The three-dimensional parabolic equation acoustic propagation model (3DPE) of Collins and Chin-Bing is adapted to investigate the propagation of a high-frequency (2-kHz) signal in a shallow-water acoustic channel under two-dimensional ocean surfaces disturbed by gravity and capillary waves. Empirically-derived surface wave-number spectra of Pierson and Moskowitz and of Donelan and Pierson are used to construct realizations of stationary Gaussian models of the sea surface. The modified 3DPE model is applied to compute the acoustic field in a waveguide consisting of two isovelocity layers (water and thick sediment). The effects of scattering by the sea surface on the vertical/azimuthal distribution of energy arriving at the receiver are investigated by (1) beamforming the computed acoustic field over subsets of the 2-D computational mesh at the receiver range and (2) computing horizontal and vertical field correlation across subsets of the mesh. For the case of an isotropic surface wave spectrum, the field correlation estimates are derived by averaging correlation samples over multiple 2-D subarrays placed over 360° of azimuth. The angle dependence of the beam responses and the depth and cross-range dependence of the field correlation are interpreted in terms of the surface scattering effects.
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