Statistical study of sound speed in the inhomogenous upper ocean
Fitzgerald, James Richard
Thornton, Edward B.
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The statistics of the fluctuations of the is-situ speed of sound in the upper ocean have been studied by analyzing the instantaneous phase difference of the output of two hydrophones separated by one meter for sounds of frequency 15 to 151 kHz. The experiment was conducted at 11 ft. in water depth of 60 ft. in low sea states at night. Comparison of the speed calculated from the time averaged phase difference, with the speed given by velocimeter or empirical relations, yielded differential speeds which deviate by 1 m/sec to 8 m/sec from the accepted values, for frequencies less than 100 kHz. Correlation and spectral analysis of the sound phase and ocean height fluctuations has shown the close relation between these two parameters. There is strong evidence of the presence and importance of bubbles in all the results, particularly of a large population resonant in the frequency range 56.3 to 71.1 kHz (radius 50 to 60 microns). Evidence is presented to suggest that bubbles appear at the surface during internal wave activity at lower depths and that for sound frequencies near the bubble resonances the sound phase is strongly modulate by, and in phase with, the ocean wave height.