Dolphins inspire underwater sonar innovation

Image: NOAA

Underwater imaging sonars are an essential technology for ocean exploration, and increasingly inspiration for new devices is coming from marine mammals such as dolphins.

A team of scientists from the National University of Singapore (NUS) has now applied dolphins' ability to acoustically scan objects underwater - and pick matching objects visually - to develop an innovative compact sonar. They demonstrated that when a dolphin’s sound bounces off an object, it contains information about its shape; they then recorded the dolphin echoes emitted when scanning an object underwater.

Based on their observations, the team built a biomimetic sonar that replicated a dolphin’s sonar. Biomimicry uses models, systems and elements of nature to solve complex human problems.

In this case, the sonar is designed to emit sharp, impulsive click sounds similar to a dolphin’s echolocation. Three transmitters send sounds in different directions and the researchers then process sounds from both the dolphin and the sonar to visualise what the echoes revealed about an object's shape.

To complement the hardware, the team came up with an innovative software that allowed the sonar to improve the visualisation of the echoes. The device boasts an echo-processing method that gives clearer underwater imaging than conventional signal-processing methods of visualising sound echoes.

Based on the hypothesis that dolphins use prior information to process their echoes, the researchers incorporated the concept of sparsity into the sonar’s software, which helps interpret sound echoes better. This assumes that of any space scanned, only a small percentage is occupied by the object.

Researchers Dr Hari Vishnu (r), Professor Mandar Chitre, and Abel Ho (l) with their dolphin-inspired sonar.

Compared to other sonars of similar sizes and purposes, the biomimetic sonar provides a better trade-off between sonar-image clarity, the number of sensors and the size of the sensor array used. Conventional methods of processing sound echoes usually break down when sensors are too few or spread out.

“Using prior information, such as the idea of sparsity, is intuitive," said Dr Hari Vishnu, senior research fellow at NUS Tropical Marine Science Institute (TMSI). "It is something humans do all the time – we turn our understanding of reality into expectations that can speed up our inferences and decisions.

"For example, in the absence of other information, the human brain and vision system tend to assume that in an image, the light on an object will be falling from above.”

The effectiveness of the software was demonstrated when it was able to visualise information from a dolphin’s sonar echoes when scanning an object, as well as sonar signals produced by their compact sonar. A conventional approach of processing both sonar echoes resulted in noisy images.

Dr Matthias Hoffmann-Kuhnt examines images of the compact sonar in operation underwater.

However, the new processing approach gave better resolution and therefore sharper images. The software is also able to generate visualisations with a mere three clicks from the sonar, allowing it to be operationally fast.

The new sonar processing method could have potential benefits in underwater commercial or military sonars. For example, it could be used to scan the seabed to search for features that can be used to aid navigation. The sonar’s compactness also makes it suitable to be mounted on underwater robots for ocean exploration.

The study was published in Communications Engineering in 2022.

The compact sonar comprises three sound transmitters which emit sharp clicks like dolphin echolocation sounds.