Target Detection in Bubble-Populated Water Using Bio-Mimetic Sonar

Abstract

Marine mammals have been observed to hunt effectively in littoral environments where man-made sonar systems have always performed poorly. Surf zones and adjacent areas which form part of the littoral environment are particularly problematic because transmitted signals are affected by microbubble populated waters.

Leighton first proposed the twin inverted pulse sonar (TWIPS) technique in 2004 [1], when he suggested exploiting the nonlinear nature of bubbles for contrast enhancement of a linear target buried in a cloud of bubbles. Leighton, together with co-workers, subsequently published numerous papers on the TWIPS technique [2-13] and also filed for an international patent application in 2006 [14]. Details of the TWIPS technique were first described in [4], where the authors showed simulation results claiming that their technique outperformed the standard sonar processing technique. The patent application report [14] provided implementation details together with simulation and experimental results on TWIPS, but there was no thorough and quantitative measure of the performance of TWIPS compared with the standard sonar processing technique. In addition, the authors only discussed examples using windowed sine wave pulses at 6 and 300 kHz, although they also claimed that their method would work for any other type of pulses (chirps, pseudo-random noise sequences or M-sequences) with different time durations and operating at other frequencies.

This thesis aims to provide a quantitative measure of the performance of TWIPS against other (simpler) signal processing techniques through the use of signal to noise (SNR) measurements and receiver operating characteristic (ROC) curves. This will be explored both by simulation and experiments in water populated by clouds of microbubbles. Apart from the standard sonar processing technique and TWIPS , several other processing techniques, namely, averaging and smoothing, bandpass filtering and standard cross correlation, are used for performance comparison. In addition, a new variant of TWIPS will be included for discussion.

To extend the scope of the techniques discussed, simulations include applications using bio-mimetic sonar signals from two cetacean species: echolocation chirps from porpoises and echolocation clicks from dolphins. In general, most species of porpoises produce echolocation chirps that have low sound pressure levels, narrower bandwidth and longer time duration compared to echolocation clicks produced by some species of dolphins. The use of these two types of bio-mimetic signals will provide insights on how bubble cloud backscatter will appear to these animals and whether the TWIPS technique would actually work if the animals do adopt TWIPS processing. Experiments were conducted on a modified setup different from the model, but it was sufficient to demonstrate that the performances of the signal processing techniques agree with simulation results.

This study showed that TWIPS does outperform the ?standard sonar processing technique? defined in Leighton et al. (2005). However, it also showed that bandpass filtering or cross correlation methods performed better or equally well against TWIPS under the conditions considered in the simulations and experiments. It is hoped that the studies here will offer alternative methods of processing sonar signals and statistical methods for the analysis of their performances. This would then help in the development of man-made sonar systems employing bio-mimetic signals that perform effectively in the littoral zone.