Acoustic interference fields in the ocean

Hurdle, Burton G. (1988). Acoustic interference fields in the ocean. PhD thesis The Open University.



Two areas of underwater acoustics are investigated: ocean-bottom scattering and acoustic fields in geometrically dispersive sound channels. The purpose is to describe and provide an understanding of the physical mechanisms in these two areas by comparing analyzed results from ocean experiments with theoretical computations.

Experiments using directive 19.5-kHz transducers illustrate temporal and spacial behavior of signals scattered from the ocean bottom. The signals fluctuate, as a function of acoustic geometry, in linear relation to source and receiver motion and to signal frequency. Spacial structure of the acoustic field depends on frequency and acoustic geometry and is independent of motion and bottom roughness. Data supporting these observations are included as well as data showing the effects of bottom type on the scattered returns, that is, the existence of subbottom returns in some data. Volume-scattering-strength profiles are also provided from data obtained in these experiments.

Continuous-wave (CW) and impulsive sources covering frequencies 5 to 260 Hz were towed and deployed respectively over ranges up to 3000 km, with reception on fixed hydrophones. Analyses of measured propagation losses of these low-frequency acoustic signals in the dispersive channel provide insights into the nature of the propagation and the acoustic channel. Both the CW signals and the arrivals of the impulsive signals are analyzed in terms of transmission loss, convergence-zone structure, source-motion effects, interference structure, and channel characteristics. The systematic variation (internal tides) of the medium and its influence on the interference field are discussed. The state of modeling, both simple and complicated, is reviewed and compared with results of the ocean experiments. Relationships are provided between this work and the broader field of underwater acoustics. Suggested areas for future research are made.

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