Dolphin research could lead to man-made sea mine detectors

Sonar systems operating in the bubble-rich waters of busy ports and harbours are subject to signal scattering, which leads to the generation of ‘cluttered’ and unclear images that make it difficult to identify mines planted on the underside of a boat.

‘Searching for mines in bubbly water is akin to looking for a rabbit on the road ahead of you when driving at night through thick fog,’ Prof Tim Leighton from Southampton University’s Institute of Sound and Vibration Research (ISVR) told The Engineer via email.

When hunting prey, dolphins have been observed to blow ‘bubble nets’ around schools of fish, which force them to cluster together and make them easier for the dolphins to pick off.

Leighton’s research group has examined whether dolphins process sonar signals in a way that enables them to distinguish between targets and clutter in bubbly water.

Dolphins emit sequences of clicks and the amplitude of each click can vary from one to the next so that some are quieter or louder than others. The team suspected that the variation in amplitude was not coincidental, but was instead a key way of distinguishing fish from bubbles. The variation in amplitude is key as it changes the echoes, which allow the dolphins to identify fish in the bubble net.

The team placed a sound source and a metal sphere target in a tank containing 200 tons of water and then surrounded the target with a bubble cloud to observe how the sound pulses were scattered.

Echolocation pulses similar to those that a dolphin would rely on were emitted and then processed using non-linear equations instead of the conventional method used to process sonar returns.

The new technique, known as Biased Pulse Summation Sonar (BiaPSS), reduced the effect of clutter by relying on the variation in click amplitude in the same way that occurs when a dolphin emits a sequence of clicks. It was shown to be an effective way of distinguishing targets from the clutter generated by bubbles.

‘Mines bought for $1,000 [£639] each have caused tens of millions of dollars of damage to ships in the last decade and loss of life,’ said Leighton.

The study, co-authored by Prof Tim Leighton, Prof Paul White and student Gim Hwa Chua, was published in Proceedings of the Royal Society A.