SAN DIEGO — The Navy’s submarine-hunting sonars have been accused of harming marine mammals. It now appears that in the brains of one of those mammals — the bottlenose dolphin — could reside the secret to even more powerful underwater sensors.
By studying how the marine mammals interpret the signals they emit and receive in the water, researchers believe they can eventually develop a short range, high-resolution sonar to detect man-made objects in the noisy coastal waters and on the littered sea floor.
Dolphins, like bats, have a biological sonar system called echolocation. They produce ultrasonic sounds that reflect off objects to “see” the environment around them. Whales have similar abilities, but the bottlenose dolphin resides in regions of the ocean where the Navy wants to deploy advanced sonar: in the littorals, or coastal waters of the world.
The research is part of the Navy’s Marine Mammal Program, which works under the auspices of the Space and Naval Warfare Systems Center San Diego.
Dolphins emit short echolocation clicks on the order of 100 microseconds in beam patterns that shift in frequencies, source levels and bandwidth. Those impulse signals range from 35 to 135 kilohertz in frequency and 80 to 100 kilohertz of bandwidth.
Dolphins are able to “tune” their sonar for various tasks. For example, in waters where snapping shrimp generate background noise — like sizzling bacon, to human ears — dolphins echolocate at higher frequencies that are optimal against the din.
“They can do things like steer the beam and change the beam width on a click-to-click basis,” says Patrick Moore, a scientist and former head of the biosonar program office.
Scientists want to build sonars that can do the same thing. It’s called environmentally adaptive sonar.
By virtue of being mobile when they echolocate, dolphins are able to perceive objects from multiple points of view.
“It swims around the target and looks at it from all these different aspects and creates an image, somehow,” says Moore.
The researchers have attempted to mimic the biosonar by putting a multi-beam, mechanically scanned sonar on the nose of an unmanned underwater vehicle. As it moves through the water, the beams pick up multiple perspectives of an object, says Steve W. Martin, senior project engineer. Those snippets of data are then assembled for image analysis.
But researchers don’t know how dolphins “see” their environment using the information from those echolocation signals.
“We don’t know if they form images,” says Martin.
That signal processing capability in their brains is something the scientists are trying to unravel.
“We’re dealing with an acoustic animal, not a visual animal. So it’s hard for us as visual creatures to try to get inside the head of acoustic animals,” says Moore.
But that’s exactly what the researchers are planning to do. The scientists have received funding from the Office of Naval Research to do brain imaging work on echolocating dolphins to discover what parts of the brain are actively engaged in processing the echoes.
There’s a theory that the dolphin has two hearing systems. One is devoted to communication sounds — the whistles and other noises they make in social settings. The scientists believe the other one is a passive system that has special timing considerations in a portion of the brain that becomes engaged when they begin to echolocate. When dolphins generate a click, they somehow know when the echo will return, and they can ignore other incoming signals until they receive the echo.
The scientists plan to experiment with the dolphins by using controlled electronic targets. They will place earphones on the jaw and the dolphin’s forehead. When the dolphin produces a click, the scientists can manipulate the sound and present it back to the animal when they believe it would return from a target. By conducting the brain images, the scientists will begin finding clues into the dolphin’s signal processing abilities.
The team works closely with the Navy’s explosive ordnance disposal community, which employs dolphins as mine hunters. The EOD units also deploy UUVs, which have met much success clearing mines in the Middle East harbors. But the sea floor there is relatively uncluttered. Such robotic technologies do not fare so well in other regions where the ocean bottom is not as pristine, researchers say. In such cases, dolphins continue to prove their mettle with their echolocation abilities.
Progress has been made in biomimetic sonar. Martin built a biosonar that could detect underwater buried objects in a previous $4.5 million project funded by the Defense Advanced Research Projects Agency. But there was insufficient funding to take the prototype to the next level, says Moore. It’s a recurring problem that frustrates the research team.
The scientists are conducting small business innovative research-funded work to apply the concept to small side-scan sonars for unmanned underwater vehicles.
“We’ll get our technology to the fleet, one way or another,” says Moore.
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