Digging in the dirt: chemical and biological sensors could aid the search for hidden land mines

Science News, March 28, 1998 by Corinna Wu

Many people regard antipersonnel land mines as the worst form of pollution on the planet. An estimated 100 million of these small explosive packages, designed specifically to maim or kill when stepped on, lie buried around the world, remnants of past military maneuvers or terror campaigns. These devices have turned fields, forests, and villages into treacherous, unusable terrain.

Nations with the severest land mine problems include Afghanistan, Angola, Bosnia, and Cambodia, where years of war have littered the landscape with the deadly devices. In many parts of these countries, the activities of everyday life threaten civilians with possible injury or death. In Cambodia, I of every 250 people has lost a limb or limbs because of an encounter with a mine. These deaths and permanent injuries among the population exact a crushing economic toll from nations that can ill afford it.

With so many mines already in the ground, the problem of removing them seems almost insurmountable. Even if no more mines were laid, it would take 1,000 years to clear them all at current removal rates, says Ron Woodfin of Sandia National Laboratories in Albuquerque, N.M. A dramatic increase in the number of workers could speed up the demining process, but many people are hoping that new technology can quicken the pace.

Some of the more promising approaches now in development employ chemical and biological sensors to detect traces of explosives emitted by mines. Such sensors could enable a demining team to survey a large area and identify hot spots quickly and safely.

Perhaps more important, sensors can indicate where there aren't any mines. The ability to see that an area is safe would allow demining teams to focus their energies on the trouble zones, says Albert M. Bottoms, president of the Mine Warfare Association (MINWARA), an educational organization in Monterey, Calif. However, "even that is beyond our current technology," he says. Bosnia alone has an estimated 18,000 minefields. "To our utter dismay, we don't know how to do a survey of even one of them, let alone 18,000," he remarks.

The various designs of antipersonnel mines form a gruesome gallery of weapons. According to the humanitarian organization CARE in Atlanta, there are more than 600 types. Explosive blast-effect mines tear off a person's lower leg and drive dirt and bone fragments into the wound. Fragmentation mines are stuffed with ball bearings and metal scraps to pepper their victims with shrapnel. Mines nicknamed "bouncing Betties" pop out of the ground and explode at waist height, shooting out fragments on all sides.

The military can often use brute force to rid an area of antipersonnel mines. "They're not trying to remove the mines as much as defeat them," says Dick Davis, director for defense programs at Oak Ridge (Tenn.) National Laboratory. A heavily shielded, remote-controlled vehicle can quickly forge a safe, though limited, path for troops by turning up and detonating mines as it rolls along.

Demining an entire area for humanitarian purposes--so that children can play and families can move about free of fear--is much trickier. Workers, usually trained civilians, slide a rod at an angle into the soil, probing gently for any suspicious objects, which include both unexploded artillery and mines. This process, required to clear an area as thoroughly as possible, is dangerous and frustratingly slow.

Bottoms sees promise in new technologies that might make mine clearance safer and faster. He recalls a videotape he once viewed of "a line of peasants, walking across a field, poking the ground with sticks. Technology has to offer us something better than that."

Finding effective detection methods is particularly difficult because most of the mines are made of plastic instead of metal, rendering standard metal detectors useless for humanitarian demining

Other types of imaging systems, such as infrared detectors or ground-penetrating radar, can alert deminers to objects buried in the ground, but they don't distinguish among mines, unexploded shells, or innocuous metal fragments. "In Afghanistan, they find 115 [harmless objects] for every one that has explosives in it," says Woodfin. Nevertheless, each hit has to be carefully probed, wasting valuable time. The large number of false positives can also cause a deminer's attention to flag, with potentially disastrous results. "False positives are bad, but false negatives are even worse," says Bottoms.

In order to make detectors that don't cry wolf so often, several groups of researchers are focusing on the essential difference between benign objects and deadly mines: the presence of explosives. Since most mines leak a little bit of explosive into their surroundings, chemical and biological sensors can sniff them out (SN: 4/6/96, p. 223). Deminers can then ignore other buried objects.

These new sensing techniques are variations on methods developed to detect chemicals and hazardous waste in the environment.

Most of the "cheap and dirty" mines contain the explosive trinitrotoluene, or TNT, says Woodfin. Some use plastic explosives such as RDX, a favorite of terrorists.