Geoacoustic inversion in shallow water

Cox, Benjamin Timothy (1999) Geoacoustic inversion in shallow water. University of Southampton, Doctoral Thesis.

Abstract

This thesis is concerned with the problem of estimating the acoustic properties of a seabed from measurements of the sound field made in a shallow ocean. It studies existing methods for performing geoacoustic inversion in shallow water waveguides, and purposes two new techniques. The sound field in the water column of a laboratory tank is predicted with a Fast Field Program (FFP) and compared in phase and magnitude, with measurements made in both range and depth. Head waves' arrival times are measured and used to infer the shear and compressional wave speeds of the tank base. The density and wave speeds of the tank base are also estimated by Matched Field Inversion using both a single frequency and an incoherent multi-frequency Bartlett processor. The FFP is used to compute the replica acoustic fields. A numerical sensitivity analysis confirms that the wave speed estimates are very dependent on the accuracy of the water depth measurement. A method for estimating the angle dependent plane-wave reflection coefficient due to Frisk et al. [J. Acoust. Soc. Am. 68, 602-612, (1980)] is shown to be too sensitive to errors in the depth-dependent Green function to be of any practical use. An investigation, using synthetic data, aims to determine the most accurate Hankel transform (HT) routine for use in shallow water acoustics. The asymptotic routine is recommended because of its speed. Both the HT and Prony's method are used to calculate modal dispersion curves from measured pressure data. A new inversion method, Matched Dispersion Curve Inversion is proposed, which is robust to error in the measurements of source, receiver and water depth. Another novel inversion method based on model dispersion curves is also presented. Measured model dispersion curves and their gradients are used to invert for the phase of the frequency and angle dependent plane-wave reflection coefficient. Both new techniques are demonstrated using the measured dispersion curves.

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