Angle–time–frequency resolution of the noise field generated by wind-induced breaking waves

Abstract

This paper describes a model of the noise field generated by waves breaking on the ocean surface and the angle–time–frequency resolution of that noise field. The model takes into account the discrete space-time structure of the noise sources and thus differs from models based on the continuous source sheet assumption. This is done using a stochastic model to specify the locations, occurrence times, and radiated noise waveforms for the individual breaking waves and then computing the noise field as the superposition of the noise contributions from those breaking waves. The angle–time–frequency resolution of the noise field is obtained by applying a time–frequency resolution algorithm to the output of a beamformer as a function of the steering angle. To describe the properties of the 3-D noise resolution, we assume a vertical array in a shallow-water acoustic environment and use recently obtained experimental estimates of the acoustic coverage and the source level and waveform-duration probability densities to determine the breaking wave source model parameters. The examples presented indicate that the breaking wave noise is largely confined to two angular sectors, the high-angle sector looking toward the ocean surface and the sector containing the discrete mode propagation energy. The high-angle noise exhibits a large temporal variability, characterized by a small number of large broadband bursts, and is only weakly dependent on the acoustic environment. On the other hand, the discrete mode noise shows less temporal variability, characterized by a larger number of smaller broadband bursts, and is strongly dependent on the acoustic environment. Finally, the angle–frequency structure of the low-level noise in the downward-looking direction represents an example of imaging the bottom with surface-generated ambient noise and suggests that ambient noise measurements might be used to estimate sediment thickness.