FINITE CORRELATION AND COHERENT PROPAGATION EFFECTS IN THE NORMAL-MODE DESCRIPTION OF BOTTOM REVERBERATION

A normal-mode scattering formulation that assumes a finite spatial cor relation length in the distribution of scattering features is used to compute single-frequency bottom reverberation for bistatic and monosta tic scattering geometries in a shallow water and deep water environmen t. Spatial correlation of the scattering features allows the superposi tion of modes scattered within each spatially correlated region and pr oduces diffraction in the scattered field that is not predicted in the limit of a zero spatial correlation length (point scattering). For bi static scattering geometries, the scattered field computed as a functi on of scattering location in the horizontal plane exhibits a pattern o f diffractive maxima and minima for nonzero spatial correlation length s. The spatial details of the diffraction pattern and its influence on the scattered energy depend on the frequency and spatial correlation length and can result in a significant reduction in the predicted leve ls of received reverberation. The greatest sensitivity to finite corre lation effects occurs for monostatic scattering geometries because the strongest diffractive effects occur in the backscattering direction. The effects of including modal interference in the incident and scatte red held propagation are also examined in this paper. The inclusion of modal interference in the propagating fields imposes on interference pattern on the spatial structure of the scattered field in the horizon tal planet and can cause the temporal dependence of the reverberated r eturn to oscillate about the levels of return predicted when modal int erference in the propagating fields is neglected. In agreement with pr eviously published results for bottom backscattering, the effects of i ncluding modal interference in the propagating fields were found to be significant for deep water environments that exhibit convergent zone propagation and to be of limited importance for shallow water environm ents in which the energy incident on the bottom is characterized by a large number of multipaths. The present work includes results which sh ow that the effects of including modal interference in the propagating fields can be important for shallow water environments that exhibit s ignificant bottom penetration. (C) 1997 Acoustical Society of America.