[Editors] MIT: Detecting land mines with sound

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
http://web.mit.edu/newsoffice/www

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MIT: Detecting land mines with sound
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For Immediate Release
TUESDAY, JAN. 23, 2007
Contact: Elizabeth A. Thomson, MIT News Office
Phone: 617-258-5402
Email: thomson at mit.edu

PHOTO AVAILABLE

CAMBRIDGE, Mass.--Researchers at MIT's Lincoln Laboratory are 
developing a highly pinpointed sound beam that can detect buried land 
mines from a safe distance. The new beam will use sound to seek out 
land mines like a bat uses sonar to hunt its prey.

The researchers built a prototype detector and tested it at the Cold 
Regions Research and Engineering Laboratory Army Corps of Engineers 
land-mine facility in New Hampshire. They were able to detect both 
metal and plastic mines but said that the system will have to get a 
major boost in acoustic power before minefield searchers can use it 
safely.

Robert W. Haupt, a technical staff member at Lincoln Lab, explores 
innovative ways to find and reduce the large number of land mines 
abandoned in war-torn countries. An estimated 26,000 people are 
killed or maimed every year by 60 to 70 million undetected land mines 
in 70 countries. Those casualties include military troops but most 
are civilians--half of them children under age 16--who step on 
uncleared minefields after a war.

Many existing prototype mine detection systems can detect only metal, 
have limited range or are impractical in the field. "Reliable methods 
that quickly and accurately locate land mines made of metal and 
plastic, unexploded ordnance and other mine-like targets are 
desperately needed," Haupt said.

Haupt and fellow Lincoln Lab staff member Ken Rolt developed a 
high-powered sound transmitter that looks like a stop sign studded 
with 35mm film canisters or prescription pill containers. This is 
called a parametric acoustic array, and Haupt and Rolt have built one 
of the most powerful ones around.

The array is made up of ceramic transducers--devices that emit a 
powerful narrow acoustic beam at ultrasonic frequencies. One meter 
away, the ultrasonic pressure level measures 155 decibels--more 
acoustic power than a jet engine. Immediately outside the beam, the 
acoustic intensity dies away to almost nothing.

By a process know as self-demodulation, the air in front of the 
acoustic beam converts the ultrasound to much lower frequency audible 
tones that sound like extremely loud tuning forks. Unlike ultrasound, 
the low-frequency sound can penetrate the ground, causing detectable 
vibrations in the mine's plungers and membranes.

"The use of ultrasound allows us to make a very narrow and highly 
directional beam, like a sound flashlight," Haupt said. It would take 
a huge number of conventional loudspeakers to do the same trick, and 
they would weigh too much, take up too much space and use too much 
power to be practical, he said. Plus, they would deafen anyone within 
earshot. "Using a narrow sound beam, we can put sound just where we 
want it, and we can minimize sound levels outside the beam to avoid 
harming the operators or people nearby," he said.

Once the sound beam "hits" buried ordnance, the vibrations from the 
mine, resonating from the sound waves, push up on the ground and can 
be measured remotely with a laser system called a Doppler vibrometer. 
The sound signature of a mine looks like a mountain range of spikes 
compared with the flat-line response of the rocks and dirt around it.

"It turns out that mines will vibrate quite differently from anything 
else," Haupt said. "You can determine what types of mines there 
are--and which countries made them--by their unique signatures."

Haupt also is working with Oral Buyukozturk, professor of civil 
engineering at MIT, to tailor the system to detect damage in cement 
bridge piers from as far away as the shore.

A paper on the land-mines work by Haupt and Rolt appeared in a 2005 
issue of the MIT Lincoln Laboratory Journal. A second paper by Haupt 
was  published in  2004 by the International Society for Optical 
Engineering.

This work is supported by Lincoln Laboratory's Line Funding and the 
Defense Advanced Research Projects Agency.