A model is developed for the calculation of the spatial properties of the noise field produced in a stratified ocean by the action of wind at the surface. The random noise sources are represented by correlated monopoles distribute d over an infinite plane located an arbitrary depth below the surface. Wave-theoretical methods are applied to derive expressions for the intensity and spatial correlation of the noise field. A normal-mode representation of the noise field is used to reduce these expressions to forms which allow physical interpretation and are suitable for numerical computation. Examples are given of intensity profiles and spatial correlation in the vertical for three generic sound-speed profiles. The results show that the sound-speed profile and the presence of the bottom can be important in determining the spatial properties of the noise field. An example is given of a calculation of the horizontal spatial correlation using the fast field program (FFP).

An expression is derived for the acoustic field scattered by a rigid sphere in an isovelocity fluid layer overlying a horizontally stratified medium. The result, valid when multiple scattering can be neglected, is expressed in terms of normal modes and plane-wave scattering functions. It allows simple physical interpretation and straightforward numerical implementation. Sample numerical calculations are given for two cases typical of shallow-water environments. Generalizations to a depth-dependent sound velocity fluid layer and to nonrigid spheres are indicated. A second derivation is given for objects of nonspherical shape in a form similar to that of the rigid sphere. Hence, if the plane-wave scattering functions for the object are known, the scattered field in a stratified medium can be calculated.

Attenuation of the coherent component of sound propagating in shallow water with rough boundaries is studied. Normal-mode theory is used in conjunction with a boundary perturbation method. The inclusion of surface and bottom scattering cause the eigenvalues to become complex and explicit mathematical expressions for the imaginary parts of these eigenvalues are derived. These expressions correspond to the individual normal-mode attenuation coefficients due to scattering. Numerical results are presented for sample shallow-water environments with depth-dependent sound-speed profiles and lossy bottom sediments. The profile structure and the acoustic properties of the bottom sediment have a significant effect on the total attenuation due to scattering. PACS numbers: 43.30.Bp, 43.30.Jx, 43.30.Gv.

Slock 20. Abstract (cont'd) than those obtained when the maximum was directed seaward. This difference or anisotropy (seaward vs. shoreward), which depends on range from the beach, on frequency and on surf intensity, was 10 dB at 300 Hz at the 9 km site during very heavy surf. Surf beat was clearly audible when the cardioid maximum was steered shoreward at ranges as great as 2 km. During heavy surf, the omnidirectional ambient noise levels also increased significantly in the same frequency range at which the anisotropy is evident. The anisotropy effects diminish both in magnitude and in frequency range with lower wave hexght but are still observable during light surf. We have concluded that intense breaking surf can contribute significantly to ambient noise in fairly deep continental shelf waters.

A model is developed for the calculation of the spatial properties of the surface-generated noise in a three-dimensional ocean. This is an extension of the work of Kuperman and Ingenito [J. Acoust. Soc. Am. 67, 1988–1996 (1980)], which used a normal-mode representation of the noise field in a stratified ocean. Noise fields are simulated for both point receivers and vertical line receivers. These examples show how the spatial and directional characteristics of the noise field are affected by the ocean environment. For example, as is apparent in ambient noise data, surface noise propagating at high angles over a sloping ocean bottom is deflected into shallower angles. Also, matched-field processing simulations in three-dimensional ocean environments can be done in a consistent manner: signals and surface-generated noise are modeled by propagating through the same environment with the same theory.