sirOb: an interactive package for inversion of parametric sonar data

A technique is presented in order to quantitatively determine the geoacoustic parameters of the uppermost seafloor strata by model-based inversion of backscattered data obtained with a parametric sonar. The technique has been implemented by means of an inversion software package that has been named “sirOb” (Seafloor Identification by acoustic Remote sensing Of the Bottom). The parameters to be identified can be grouped in two classes: deterministic parameters (acoustic impedance, compressional wave attenuation) and stochastic parameters (bottom surface roughness, volume inhomogeneities). The “BoRIS” code, that relies on the Kirchhoff approximation and on the small perturbation theory, has been selected as the forward model. Field data are obtained with a parametric sonar instrument, having a peak difference frequency at 8 kHz and 4 kHz bandwidth (both sides). A specific wavelet transform has been defined to obtain a significative measure of discrepancy between the data and the model. This discrepancy is minimized as a function of the bottom parameters (deterministic and stochastic). A divide-and-conquer strategy has been implemented, by first recovering the surface-related parameters (acoustic impedance, surface roughness) and then recovering the subbottom parameters (changes in acoustic impedance, volume inhomogeneities, compressional wave attenuation). This artificial division is made possible by an appropriate weighting of the discrepancy measure. The technique has been tested on simulated data, and then it has been applied on field data collected in shallow water areas of the Mediterranean Sea, where independent ground truth data were available