Portland, Ore. A breakthrough in sonar technology may soon make it possible to more accurately detect and map underwater artifacts, such as mines, even when they are buried.
"Our results are from simulations, but they show that some relatively simple changes to standard sonar equipment will result in a much simpler and effective means of detecting underwater mines," said David Pierson, a Johns Hopkins University scientist who invented the
technique for his dissertation under professor David Aspnes at North Carolina State University.
The scheme works by rendering the ocean floor transparent, thereby making an artifact stand out in stark relief even if it is buried. The key is time-reversing the echo from a first sonar ping and broadcasting it in lieu of a second sonar ping. The background of the subsequent echo from that second broadcast is canceled out, Pierson said, effectively rendering the ocean floor transparent. As a result, after several such cycles, even the faintest echoes from buried mines, once masked by obstacles, stand out on a grid of ocean floor.
"Each time we transmit a time-reversed echo, we find the seafloor gets suppressed, but the echo from buried objects is enhanced even when the signal from the buried object was too small to be detected at first," said Pierson. "Today, navies use everything from dolphins to divers to software modeling with elaborate sonar arrays. Now we think we can do better."
Since 1776, when mines were invented, navies have sought ways to detect them. The most sophisticated sonar setups today use multiple transmitters and receivers to map out items on the ocean floor. Pierson's method is simpler, requiring only a single standard sonar rig, rather than an elaborate setup of multiple transmitters and receivers. It even detects buried mines that may be missed by multiple-transceiver rigs.
"We don't need arrays of sonar transmitters and receivers or any modeling software. Instead, we just record the return echo of a sonar transceiver, time-reverse it and transmit it back out," said Pierson.
Pierson's approach is not the first use of the reversed-echo technique just its first use for mapping. Both optical and acoustical experiments have been performed using reversed broadcasts, according to Pierson, but his is the first to use isolating targets in back-scattered signals.
Pierson's work at North Carolina State was supported by a grant from the Office of Naval Research. At his current job at Johns Hopkins, his work is supported by the U.S. Navy.