Observations of clustering inside oceanic bubble clouds and the effect on short-range acoustic propagation.

It has recently been shown [Weber, T. C. et al. (2007). "Acoustic propagation through clustered bubble clouds," IEEE J. Ocean. Eng. 32, 513-523] that gas bubble clustering plays a role in determining the acoustic field characteristics of bubbly fluids. In particular, it has been shown that clustering changes the bubble-induced attenuation as well as the ping-to-ping variability in the acoustic field. The degree to which bubble clustering exists in nature, however, is unknown. This paper describes a method for quantifying bubble clustering using a high frequency (400 kHz) multibeam sonar, and reports on observations of near-surface bubble clustering during a storm (14.6 m/s wind speed) in the Gulf of Maine. The multibeam sonar data are analyzed to estimate the pair correlation function, a measure of bubble clustering. In order to account for clustering in the mean acoustic field, a modification to the effective medium wave number is made. With this modification, the multibeam sonar observations are used to predict the effect of clustering on the attenuation of the mean field for short-range propagation (1 m) at frequencies between 10 and 350 kHz. Results for this specific case show that clustering can cause the attenuation to change by 20%-80% over this frequency range.