Tomographic Inversions in Shallow Water Using Modal Travel Time Measurements

The paper deals with an inversion approach based on modal travel time measurements for the estimation of water column and bottom properties in shallow water, by acoustic means. The modal structure of the acoustic field measured at a single hydrophone is defined on the basis of group velocity predictions at the central frequency of a broadband signal (tomographic signal). In the proposed approach, the assumption of good a-priori knowledge of the parameters to be recovered which is normally applied in modal-travel time inversion schemes, is waived and a relatively wide search space is considered. Following an identification process to assign modal arrivals to the peaks of the measured signal, a matching procedure is adopted based on the minimization of a cost function relating the travel time differences of the identified peaks with respect to the modal travel times for the same peaks. This procedure leads to a set of environments, which are considered close to the actual one, among which a reference environment for the application of a linear inversion procedure is chosen on the basis of the convergence of the local search. Thus, the inversion procedure adopted is hybrid in the sense that an optimization procedure for the estimation of the most probable reference environment is combined with a linear inversion method to estimate the unknown parameters. The approach can be applied in shallow water tomographic experiments involving a single source-receiver pair and leads to the simultaneous estimation of water and sediment sound velocities.