A study of ship wakes and the enhancement of sonar within them

Abstract

Gas bubbles underwater are powerful scatterers of sound, particularly when the frequency of that sound coincides with the resonance frequency of the bubble (which is inversely proportional to bubble size for air bubbles greater than 25 μm in radius, close to the sea surface). Such powerful scattering of sonar by bubbles might hinder sonar detection and identification of other scatterers that could be targets of interest, for which bubbles are clutter. An example would be when submerged cargo that has fallen off a container ship at night (a hazard that would normally be easy for collision avoidance sonar on a following vessel to detect in bubble-free water) is difficult to detect if still in the wake of the vessel because of the clutter provided by the bubbles. Similar problems occur when underwater hazards are hidden from sonar near the shore by the bubbles of breaking coastal waves. Furthermore, the potential of bubbles to act as sonar clutter has raised questions as to whether techniques to mitigate against this are used when echolocating dolphins blow bubble nets whilst hunting fish.

 In recent years, two new sonar techniques have been proposed, both using two (or potentially more) pulses which are identical in all but one aspect (phase or amplitude), to detect objects hidden in bubble clutter: Twin Inverted Pulse Sonar (TWIPS) and Biased Pulse Summation Sonar (BiaPSS). They do this by exciting nonlinear scatter from the bubbles using high amplitude sonar, and separating it out from the linear scatter from the targets of interest. TWIPS and BiaPSS rely upon bubbles being driven to large nonlinear pulsations, but this is dependent upon the availability of a high-amplitude source.

 This thesis experimentally validates TWIPS and BiaPSS using a commercially available underwater acoustic source. In former studies, purpose-built sources were used. The aim of the experiments in this thesis was to demonstrate the level of relevance this novel technology has and how/whether existing hardware could be used to adopt these new techniques. Recent access to a broadband high-power source, harvested from an acoustic doppler profiling system, has allowed exploitation of bubble resonances across a large bubble size range (the size range of near-surface ocean bubbles under waves and wakes being broad), therefore allowing bubbles to expand sufficiently, in the rarefaction phase of a driving signal. As such, the contrast between linear and nonlinear scatter is enhanced, improving target detection capability. At some point within the duration of an entire broadband pulse, each bubble in the cloud becomes excited at its resonance frequency, whilst at all other times every bubble is off-resonance (and so likely to scatter only linearly). However it is shown in this thesis that exciting resonances in this way can be enough to enhance sonar target detection capability in bubbly water.

 Having achieved experimental validation of the broadband potential of TWIPS and BiaPSS, this investigation exploits the capability of using broadband linear sine sweeps to study when TWIPS and BiaPSS processing can work on a simulated Atlantic Bottlenose Dolphin (Tursiops truncatus) echolocation click too.

 The thesis then proceeds to propose and test new sonar systems: Time-Reversed Pulse Pair Sonar (TRePPS) explores the area of commonality between these two-pulse sonar systems and time-reversal to explore whether a synergy can be found.

 Notably, TWIPS and BiaPSS required a critical period of no emission between projecting the first and second pulse of the pair, that period being long enough to allow the bubble pulsations excited by the first pulse to damp out before the onset of the second pulse, but short enough to prevent evolution of the entire population of bubbles present in the sonar field-of-view. A new sonar is devised which does away with this off-time, so that the two pulses are presented simultaneously (TRePPS).

The sonar systems are tested in simulation, tank tests, and field tests.

Testing of these novel sonar systems requires a bubble field, for these simulations, tank tests, and field tests. In keeping with the scenario outlined at the start of this abstract, the bubble field chosen for field tests is a ship wake.

 A wealth of information exists on bubble populations generated through natural physical processes. However, the interest in bubble populations generated from wakes at sea has yielded relatively fewer publications, in the public domain. Therefore, to understand the composition and evolution of a ship wake, the thesis conducts a review of existing knowledge of ship wakes, and take acoustic absorption and backscatter measurements on real ship wakes. An acoustic model of a ship wake is produced, to simulate the scatter from an insonified wake and has the potential to be used in the testing of the novel sonar systems. In this thesis, a real wake is used to experimentally validate TWIPS.

 The novel outcomes of the research presented in this thesis comprise of:

 • an extension of a review of the existing open-source literature on the acoustic properties of the bubble wake behind travelling sea-surface vessels (originally conducted by Leighton & White, 2008),

• a numerical model to represent a generalisation of those acoustic qualities and how this might be used in sonar simulations,

• the examination of existing two-pulse sonar techniques, using a commercially available underwater acoustic source,

• the study of these techniques in a field trial, using boat wakes as the source of bubbles, and

• the formation of new techniques, inspired by the former inventions and motivated by the need for such techniques to be employed by existing platforms with ease.

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Identifiers

ORCID for Paul White: orcid.org/0000-0002-4787-8713

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