Low frequency sound propagation in sea surface mixed layers and in the presence of internal waves

Prior, Mark Kevan (1996) Low frequency sound propagation in sea surface mixed layers and in the presence of internal waves. University of Southampton, Doctoral Thesis.

Abstract

The propagation of low frequency sound trapped in near-surface mixed layers of the ocean is investigated. First, a review of numerical methods is made to highlight the strengths and weaknesses of the various available methods. The need is identified for simpler methods which provide more physical understanding than numerical methods. The propagation of low frequency sound in sea surface mixed layers is investigated using both numerical and simpler approximate methods. Emphasis is placed on the low frequency process of diffraction leakage. A new method for predicting diffraction leakage from sea surface mixed layers is proposed. The study of diffraction leakage from sea surface mixed layers is extended to layers whose characteristics vary with range. The new method for predicting diffraction leakage is shown to allow the acoustic impact of these changes with range to be predicted.

The scattering of sound from random sound speed inhomogeneities arising from internal waves in the ocean is then studied. A normal mode method is used, partly because of its suitability for low frequencies but mainly because the method allows physical insight into the processes involved in scattering. A new method of predicting the effects of scattering from internal waves is proposed and is shown to agree with a numerical, Monte-Carlo type approach. The new method allows the statistics of the amplitudes of the acoustic normal modes to be directly related to the statistics of the amplitudes of the internal waves.

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