Spotting Mines With Dolphin Sonar

NORFOLK, VIRGINIA--Dolphins can easily locate a meal of razor fish or eels hiding beneath the ocean floor by emitting chatterlike sonar clicks and listening to the echoes. Now researchers have created an artificial sonar that mimics dolphins' technique and may soon be used to sound out less savory quarry: underwater explosive mines buried by silt or sand. A new dolphinlike sonar system, presented at a meeting of the Acoustical Society of America here this week, can peer through sand and discern objects buried tens of centimeters below.

"Buried mines are one thing the [Navy] is really worried about," says Whitlow Au, a dolphin expert at the Hawaii Institute of Marine Biology in Kailua. Some mines are made of fiberglass and programmed to detonate only when they detect a boat overhead. That makes them hard to sweep or find with a metal detector. "During the Gulf War, several big Navy boats were severely damaged by mines," Au says. Most long-range Navy sonar uses long wavelength pulses that can't tell a buried mine from an old shell casing, Au says. The Navy has an active program of bottlenose dolphins trained to find both moored and buried mines.

Hoping to emulate the dolphins, Au and colleagues built a sonar system that made brief clicks composed of a broad range of short wavelength sounds. Such short waves, they reasoned, should be able to precisely probe the shape of a buried object by caroming off its contours and crevices; longer wavelengths would give a blurrier picture. As a test, the team buried a steel sphere and a cast-iron pot in elbow-deep silt, along with "red herrings" such as a glass jar and a hunk of coral. Mounted on a wheeled cart, the sonar prowled the ocean floor while a specially programmed computer deciphered the echoes. The system correctly identified the mine-shaped sphere 90% of the time.

"I think this sort of thing is the way to go," says Robert Gragg, a physicist with the Naval Research Laboratory in Washington, D.C. But searching a minefield with portable sonar would take ages, cautions John Osler, a seismologist with the Saclant Undersea Research Center in La Spezia, Italy. Ship-mounted devices might be able to cover ground more quickly by sending sound waves glancing off the ocean floor, he says. Such waves, which are shorter wavelength than standard sonar, partially penetrate the surface, but so far no one has figured out how to capture these tiny echoes or put them to use.