Determination of ocean bottom sound‐speed profiles using a modal inverse technique in a range‐dependent shallow water environment

Abstract

A modal inverse technique for determining ocean bottom sound‐speed profiles is applied to experimental data obtained in a range‐dependent shallow water environment. The method consists of towing a cw source away from moored receivers to obtain a well‐sampled measurement of the complex pressure versus range. The Hankel transform of the measured field then yields the depth‐dependent Green's function versus horizontal wavenumber, which contains distinct peaks at the modal eigenvalues. The eigenvalues are used as input data to a perturbative inverse scheme that generates the sound‐speed profile of the bottom. Data at 140 and 220 Hz were obtained in May 1984 in Nantucket Sound, MA in an area where the depth changed abruptly at about the midpoint of the experimental track. An associated change in bottom properties was assumed, and the concept of local modal eigenvalues yielding local geoacoustic parameters was then used to determine separate models for the two regions. Excellent agreement between the pressure fields computed for this range‐dependent model using adiabatic mode theory and the experimentally measured fields is demonstrated.