For decades, fishermen have used underwater listening devices to find schools of fish, and sonar operators have bounced sound waves off of enemy submarines to reveal their presence. Both techniques work because sound travels easily through water. Now researchers may have found a way to take undersea listening to the next level, using the ambient noise in the ocean. The technique, called acoustic illumination, is similar to computed tomography, in which medical images are derived from the way bone and tissue distort x-rays passing through the body. Today in Geophysical Research Letters, researchers describe how they installed lines of underwater microphones at two different locations in the Pacific Ocean. Then the scientists identified and isolated ambient underwater sounds and tracked them back to their sources by computing the differences in time it took for the sounds to reach the two arrays. Using the method, scientists could track underwater temperature changes for wide swaths of ocean—because sound's speed depends on water temperature--or they could record the changing migration patterns of fish, whales, and other marine life.
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