Unlike current mine-detection techniques, the patented methodology finds objects buried in the ocean floor without the use of complex, unreliable modeling and without the usual arrays of sonar transmitters and receivers. Instead, the method records the return echo of a sonar transceiver's "ping," then time-reverses and transmits that signal. The following echo clearly shows buried objects, and suppresses the response from the seafloor itself, making the underwater terrain "transparent."

Dr. David M. Pierson, then a doctoral student in physics at NC State, demonstrated the new approach in research he conducted with Dr. David E. Aspnes, Distinguished University Professor of Physics, in late 2003. The project was supported by a grant from the Office of Naval Research. Pierson has since joined the Applied Physics Laboratory of Johns Hopkins University in Baltimore, where his work is supported in part by the U.S. Navy.

"The method has not been explored as a solution to this problem until now," said Pierson. "Using time reversal on the return echoes back scattered by buried mines gave us results we considered amazing."

According to Aspnes, the young physicist's research is a breakthrough. "Time reversal is a technique that has been used before in various contexts, including optics and acoustics, but before Pierson's work the advantages of time reversal for isolating targets in backscattered signals was never before recognized."

Using time reversal to find buried mines requires only one transceiver, said Pierson, although more can be used, and the method isn't limited by the composition of the ocean floor. "Previous methods had to incorporate a lot of complex modeling of the seafloor and the ocean environment," Pierson said, "and required sophisticated software and hardware systems. My time-reversal technique not only simplifies the needed equipment, but also can be implemented using existing sonar equipment, with minor software changes. More elaborate analyses of echoes are also made possible."

What Pierson has done, said Aspnes, is to demonstrate a new approach that uses sonar but is simpler and works better than any previous method. "In Pierson's approach," he said, "a 'ping' is first transmitted from a sonar transceiver. The return echo is then recorded, time-reversed, and transmitted. He discovered that in the next echo the response from the seafloor was suppressed, but the echo from buried objects was enhanced. This enhancement is seen even if the signal from the buried object is too small to be detected in the first return."

The NC State discovery should please naval mine-detection experts, who now use everything from dolphins to divers to sophisticated software modeling and elaborate sonar arrays in their grim work. And it should send those who design such mines back to their equally grim drawing boards.

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