Target discrimination in bubbly water

Abstract

A man-made sonar that can operate in bubbly water is relevant: decades of active sonar experience built up for deep water applications are insufficient for shallow water operations required of a changed maritime security landscape. In many instances, military-trained dolphins represent the only viable sonar option, and this study adds to the limited number of solutions that could in future be developed into standard military tools. A series of laboratory tests is described which examines the ideas put forward. The necessary sonar simulation model (based on current state-of-the-art techniques) is also developed. The efficacy of the Twin Inverted Pulse Sonar (TWIPS), is first demonstrated with a linear frequency modulated (LFM) waveform. While TWIPS arose in response to a video showing dolphins generating bubble nets when hunting, existing evidence of TWIPS-like pulses produced by odontocetes suggests their amplitudes appear too low for TWIPS. Instead, the review of the echolocation behaviour of other species of odontocetes yields several results and techniques, which can possibly enhance sonar performance in bubbly water. Parameters like chirp structure and pulse duration are found to be important because of the nonlinear time-dependent characteristics of bubbles. A two-pulse technique, the Biased Pulse Summation Sonar (BiaPSS), which arose from the review, is extensively tested in both simulations and measurements here. Like TWIPS, BiaPSS exhibits the primary advantage of distinguishing a solid target (linear scatterer) from the bubble cloud (nonlinear scatterers) by exploiting nonlinearities with a secondary advantage of outperforming standard sonar processing in target detection. While this does not conclusively prove that dolphins use such nonlinear processing, it demonstrates that a man-made system can classify and detect targets in bubbly water using dolphin-like sonar pulses, raising intriguing possibilities for dolphin sonar when they make bubble nets

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ORCID for Timothy Leighton: orcid.org/0000-0002-1649-8750

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