must have seemed the oxcart to nowhere, lumbering back and forth across a field near the village of Joya de Ceren. But to the scientists guiding it, the cart's oil-drum-sized radar unit represented an experiment that could help revolutionize the way archaeologists uncover the past.

The researchers hoped during that visit to El Salvador in 1979 that the technology, known as ground-penetrating radar, would reveal the extent of a Mayan hamlet buried beneath as much as 17 feet of volcanic ash. Sixteen years later, the now-suitcase-sized radar with vastly improved software has allowed scientists to see for the first time the true extent of the settlement -- without turning a spade.

''We first thought that the occupants of the buried site were a couple of poor farming families out in the middle of a rain forest,'' says Larry Conyers, a research associate at Ceren, which was first discovered in 1976. But surveys in 1990 and 1994 using a new generation of ground-penetrating radar uncovered a vastly larger village, complete with a plaza and a religious complex. ''There had to be 40 to 50 families to generate enough labor to have built all this stuff,'' Dr. Conyers says enthusiastically. ''We've found 30 buildings so far, and there may be 100 or more out there.''

The findings were reported over the weekend during a conference on ground-penetrating radar and archaeology at the University of Colorado at Boulder. That such a conference was held at all suggests the technology's growing importance in discovering and preserving important cultural sites such as Ceren, which has been called the Pompeii of the Western Hemisphere.

The radar not only can help pinpoint the most interesting structures to excavate, saving time and increasingly scarce research money, but it also can tell researchers where not to build the visitors' center. Most important, say archaeologists, it is non-destructive, allowing them to determine which features to leave untouched.

First developed by the Pentagon during the Vietnam War to detect underground tunnels used by the Viet Cong, the radar sends pulsed signals into the ground and then records the intensity of the echo and the time it takes the echo to return. The echoes are generated when the signal hits boundaries between materials of different densities and moisture content. As the echoes return, a computer records and analyzes them, generating two-dimensional images that represent vertical slices of the ground.

The technology has been used extensively in detecting and tracking underground plumes of pollutants, says Farouk El-Baz, director of the Center for Remote Sense at Boston University. It has also been used to detect everything from unexploded World War II land mines to defects in airport runways and concrete structures.

Archaeologists began experimenting with the radar in the 1970s, Conyers says, and it has helped in notable discoveries. Dr. El-Baz used the technology at Egypt's Great Pyramid in 1989 to locate the underground chamber containing the disassembled boat of the pharaoh who built the pyramid. And the technique is gaining use among archaeologists who conduct preconstruction site surveys. Several colonial-era sites in Boston were discovered in surveys for a major highway reconstruction project.

But the effort at Ceren, where the Mayan village was buried by a volcanic eruption in AD 590, represents one of the first applications using the technology for wide-scale surveys.

Once the data is collected and printouts are generated, analysis can take months. But the results can spot items as small as a shoebox. ''We can see buildings, platforms, columns, even some of the furrows in the cultivated cornfields, in extraordinary detail,'' Conyers says.

But human eyes roaming the printouts can still miss interesting details. Dean Goodman, a friend and collaborator of Conyers who works for the Japanese government conducting preconstruction archaeological surveys, has developed software that can ''massage'' the data to reveal patterns that the eye might miss.

The next step in using the technology, Conyers says, would be to combine ground-penetrating radar with information from global-positioning satellites, which could give the radar's location to within centimeters. Up to 75 percent of the time spent on using the radar goes to surveying the site and setting up a grid to locate the radar above ground. That time could be cut dramatically by using the satellites to identify position. Moreover, the radar itself is getting smaller still.

During the weekend, Conyers and Mr. Goodman took to the Colorado hills to test a backpack-sized unit that combines the radar with a small computer.

''After a half-mile hike, we set up a ... grid in an old coal-mining camp south of Boulder and found the cellars of six houses. All the data was acquired and processed in the field,'' Conyers says. The only thing missing for on-site analysis was a printer.