Frequency dependence of sound propagation in shallow water: Theory and experiments

Abstract

Experimental observations of broadband acoustic propagation in a known geological region of the Atlantic Generating Station (AGS) site [Badiey etal., J. Acoust. Soc. Am. 96, 3593–3604 (1994)] has prompted new approaches to understanding frequency-dependent behavior in shallow-water regions. First, recent acoustic observations and detailed geological borehole measurements are reviewed, along with a three-dimensional model of the geoacoustic data that have been developed using the kriging method. Parabolic equation modeling, including range-dependent sound speed and attenuation, is performed for both cw and broadband signals in this region. This is accompanied by normal mode investigations in which trapped modes in the layered media and range-dependent mode coupling are examined. A modal-based theory is presented to explain quantitatively the interference patterns observed in the experimental data (transmission loss versus frequency) in terms of waveguide parameters. It is shown how layered shallow-water waveguides act as bandpass filters in which broadband acoustic energy is selectively broken into narrow-band components, thereby providing new insights applicable to existing inverse techniques.