Sound speed dispersion and fluctuations in the upper ocean : project BASS
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Simultaneous measurements of ocean microstructure and sound phase shift from a stable platform in Bass Strait, Australia, have provided new relations between the statistics of the medium and the statistics of the local sound phase speed near the sea surface in the open ocean. Because of dispersion due to ambient bubbles, average phase speeds in the frequency range 15 to 100 kHz differ as much as 2.5 m/sec from the accepted 3MHz "precision" velocimeter values down to depths of 6.76m in the presence of wind speeds of 25-30 knots. These differential speeds imply average bubble volume fractions of the order of 10 with standard deviations approximately one-fifth of the mean value. The differential sound speed is now shown to increase approximately proportional to the wind speed. The third power decrease of differential speed with increasing depth is roughly verified. Under these experimental conditions the predominant cause of the local phase fluctuations at 24.4 and 95.6 kHz is shown to be bubble activity rather than temperature fluctuations. At 24.4 Khz the activity is the random change of number of bubbles. At a frequency such as 95.6 kHz, where there is a large resonant bubble population, the predominant part of the frequency spectrum of the sound phase modulation is shown to be caused by changing bubble radius due to the fluctuating ocean surface wave height. The sound phase spectrum mimics the wind wave spectrum given by Pierson and Moskovitz t to two octaves beyond the frequency of the peak energy, at which point the surface pressure effect has dropped low enough for temperature fluctuations to take over. A theory is presented for prediction of these microsturctural sound phase fluctuations from a knowledge of the surface wave height spect
NPS Report NumberNPS-61Md73101A
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