Abstract:
Breaking surface waves produce oscillating bubbles which generate high-frequency acoustic energy in the ocean. An experiment was conducted in May 1996 to examine the spatial and temporal evolution of the acoustically active regions beneath breaking surface waves within the frequency band 8--80 kHz. A multibeam, high-resolution, passive sonar, originally designed for acoustic daylight imaging, was aimed at the surface with a slant range of 45 m. The surface footprint was 9 m in diameter and contained 126 beams, each 0.75 m wide between the -3-dB points at the highest operating frequency. With this resolution, pictorial images have been obtained which show the spatial distribution and the spectral structure of the acoustic sources within individual breaking events. The spectral structure of the sources evolves on a time scale that is at least as fast as 40 ms, which is the temporal resolution of the system. The experimental site was 25 miles from the coast of southern California, with wind speeds rarely exceeding 15 kn. Examples of breaking-wave images will be presented and interpreted in terms of wind speed, wind direction, and horizontal anisotropy in the ambient noise field. [Work supported by ONR.]