Determination of compressional wave speed profiles using modal inverse techniques in a range-dependent environment in Nantucket Sound

Abstract

An inverse method for determining geoacoustic properties in a horizontally stratified, shallow-water waveguide is extended to the case of a weakly range-dependent environment. The technique consists of estimating the local modal eigenvalues from the beam-formed output of a horizontal array and using these data as input to modal inverse methods for obtaining the local bottom parameters. Specifically, the approach is applied to data at 140 and 220 Hz obtained in a shallow-water environment with a known abrupt change in bathymetry. First, a range-independent medium is assumed, and both iteration of forward models and perturbative inversion methods are applied to the modal data to obtain estimates of the bottom sound velocity profile. Although the perturbative inversion results are clearly superior, neither approach reproduces the full dependence with range of the observed pressure fields or the complete modal peak structure. In particular, the data exhibit an apparent splitting of the modal peaks which is interpreted, within the context of adiabatic mode theory, as the superposition of the modal contributions from the two segments of the waveguide with differing depths. When the assumption of a range-independent medium is relaxed, and the perturbative inverse technique is applied to the eigenvalue data from each portion of the waveguide, distinctly different bottom profiles are obtained for each part. These result in significantly improved agreement between theory and experiment, thereby demonstrating the effectiveness of the range-dependent inversion procedure.