The Theoretical Response of a Vertical Line Array to Wind-Generated Noise in Shallow Water

In this NATO-furnished report a wave-theory model for the propagation of noise from a surface source layer was used to investigate the directional response of a vertical array of hydrophones to wind noise in shallow water. The model assumes an infinite layer of sources radiating sound with pressure directionality of the form COS (superscript m sub alpha), where m or = 1 and alpha is an angle measured from the downward vertical. The field incident on the array is inhomogeneous and directional responses have been calculated from the (non-Toeplitz) spatial correlation matrix for various bottom types, sound speed profiles, and values of m up to 3. Results show that both components of the noise field - the discrete normal modes and the continuous spectrum - can contribute significantly to the total noise level and to the array response. Shallow-water noise levels are found to exceed those of an idealized, infinitely-deep ocean by approximately the amount contributed by the discrete modes. Furthermore, the discrete mode arrivals give high-array responses in near-horizontal steered directions. Thus the directional response pattern produced can be of considerably different form from that predicted by a simple correlation model for ideal conditions. The extent of these differences in both noise level and array response is strongly dependent on bottom type and on the value of the parameter m, and slightly dependent on sound-speed profile.