R. A. Stephen
Woods Hole Oceanogr. Inst., Woods Hole, MA 02543
Gaussian beams are often used for the insonifying field in scattering problems from interfaces. They have the advantage of restricting the grazing angle content of the incident field while localizing the scattering region on the interface. However a disadvantage is that the beams spread significantly. For oblique incidence, the width of the beam can vary significantly over the region of interaction on the interface and the scattering problem defined in these terms has an inherent propagation component. Analytical work has been carried out for the propagation of Gaussian beams in homogeneous and heterogeneous media [for example, Cerveny et al., Geophys. J. R. Astron. Soc. 70, 109--128 (1982)]. For a given propagation distance there is an optimal initial beamwidth that will minimize the width of the beam along the whole path. Consequently for a given angle of incidence and a given incident amplitude threshold, there will be a minimum surface scattering area. Optimum beamwidths can be defined and these should be used by all investigators in order to standardize results. [Work supported by Office of Naval Research.]