Sound sense

Now sound waves can be used for tracking even the most elusive objects under sea

Published: Tuesday 15 October 1996

a technique for creating underwater images using hydrophones (a device that detects sound waves transmitted though water) instead of video cameras is being developed by a professor at the University of California ( uoc ), us . The acoustic daylight imaging system ( adis ) may also be able to find wrecks, locate faults in oil-rigs, pinpoint underwater cables and navigate seabed vehicles. With adis, underwater warfare will never be the same again. It provides a new way of locating mines under water.

The system, pioneered by Michael Buckingham, professor of ocean acoustics in the uoc, creates pictures of underwater objects by using an array of hydrophones. It works on the basis that oceans are full of ambient sound which is the background noise created by waves, rain, ships and fish.

A series of hydrophones will pick up this sound, but if there is an object in the way, the sound waves received by the hydrophones will be higher because all objects in the sea have a sound that is added to the ambient noise. Buckingham is using a three metre-dish equipped with 126 hydrophones, each of which receives differing levels of sound when an object comes in their range. Each acoustic level is converted into a pixel image by changing the sound into electronic signals.

A computer programme then processes the 126 pixels to produce a picture of the object on a screen which is refreshed 30 times a second to create a moving image. It is a bit like a satellite dish, but instead of having one point of focus, it has 126 points. It is far more sophisticated than sonar, which can only track an object but cannot tell the operator what the object is. An environmental advantage of the system is that it is completely passive. Unlike sonar, it does not generate any sound.

Buckingham and his team are now working on a 1,000 hydrophone-system that will provide images of much higher quality. The system can be used on any underwater vehicle, manned or unmanned. With the right kind of software, an unmanned vehicle could be given an intelligence to navigate itself around obstacles.

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