Geoacoustic Inversion in Laterally Varying Shallow-Water Environments Using High-Resolution Wavenumber Estimation

Kyle Becker, Ph.D., 2002
George Frisk, Advisor

An experimental method for extracting horizontal wavenumber spectra for point-source acoustic fields in laterally varying shallow water waveguides is discussed. The experiment was designed to provide input data for a perturbative inversion method for inferring the geoacoustic properties of the bottom. The input data are discrete values of horizontal wavenumbers that correspond the propagating modes in a shallow-water waveguide. Based on the asymptotic Hankel transform pair relationship between the complex pressure field and the depth-dependent Green's function, a high-resolution autoregressive (AR) spectral estimator is applied to extract wavenumber content. The estimator is characterized in terms of it performance on short-aperture noisy data. The estimator is then used on short-aperture synthetic acoustic data for extracting local horizontal wavenumber content for doing range-dependent geoacoustic inversion. Results are discussed in terms of discrete and continuous changes in the waveguide environment. The estimator is then applied to real data for a fixed receiver and towed source. For a source speed of 2 m/s, a shift was observed in the measured wavenumbers. Based on this observation, a method for measuring modal group velocity is presented from measurements of the shifted wavenumbers for a source towed out and back along the same track.