Near-Surface Ocean Bubble Distributions Measured in Situ by Acoustic Techniques

Abstract

An in situ acoustic measurement is highly specific for the detection of bubbles in water because of the very large values of their resonant cross sections for sound absorption and scatter. A pulse-echo “bubble spectrometer” has been used to depths of 50 ft in shallow water during sea states 1 and 2. The pulse-echo oscillograms have been analyzed both for excess attenuation (relative to clean water calibration) and backscatter between echoes (relative to a rigid reflector) to provide data over the frequency range 20 to 200 kHz (inferred bubble radius 200 to 18 μ). The number of sea-level bubbles (of radius between R and R+1 μ) per unit volume shows the peak at approximately R=60 to 80 μ, as observed by Glotov et al. for wind-agitated waves in a labortory tank. In addition, there is strong evidence of a significant distribution of very small bubbles (R<40 μ) whose number per unit volume, being proportional to R−1, will play an important part in high-frequency attenuation in well-mixed shallow ocean waters. It is postulated that the surfaces of these small bubbles are so encumbered by soluble organic matter and detritus, that their numbers are determined more by buoyancy and turblent entrainment than by rate of gas diffusion across the bubble walls. (This analysis is based on data collected by S. Buxcey, RCN, James E. McNeil, RCAF, and Robert H. Marks, Jr., USN.) [Support was obtained from the Office of Naval Research.]