Full waveform inversion of field sonar returns for a visco-acoustic Earth; a comparison of linearized and fully nonlinear methods

The techniques of linearized least squares inversion (LLSI) and simulated annealing (SA) are both used to invert a series of synthetic and real normal-incidence, geo-acoustic sonar returns for estimates of impedance versus two-way travel time in the top several meters of ocean floor sediment. The objective is to determine the better (faster, more accurate) method for inverting this class of data. LLSI uses an over parameterized earth, i.e., one composed of layers whose thickness corresponds to a travel time equal to the sample interval. This makes the inverse problem quite large, but also makes it nearly linear. SA uses a more efficient parameterization, one whose layers have variable thickness as well as variable impedance. Because of the relatively narrow frequency band (~1 octave at 20 dB down from the peak) the time domain signal is oscillatory and inversion for layer thickness is nonlinear. Results show greater time efficiency in solving the large linear problem (LLSI) than in solving the small nonlinear problem (SA). However, in both cases almost all of the waveform energy was modeled, indicating that essentially all the information in the data had been successfully recovered. The inversions are applied to 10-20 kHz field data acquired offshore Florida, and several techniques are employed to enhance the effectiveness of each inversion method