Modal mapping in range and azimuth for shallow-water waveguides

Abstract

It is well established that the normal mode solution for sound propagation in shallow water is a function of the local waveguide properties. Much work has been done on the inverse problem where the local modes are measured and used as input data for geoacoustic inversion methods. In the range-dependent case, short sliding-window transform methods based on the Hankel transform have been implemented to extract the corresponding range-varying wave number spectra. A major objective of this work is to extend the range-dependent work to fully three-dimensionally varying environments. Recently, experiments were conducted using cw sources operating in the frequency range 20–475 Hz. The acoustic field was measured on an array of freely drifting hydrophone buoys equipped with GPS navigation. Using the complex pressure measured on these synthetic aperture horizontal arrays, local mode information is extracted as a function of range and azimuth from the source. Emphasis is placed on modal variability for tracks both along and across bathymetric contours at several frequencies. [Work supported by ONR.]