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Title: An investigation into the effect of surface waves on time reversed signals in a shallow-water waveguide and the use of chaotic signals for acoustic detection
Author: Adamson, J. E.
Awarding Body: University of Bath
Current Institution: University of Bath
Date of Award: 2007
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The object of this thesis is to investigate experimentally the effects of surface waves on time reversal in a shallow-water waveguide and to use chaotic signals to detect resonant objects. These two areas of research are separate but complementary. Chapters 2. to 4 will explain the background of acoustic time reversal techniques, and the associated experiments performed during this thesis. Chapters 5 and 6 will then focus on chaotic signals, arid how they can be used to detect resonant objects underwater. The synergy of these two separate approaches is explored in Chapter 7, which summarises the findings for both areas of research and makes recommendations for further work in these areas. 1.2 Overview of research into the effects of waves on time reversed signals. Time reversal is a powerful self-adapting technique capable of focusing (or refocusing) energy in media. The strength of time reversal is that it is an adaptive technique that can compensate for errors caused by both geometric distortions within an array of sensors (hydrophones in this case), and wave field distortion caused by inhomogeneities within a media WithOlit any prior knowledge ofeither the array or the medium. This ability makes time reversal a very attractive technique for focusing energy in areas such as waveguides, where the performance of conventional methods (such as beamforming) will be significantly reduced by multiple reflections, signal scattering etc. In the field of underwater acoustics, this can be applied to communications in complex environments and to mine hunting, especially in areas where other techniques may fail such as shallow water channels and littoral zones. One of the assumptions of time reversal is that the medium can be considered !., invariant over the time of the process. In shallow-water channels and littoral zones, water depth fluctuations due to surface waves can become a significant proportion of the total water depth, and in these conditions the assumption of a static medium breaks down. My research is to determine what effect surface waves have in shallow water waveguides, so that it might be used to determine the feasibility of using time reversal for mine-hunting and/or communication in these conditions. 1.3 Overview of research into the use of chaotic pulses for acoustic detection. This research was carried out in collaboration with (and with partial funding from) QinetiQ. Dr. Alan Fenwick (QinetiQ) and his team had carried out simulations of chaotic (acoustic) pulses and their use in detecting resonant objects. The research presented in this thesis aimed at providing experimental validation of these simulations. As with the research into surface waves on time reversal, this area of research has applications in the field of mine hunting. In conditions of harmonic resonance, chaotic signals produce sporadic higheramplitude responses (called 'bursts') from a target. The present research aimed at using this characteristic to produce a method assisting in the detection of resonant objects. My research provided the necessary experimental validation and research was then taken further to design a method of detection.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available