High-Resolution Acoustic System Detects Objects Buried In Soil

"There are thousands of potential building sites in the United States that must be carefully assessed before construction can begin," said William O'Brien Jr., a UI professor of electrical and computer engineering and the director of the Bioacoustics Research Laboratory at the university's Beckman Institute for Advanced Science and Technology. "What's needed is a quick and easy technique to identify those sites that contain important cultural or archaeological artifacts."

Current "dig and sift" methods are too time-consuming, expensive and imprecise, O'Brien said. Ground-penetrating radars don't work well in wet soils or for non-metallic objects, such as arrowheads, pottery shards or human remains. And technologies used in seismic exploration lack the resolution needed to identify small artifacts. The use of sound waves seemed like a good alternative.

To perform a feasibility study, O'Brien and his colleagues -- professors David Munson and Robert Darmody, and graduate students Catherine Frazier and Nail Cadalli -- used a single-element transmitter to send pulses of sound into the ground at a frequency of 6 kilohertz. For their receiver, the researchers used a 52-element acoustic array from the head of a torpedo. They also developed special image-reconstruction software to convert the sound waves reflected from buried objects into pictures.

"In principle, our technique 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," O'Brien said. "Because we use a much higher frequency, however, our resolution is much greater."

Currently, the device can penetrate about a foot underground and resolve objects that are 2 inches in diameter. Future improvements are aimed at increasing both penetration depth and resolution. One goal is to build a better transduction device that would send more sound into the ground.

"The torpedo head was designed to propagate sound into water, not into soil," O'Brien said. "We are developing a transduction device that is much better matched to the impedance of soil."

The researchers are also experimenting with a transmitter array that could provide focusing of the transmit beam. "With a focused source, we could transmit more energy into the region of interest," O'Brien said. "That would allow us to penetrate farther and obtain better image quality."

With additional modifications, the system also could be used to detect land mines, O'Brien said.

The researchers described the acoustical imaging system in the July issue of the Journal of the Acoustical Society of America. The work was supported by the U.S. Army Construction Engineering Research Laboratory.