ASA 126th Meeting Denver 1993 October 4-8

1pUW3. Long-range measurements of the strength and spectral character of low-frequency surface reverberation.

Roger C. Gauss Raymond J. Soukup David M. Fromm

Naval Res. Lab., Washington, DC 20375-5350

Joseph M. Fialkowski

Planning Systems, Inc., McLean, VA 22102

Long-range monostatic measurements of low-frequency (185--985 Hz) and low-grazing-angle (nominally 5--15 deg) acoustic surface scattering were made in the Gulf of Alaska in February of 1992. The upward-refracting sound-speed profiles permitted ensonification of the sea surface at ranges of several tens of kilometers without interacting with the ocean bottom. Reverberation returns from these ranges were used to quantify both the strength and spectral character of surface/near-surface reverberation as functions of frequency and environmental conditions. Spectral results over the range of wind speeds (4--17 m/s) have revealed a dominant zero-Doppler component and a weaker-than-expected dependence of spread on both wind speed and frequency. Overall, the results are consistent with sub-surface bubbles as the driver for surface reverberation when whitecaps are present, and will provide data for comparison with theoretical and numerical models, which in turn will give insight into the physical mechanisms responsible for the observed acoustic scattering. [Work supported by SPAWAR (PMW-182) and ONR (Code 4532).]