A nonlinearized iterative approach of the acoustic characterization of defects within a half-space

Acoustic characterization of a defect of constant speed-of-sound but unknown position and shape found within a known half-space of the same density is considered. Only P-waves are assumed to be propagated. Data are aspect-limited and frequency-diverse, i.e., scattered fields observed above the half-space when it is illuminated by plane waves at several frequencies. A nonlinearized iterative inversion allows its evaluation by taking into account that what is sought is a distribution in space of binary contrast values 1 (defect) or 0 (embedding). The procedure is inspired from a modified gradient method developed earlier for the characterization of a scatterer in free-space but it has the complication that the needed functional derivatives with respect to the contrast do not exist (which precludes direct application of the original method). Key points of the procedure and numerical results for typical cases are presented and possible extensions envisaged