The Sound of Deadly Danger Researchers combine acoustic waves with radar to detect buried land mines. By John Toon

By simultaneously using sound waves to create tiny soil disturbances and precision radar to measure the resulting movement, researchers at the Georgia Institute of Technology have developed a new method for detecting land mines buried in the soil.

photo by Gary Meek Dr. Waymond Scott shows examples of the deactivated low-metal mines that can be detected with the experimental system shown behind him. The mines range in size from tiny anti-personnel weapons to anti-tank mines nearly a foot in diameter.

The technique could be combined with other advanced location methods to create a new generation of land mine detector able to find different types of buried weapons across a broad range of soil and environmental conditions. The work was sponsored by the U.S. Army Research Office and the Office of Naval Research, and was described at the 139th meeting of the Acoustical Society of America in Atlanta in June.

With more than 100 million land mines buried throughout the world causing some 26,000 injuries and deaths each year, the stakes are high. But existing mine detectors do not work under all conditions, and have particular difficulty finding small anti-personnel mines made mostly of plastic.

"Detecting land mines is a very difficult thing to do," says Dr. Waymond Scott, an associate professor in Georgia Tech's School of Electrical and Computer Engineering. "Every existing method for mine detection has conditions under which it will work very well and conditions under which it will fail."

Scott and collaborators Peter Rogers, Gregg Larson, James Martin and George McCall – all of Georgia Tech's School of Mechanical Engineering – use a transducer to create seismic waves that travel through the soil containing land mines. This special class of elastic waves causes the soil and everything buried in it to be displaced slightly.

That tiny movement in the surface of the soil – less than one micrometer (one ten-millionth of an inch) – can be detected by electromagnetic waves from a small radar system that scans just above the surface of the soil.

"The properties of the mine are very different from the properties of the soil around it," Scott explains. "That causes the displacement around the mine to be different from the soil, because of a very strong interaction of the waves with the mine."

The technique differentiates mines from other buried objects, such as rocks or sticks, because of the different mechanical properties of the mines. The interaction and unique resonance created by the waves interacting with the mines' hollow shell and their complex trigger and explosive mechanisms make them stand out from other solid objects.

This ability to differentiate mines from other buried objects reduces the risk of false alarms, and could give the new technique an advantage over other detection methods.

"There are so many things in the ground that can look like mines," Scott says. "The problem is often not finding the mine, but differentiating it from the clutter around it."

photo by Gary Meek Using a pit containing 50 tons of damp sand, Georgia Tech researchers have demonstrated they can detect seven different types of buried land mines. Dr. Gregg Larson, left, Dr. Waymond Scott, right, and graduate student Christoph Schroeder, background, are detecting the deactivated weapons by simultaneously using sound waves to create tiny soil disturbances and precision radar to measure the resulting movement.

The technique has been shown to sense soil movement through light vegetation and ground cover such as pine straw, and may be able to sense soil movement through denser ground cover.

Using a pit containing 50 tons of damp sand, the researchers have demonstrated they can detect seven different types of buried mines. The deactivated weapons range from small anti-personnel mines just a few inches in diameter planted near the surface to much larger anti-tank mines buried more deeply.

These experimental results closely match computer modeling done to help the researchers understand the complex interaction between the elastic waves in the soil and buried objects.

Before the technique can be practical, however, researchers have many hurdles to overcome. First, they must study the wave interaction in many different soil types and environmental conditions. Now, for instance, the elastic waves have only been studied in damp compacted sand. Different soil types and conditions may require different frequencies and adjustments to detection methods.

The detection process must also be made much faster. To facilitate that, the researchers have begun developing a beam-forming array that would eliminate the need for the radar to make several scans above the soil surface. They are also considering an ultrasonic technique for detecting soil displacement.

The waves now must be propagated by a transducer source placed in contact with the soil. The researchers are investigating non-contact wave sources, such as an electric arc, loudspeaker, microwave, laser and water jet.

The system will also have to be made portable and robust enough to work reliably under rough field conditions.

Ultimately, Scott expects the acoustic-electromagnetic detection method to be combined with other technologies, such as detectors that sniff the chemicals given off by explosives in the mines, existing metal detectors and ground-penetrating radars. He believes only a combination of methods will offer reliable results over a wide range of devices and conditions.

The combined acoustic-electromagnetic detection technique had been proposed as far back as the 1960s, but only recently has technology become available to do it.

"The complicated structure of the soil makes it difficult to detect buried mines," Scott explains. "You could put this mine 1,000 miles out in space and be able to detect it more easily than if you put it one centimeter under the soil. That's because space is a more uniform medium."

The researchers hope to field test a prototype system within several years, but say it will be longer before the technique can be used to locate and remove the live mines buried worldwide.

Also participating in the research are Christoph Schroeder, Ali Behboodian, Kangwook Kim, Seungho Lee, Andrew Overway and Cheng Jia, in the School of Electrical and Computer Engineering, and Blace Albert, Fabien Codron and Andrew Slack from the School of Mechanical Engineering.

For more information, contact Dr. Waymond Scott, School of Electrical and Computer Engineering, Georgia Tech, Atlanta, GA 30332-0250. (Telephone: 404-894-3048) (E-mail: waymond.scott@ece.gatech.edu);   or Dr. Gregg Larson, School of Mechanical Engineering, Georgia Tech, Atlanta, GA 30332-0405 (Telephone: 404-894-6026) (E-mail: gregg.larson@me.gatech.edu).

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Last updated: Sept. 10, 2000