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. Last Updated: 07/27/2016

New Sonar Goes Under Sea's Floor




PANAMA CITY BEACH, Florida -- Finding lost objects on the ocean floor is becoming almost commonplace. Witness the discovery in recent years of the Titanic and the recovery of a Mercury space capsule and wreckage from plane crashes.


Now the Navy has developed a new kind of sonar to take the next step: finding objects buried beneath the sea bottom.


Known as synthetic aperture sonar, the experimental device also could be used to look under the ocean floor for unexploded ordinance as well as pipelines, shipwrecks or even treasure.


"It's in operation in this experimental mode now, and we've gone out and demonstrated it in a number of exercises," said Kerry Commander, head of acoustical sensing research at the Navy's Coastal Systems Station near Panama City Beach.


But there are some problems and further research is being conducted before the new sonar will become operational.


Sonar sends out sound signals and analyzes the echo that bounces back. Synthetic aperture sonar, or SAS, combines multiple signals while being towed through the water by a ship.


The result is a video-like image of the ocean floor. The same image can be accomplished by having a large sonar array about 15 meters long. Signal processing, however, shrinks the length to a more practical 1 meter.


"We don't understand precisely what happens to the image when it gets into the sediment," Commander said. "It distorts the signals coming back."


There are two competing theories about sound movement through sediment. The older and simpler theory is that porous sandy sediment is just a fluid that is heavier than water. The newer theory is that ripples and other irregularities in the sediment surface change sound penetration.


That may explain why the SAS has detected objects in certain situations when, according to the old theory, it should not have, Commander said.


However, buried objects often appear too fuzzy to determine what they are.