FULL-WAVE-FORM 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 simula ted annealing (SA) are both used to invert a series of synthetic and r eal normal-incidence, geo-acoustic sonar returns for estimates of impe dance versus two-way travel time in the top several meters of ocean fl oor sediment, The objective is to determine the better (faster, more a ccurate) method for inverting this class of data, LLSI uses an over pa rameterized earth, i.e., one composed of layers whose thickness corres ponds to a travel time equal to the sample interval, This makes the in verse problem quite large, but also makes it nearly linear, SA uses a more efficient parameterization, one whose layers have variable thickn ess as well as variable impedance, Because of the relatively narrow fr equency band (similar to 1 octave at 20 dB down from the peak) the tim e domain signal is oscillatory and inversion for layer thickness is no nlinear, Results show greater time efficiency in solving the large lin ear problem (LLSI) than in solving the small nonlinear problem (SA), H owever, in both cases almost all of the waveform energy was modeled, i ndicating that essentially all the information in the data had been su ccessfully recovered, The inversions are applied to 10-20 kHz field da ta acquired offshore Florida, and several techniques are employed to e nhance the effectiveness of each inversion method.