Acoustical identification of the characteristics of ocean fronts

Radcliffe, Stewart Alexander (1997) Acoustical identification of the characteristics of ocean fronts. University of Southampton, Doctoral Thesis.

Abstract

This thesis studies the effect on acoustic propagation of the thermal front associated with the western edge of the Gulf Stream. A parametric model is used to separate the features of the front so that their individual effects can be examined. This model has not been used previously to study the effect of a front on acoustic propagation. Methods are discussed which allow the simulation of acoustic propagation through a range dependent sound speed environment. A ray tracing model and a parabolic equation model are chosen, and are found to produce comparable results for the prediction of propagation angles and arrival sequences.

Optimisation methods based on Newton's method are introduced and those which make use of the gradient of a function are described. A nonlinear least squares method is used to fit a parametric model of the ocean front to oceanographic data produced by an ocean circulation model and by an oceanographic survey. The model is shown to be useful for comparing Gulf Stream fronts as it allows the front features to be quantified.

The acoustic propagation models are then applied to the parametric model of the Gulf Stream and the effects of the front features on acoustic propagation are examined. Both time domain and frequency domain descriptions of the propagation are derived by using the two models. It is found that all of the parameters that define the thermal structure of the front influence acoustic propagation.

The changes in acoustic propagation resulting from the features of the Gulf Stream front are used to develop a method to infer the thermal structure of the front from acoustic measurements. In the course of the investigation it is found that the parameters in the front model are not independent, and not all parameters may be found solely by using acoustic measurements. However, certain important characteristics of the ocean front, such as its range and width, can be deduced successfully by using inverse methods.

It is concluded that the parametric model of the Gulf Stream front is a valuable tool for studying the effects of the different front features on acoustic propagation. Future work is suggested whereby the methods proposed for inferring the structure of the front from acoustic measurements may be further tested.

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