Distributed source method for retrieval of the cross-sectional contour of an impenetrable cylindrical obstacle immersed in a shallow water waveguide

The nonlinear retrieval of me cross-sectional contour of a sound-soft or so und-hard, closed cylindrical obstacle which is fully immersed in the water column of a shallow water waveguide is investigated. Time-harmonic pressure field data are acquired on two nearby vertical arrays when the obstacle is illuminated by a line source operated at one or at best two discrete frequ encies. These data are inverted by a distributed source method. This involv es the minimization of a two-term cost functional which is characteristic o f the fit between data and wavefield associated to a test obstacle, and of the satisfaction of the boundary condition on the contour of this test obst acle. The method relies on the modeling of the acoustic wavefields as a sup erposition of elementary waves (the Green's functions of the waveguide) and uses the smoothness of the sought contour as a main constraint. Numerical experimentation shows that it yields good shape reconstructions for soft, a nd at a lesser extent hard, obstacles above a sedimentary fluid-like sea bo ttom of low as well as of high contrast using a fairly limited dataset (wit h some substantial improvement using the two frequencies), whereas it has t he potential for generalization to elastic bottom layers as well as to obst acles fully buried in the bottom.