Sound-speed measurements near the ocean surface

Abstract

The results of two field experiments conducted in the fall of 1992 on the measurement of sound speed near the ocean surface are reported. The measurement technique makes use of the travel time of short acoustic pulses between a transmitter and a receiver separated by 40 cm [Lamarre and Melville, J. Atmos. and Oceanic Tech. (1994)]. The instrument is capable of making simultaneous measurements of the sound speed at six depths, starting at 0.5 m, over a frequency range from 5 to 40 kHz. Time series of sound speed show dramatic fluctuations over time periods on the order of minutes or less. These are attributed to the formation of bubble plumes or the passage of bubble clouds. In two particular instances, a newly created bubble plume generated a sound-speed reduction of 800 m/s at a depth of 0.5 m; and a bubble cloud 25-s old generated a reduction of 400 m/s. Frequent occurrences of sound-speed reductions greater than 100 m/s are observed during moderate wind conditions (8 m/s). The signals at various depths are highly correlated and mostly coherent at frequencies below 0.05 Hz. The time-averaged (20 min) sound-speed profile is found to be significantly more pronounced and shallower than previously reported. The average sound-speed reduction is found to correlate with wind speed. Simultaneous measurements at several acoustic frequencies show that the sound speed is nondispersive below 20 kHz for moderate wind conditions of up to 8 m/s. Upward-looking sonar data reveal the presence of bubble clouds which correlate with the large sound-speed reductions observed. However, even though bubble clouds are present down to a depth of 3 m and greater, significant sound-speed reductions are confined to the first meter below the surface.