Characteristics of near-surface, low-frequency sound speeds

Abstract

The bubble size density for radius of 34 to 1200 μm 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.]

NCPA Planning, Organization and Research FY 92

“…Hence, using the result in Eq. (36) The results for oc(O) inferred from backscatter data are consistent with the in-situ measurements of and Cartmill and Su (1992). Using acoustic measurements of It should be noted that although the Von Karman spectrum used here is isotropic at all scales, isotropy is a plausible approximation only in the inertial subrange.…”

“…Since there was little improvement, in all four cases in Fig. 2 we have used an e-folding depth of L=1.5 mn, a value that is consistent with oceanographic measurements (Thorpe, 1982;Farmer and Vagle ,1989;Cartmill and Su, 1992;Su et al, 1993).…”

“…The larger bubbles rise to the surface rapidly, leaving behind a turbulent, smoke-like cloud of microbubbles with radii of 10-100 microns (Su and Cartmill, 1992;Su et al, 1993). Since the microbubbles may stay in the water for some time before dissolving, they can be transported downward by turbulent downwelling currents (Thorpe, 1982(Thorpe, , 1984Thorpe and Hall, 1983;Osborn, et al, 1992).…”

“…Although experimental measurements indicate that the average concentration of bubbles decays approximately exponentially with depth , it is clear from the uplooking sonar records (Thorpe, 1982;Farmer and Vagle, 1989;Farmer, 1992) and in-situ measurements Cartmill andSu, 1992, Su et al, 1993) that there are significant fluctuations about the average, both horizontally and Svertically. Hence we write ki as the sum of an average value and a fluctuating part that I depends stochastically on both depth and horizontal distance.…”

“…Because cf ",'"b!e disso!ttion, both <k,(z)> and 8kt2,.) are týd decrease with increasing depth (Su,1992;Cartmill and Su, 1992;Su et al, 1993).…”