Abstract:
Modeling the effect of a rough air--sea interface on an acoustic signal is of considerable practical importance; for example, it is relevant in the detection of underwater mines, torpedoes, and submarines. Several promising theories have been developed for rough surface scattering. Among the most successful is the small slope approximation (SSA) [A. G. Voronovich, Sov. Phys. JETP 62, 65--70 (1985)]. Recent numerical studies [see S. L. Broschat and E. I. Thorsos, submitted to J. Acoust. Soc. Am. (1996), and references therein] have shown that the higher-order SSA gives excellent results over a broad range of scattering angles for rms surface slopes up to 1. However, at the lowest forward grazing angles the results become somewhat high. At these angles multiple interactions with the surface at distant points have an appreciable effect on scattering levels. Voronovich recently modified the SSA to extend the nonlocal scattering effects [A. G. Voronovich, Waves in Random Media 6, 151 (1996)]. In this paper, a preliminary numerical study of the nonlocal small slope approximation (NLSSA) is presented. The equations for the lowest-order bistatic scattering strength are derived, limiting cases are considered, and numerical results for a Gaussian spectrum are presented and compared with both original SSA and integral equation results. [Work supported by ONR.]