New Technology Uses Sound to Find Land Mines

Bijal P. Trivedi
March 6, 2002
The U.S. Army is testing a new technology for land mine detection that
is based on the use of sound waves. Officials say it could prove to be
significantly more effective and safer than the best tools now
available, metal detectors and titanium probes, which entail prodding
gingerly for the presence of mines.

de-mining—direct probing of risky areas, inch by inch—is a
very slow, difficult, dangerous, and laborious process. But it has been
the only reliable method," said Jim Prudhomme of the United Nations
Mine Action Service in New York.

Mine-ridden land must be stripped of all vegetation and searched with metal detectors. When a potentially dangerous object is found, it must be carefully unearthed. If it turns out to be a mine, it must be disarmed.

According to experts in the field, dogs have been particularly effective in locating land mines in Afghanistan, but their ability to detect mines is highly dependent on the soil conditions and they are susceptible to fatigue.

The extent of the land mine problem is daunting. There are an estimated 60 to 70 million land mines buried in about 70 countries throughout the world. Mines kill or maim more than 26,000 people every year, according to the United Nations Association of the USA.

"A metal detector works very well. But in the last few years the metal casing has been replaced with plastic and wood, which makes mines much more difficult to detect," said a physicist at the Night Vision and Electronic Sensors Directorate (NVESD) of the U.S. Army, based at Fort Belvoir, Virginia. The Army requested that the scientist not be named.

Greater Accuracy

The new acoustics-based technology, which is being developed by Stevens Institute of Technology in Hoboken, New Jersey, has advantages over other mine-detection advances now on the drawing board.

Much of the newer technology under development—including metal detectors, infrared imaging, ground-penetrating radar, and nuclear quadrupole resonance—has a fairly high rate of false positives, Prudhomme explained. That is, they regularly mistake common objects such as tree roots, rocks, shrapnel, bullets, and other debris as mines.

"That's the advantage of the acoustic system developed by the Stevens Institute of Technology," said the Army physicist. "It is sensitive only to mines and is not fooled by natural clutter, like rocks and tree roots, at least to the extent that we have tested."

In Army field tests last year, the new system successfully identified seven types of mines buried in different soil types and under different weather conditions. "The mines were detected successfully 100 percent of the time with no false alarms," said Stevens physicist Dimitri Donskoy, who designed the new technology.

"It looks extremely promising," the NVESD physicist agreed.

Donskoy's system works by using a loud speaker to simultaneously blast two deep frequencies over the minefield. The sound waves cause the mine to vibrate, and these vibrations disturb the surface soil directly above the mine. A laser sensor detects the minute surface vibrations, which are recorded by a computer and used to pinpoint the location of the mine.

Unique Vibrations

The "nonlinear seismo-acoustic" land mine detection system, as Donskoy calls it, can distinguish mines from other buried debris because of a "bouncing effect" that occurs at the interface of the flexible mine container and the soil. The activity distorts the two original frequencies and produces a new frequency that causes the vibrations at the surface of soil.

"Rocks and other solids do not distort the two frequencies because they do not vibrate as easily when hit with sound waves," Donskoy explained.

A test of more than 50 types of mines—plastic, wooden, and metal—revealed that each had a different vibration when excited with the two frequencies, and then responded by producing its own distinctive "acoustic signature."

"This is our best near-term bet," said the NVESD physicist, noting that if further field tests are successful, the technology could be ready for use in the next couple of years. The next phase of field tests is scheduled for mid-April.

Scientists led by James Sabatier at the University of Mississippi have developed a similar acoustic technology that uses one frequency at a time rather than two, said the NVESD scientist. While Sabatier's technology is faster and better at detecting larger anti-tank land mines, it had a much higher rate of false hits when detecting smaller anti-personnel mines, he added.

Mine detection is a very difficult task and a successful system will probably integrate several types of sensors, said the NVESD scientist. "Both the Stevens and Mississippi technology are still under investigation, but if they are successful they will probably be combined into a single system," he said. Army funding for these two projects over the last five years and the current year has been almost $9 million.

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