Sound-speed measurements in the surface-wave layer

Wave breaking at the surface of the ocean entrains bubbles, significantly modifying the phase speed and attenuation of acoustic waves propagating through the resulting two-phase medium. An autonomous buoy system 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 travel-time technique. Simultaneous measurements at each depth are obtained at a 2-Hz rate, allowing observation of the unsteady sound-speed field from individual bubble injection events, as well as the calculation of mean sound speeds. The travel-time technique allows a direct measurement of the sound speed, eliminating the uncertainties common with inferring sound speeds from bubble population data. The sound speed buoy was deployed in the North Atlantic during the winter of 1993–94 as part of the Acoustic Surface Reverberation Experiment (ASREX). Our aim was 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 significant wave heights reaching 20 m/s and 8 m, respectively. During periods of intense wave breaking, average sound speeds below 1000 m/s were observed at the 0.7-m measurement depth while instantaneous sound speeds during individual events approached values as low as 300 m/s. Furthermore, 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.