Seeing through Soil with Sound

In contrast to the swashbuckling adventures of Indiana Jones, archaeologists actually spend the vast majority of their fieldwork time doing something rather mundane: moving dirt. Although their traditional dig-and-sift method works well, it is labor-intensive and time-consuming--and therefore rather costly. As a result, researchers have increasingly enlisted the aid of imaging techniques such as ground-penetrating radar (GPR) and seismic exploration technologies to help narrow their search. Now another tool is in the works. According to a recent paper in the Journal of the Acoustical Society of America, University of Illinois researchers have developed a high-resolution imaging system based on sound waves that can detect small, buried objects. Additionally, they report, this acoustic technique could be adapted to search for land mines.

The new method "is similar to those used in seismic exploration, where an explosive charge is detonated and the reflected sound waves are picked up by an array of receivers," explains team member William O'Brien. "Because we use a much higher frequency, however, our resolution is much greater." The new method also outperforms GPR when the ground is wet. Currently the acoustic device can "see" about a foot underground, and resolve objects that are at least five centimeters in diameter, such as the buried, air-filled pipe in the image at the right. As such it probably won't be able to differentiate small artifacts like arrowheads from similarly sized rocks. (Land mines, on the other hand, may be easier to spot, owing in part to their larger size.) But the team is trying to hone the method. One way to achieve that, O'Brien notes, might be to incorporate a transmitter array that could focus the transmit beam. "With a focused source we could transmit more energy into the region of interest. That would allow us to penetrate farther and obtain better image quality."