Do dolphins benefit from nonlinear mathematics when processing their sonar returns?

Abstract

Dolphins have been observed to blow bubble nets when hunting prey. Such bubble nets would confound the best man-made sonar because the strong scattering by the bubbles generates 'clutter' in the sonar image, which cannot be distinguished from the true target. The engineering specification of dolphin sonar is not superior to the best man-made sonar. A logical deduction from this is that, in blowing bubble nets, either dolphins are 'blinding' their echolocation sense when hunting or they have a facility absent in man-made sonar. Here we use nonlinear mathematical functions to process the echoes of dolphin-like pulses from targets immersed in bubble clouds. Dolphins emit sequences of clicks, and, within such a sequence, the amplitude of the clicks varies. Here such variation in amplitude between clicks is exploited to enhance sonar performance. While standard sonar processing is not able to distinguish the targets from the bubble clutter, this nonlinear processing can. Although this does not conclusively prove that dolphins do use such nonlinear processing, it demonstrates that humans can detect and classify targets in bubbly water using dolphin-like sonar pulses, raising intriguing possibilities for dolphin sonar when they make bubble nets.

Dolphin-Inspired Target Detection for Sonar and Radar

“…We developed a practical sonar system using this BiaPSS (Biased Pulse Summation Sonar) that, like TWIPS, was effective at enhancing the detectability of fish in bubble clutter, and distinguishing them from clutter (Leighton et al, 2012b). Given that modern amplifiers and emitters tend to more easily produce high fidelity copies of pulses that have differing amplitudes, as opposed to opposing phases, BiaPSS sources were more practical to produce than TWIPS sources.…”

“…We proposed that, rather than being an accident, this amplitude variation might purposefully supply pairs of pulses with another difference that could exploit the fact that bubble clutter scatters nonlinearly, whilst the fish do not (Leighton et al, 2012b). We tested a range of pulses, in test tanks and simulation Leighton et al, 2012b). Figure 5 shows a pulse pair based on clicks, where each click in the pair consists of two synchronous downchirps, each covering a distinct frequency range, and as such is a commonly accepted to typify a form of emission from the Atlantic bottlenose dolphin (Tursiops truncatus; Capus et al, 2007).…”

“…Such variation has been known about for years, but without adequate explanation. We proposed that, rather than being an accident, this amplitude variation might purposefully supply pairs of pulses with another difference that could exploit the fact that bubble clutter scatters nonlinearly, whilst the fish do not (Leighton et al, 2012b). We tested a range of pulses, in test tanks and simulation Leighton et al, 2012b).…”

“…In the following discussions, the pulses will be represented by numbers, for illustrative purposes: for the full explanation, where the waveform is mathematically described, see references 2012b). Consider the TWIPS signal: if the first pulse is represented by '+1', then the second pulse is represented by '−1' since it is identical except that it has opposite phase (Fig.…”

Radar clutter suppression and target discrimination using twin inverted pulses

“…Gaussian pulses are used in this particular study, although the principle was shown to work using BiaPSS for a range of other pulses, including a linear frequency-modulated pulse and dolphin-like clicks [6]. These could be of greater use if, for example, the resonance frequencies of components were not well known before the test, or if a particular target possessed several resonances that were to be used to identify it.…”