Abstract:
Bottom geoacoustic properties were determined from inversion of broadband (200--800 Hz), multitone pressure fields measured at a shallow-water site in the western Mediterranean (YELLOW SHARK 94). Extensive oceanographic and geophysical information was obtained in situ to support and validate the inversion. Matched-field processing was applied for each tone frequency. Optimization of the environmental parameters was performed simultaneously across all propagated frequencies by minimizing a maximum-likelihood objective function with genetic algorithms (SAGA). The following conclusions were drawn from this study. (a) For a fixed source-vertical array configuration broadband tomographic measurements were a sine qua non condition to obtain meaningful inversion results. (b) The broadband inversion provided considerable stability and robustness with respect to volume and bottom variabilities. (c) Corresponding single-frequency inversions performed under the exact same conditions produced erratic results. (d) Integral geoacoustic properties were effectively determined within the constraints imposed by the bottom parametrization. (e) More detailed and accurate properties were obtained by including the range dependence of ocean sound-speed profile (SSP) in the forward modeling. (f) The broadband inversion provided robust, unique, and most likely correct, geoacoustic parameters, i.e., SSP, attenuation, density, and thickness of the top clay layer, and sound speed of the underlying silt layer.