SOUND-SPEED MEASUREMENTS IN THE SURFACE-WAVE LAYER

Wave breaking at the surface of the ocean entrains bubbles, significan tly modifying the phase speed and attenuation of acoustic waves propag ating through the resulting two-phase medium. An autonomous buoy syste m was developed that directly measures sound speed at 3.33, 5, and 10 kHz at seven depths ranging from 0.7 to 7 m through the use of a trave l-time technique. Simultaneous measurements at each depth are obtained at a 2-Hz rate, allowing observation of the unsteady sound-speed fiel d from individual bubble injection events, as well as the calculation of mean sound speeds. The travel-time technique allows a direct measur ement of the sound speed, eliminating the uncertainties common with in ferring sound speeds from bubble population data. The sound speed buoy was deployed in the North Atlantic during the winter of 1993-94 as pa rt of the Acoustic Surface Reverberation Experiment (ASREX). Our aim w as to characterize the highly variable near-surface sound-speed field under varying environmental conditions. Forty-three days of data were obtained spanning several storm cycles with wind speeds and significan t wave heights reaching 20 m/s and 8 m, respectively. During periods o f intense wave breaking, average sound speeds below 1000 m/s were obse rved at the 0.7-m measurement depth while instantaneous sound speeds d uring individual events approached values as low as 300 m/s. Furthermo re, the data suggest that the dispersive effects of bubbles may extend to frequencies as low as 5 kHz near the surface during storms. Strong correlations of the mean and rms sound speed with the overlying wind and wave fields were found. (C) 1997 Acoustical Society of America.