John Cartmill
Planning Systems, Inc., Slidell, LA 70458
Ming-Yang Su
Naval Res. Lab. Detachment, SSC, MS 39529-5004
The bubble size density for radius of 34 to 1200 (mu)m in near-surface ocean from depths of 0.25 m down to 7.00 m were measured by an acoustic resonator array during a 1992 field experiment in the Northeast Pacific. The void fractions were computed by integrating those bubble densities. The low-frequency (less than 1 kHz) sound speed, averaged over 3.5 s, is then estimated from the void fraction based on a theoretical computation. A set of more than 50 time series of the low-frequency sound speed, each of about 12 min in length, were then subjected to several statistical analyses. The statistical characteristics include (a) the probability density which provides, in turn, the mean and standard deviation, skewness and kurtosis, (b) Gaussian distribution fit, and (c) autocorrelation coefficient. The mean sound-speed deficit from the nominal sound speed of 1500 m/s and the corresponding standard deviation (in the order of 100 m/s) are at least 10 times larger than expected earlier. Large zero-crossing times of the autocorrelation on the order of 100 s for depth <1 m are also found for wind speed between 10 and 15 m/s. The autocorrelation analysis further indicates the evidence of coupling between the fluctuation in the bubble density and the prevailing dominant wave train. [Work supported by ONR Acoustical Reverberation SRP.]