Shock Waves Proposed As Inexpensive Method to Detect Plastic Land Mines

BUFFALO, N.Y. -- Plastic land mines are a guerrilla fighter's dream: they cost as little as a dollar, they're easy to make, they're often lethal and they cannot be detected by current technology.

However, an accurate and inexpensive detection method effective for land mines in either plastic or metal casings may be on the horizon as the result of computer simulations conducted by Surajit Sen, Ph.D., assistant professor of physics at the University at Buffalo.

The research, to be published in the February 1998 issue of Physical Review E, indicates that weak shock waves sent into granular beds, like soil, will cause acoustic signals containing critical information to be reflected off buried objects, such as land mines.

The findings build on previously published work (in Physical Review Letters, vol. 74, pp. 2686-2689, 1995 and in Physical Review E , vol. 54, pp. 6857-6865, 1996) in which Sen found that in model systems, shock waves travel through granular media as solitons, tight bundles of energy that travel without dispersing.

Only after the publication in 1995 of the initial findings and after watching a television news report about land mines did Sen begin to think his research might be applicable to land mine detection.

The earlier work showed that solitons, which are encountered in only a handful of known physical systems, are very stable; they travel compactly, interacting very weakly with the systems through which they travel.

Sen said the weak shock waves used in the more recent UB studies are soliton-like signals.

"These signals are softer bundles of energy, which allowed us to strike a middle ground, generating signals that, like solitons, travel well in granular systems, but which also have some interaction with these systems.

"For typical sound waves traveling through any granular system, you would see some reflection of sound waves from a buried object, but it would not reveal many details about the size and shape of that object," said Sen.

"What our computer simulations have demonstrated is that when a weak shock wave penetrates into granular media and hits an object, the reflected pulse carries a lot of information about what it hit."

That information includes the size and shape of the object, critical features in determining whether or not a land mine has been detected.

According to Sen, the weak shock waves are detecting differences in density. For example, sand grains have typical densities of 2700 kilograms per meter cubed, while a piece of plastic has a density of only 1100 kilograms per meter cubed.

"So a plastic object buried in a solid would be detected as a light object lying in a heavier medium," he said.

In their computer simulations, the researchers showed the reflected soliton-like pulses split in a particular way, depending on whether they collided with a light or heavy object.

"Our models tracked the motion of individual grains as the pulse traveled along," he said. "After it hit a buried light or heavy object, relative to the soil, the pulse split up and the reflected pulse had distinctly different properties, depending on whether it was reflected by a light or heavy object. We want to take that knowledge and see if it can be used to detect land mines."

The system envisioned by Sen and his collaborators would consist of a special device that would send weak acoustic shock waves deep into soil and detect the pulses that are returned after hitting an object. The pulses would provide information about the weight and shape of the object, which would reveal whether or not it likely is a land mine.

Sen added that a key selling point for the proposed technology would be its low cost, a critical factor in developing detectors for countries where plastic land mines are common, such as Cambodia, Afghanistan and Bosnia.

He cautioned that so far, the distinguishing characteristics of the reflected pulse only have been seen in simulated systems. The next step will be to try to observe the phenomena experimentally with high reliability by sending weak shock waves through a box filled with sand or soil.

Sen's co-authors on the project are Michael J. Naughton, Ph.D., UB associate professor of physics and chemistry; Marian Manciu, a doctoral candidate in the UB Department of Physics, and James D. Wright, a master's candidate in the Department of Physics at the University of Kent in Canterbury.

The work is being funded by the U.S. Army Corps of Engineers.