Use of high frequency acoustics to study the spatial distributions of bubbles

Abstract

It is well understood that the presence of bubbles in the ocean can dramatically change the sound speed, attenuation, and scattering of underwater sound over a range of frequencies. Over the last few decades, rational theories have been developed and tested that describe the complex dispersion of sound through spatially homogeneous bubbly mixtures as a function of the sizes and densities of bubbles. However, it is clear that in the ocean, the size distribution of the bubbles will evolve with a number of different temporal and spatial scales as a result of both the physics which govern their formation (breaking wind waves, breaking waves in the surf, rain generated bubbles, or ship wakes) and the physical processes which control their lifetimes: turbulent mixing, bubble rise speed, and gas dissolution. One approach to measuring their distribution in space is the application of O(1) MHz range-gated sonars which can resolve scales of O(1) cm. These high frequencies are also advantageous to work with since the scattering cross section of the bubbles is approximately proportional to the second moment of the size distribution. Results of field measurement efforts to characterize the spatial scales of bubbles from O(1) m to O(1) cm will be presented and discussed. [Work supported by the ONR Underwater Acoustics and Marine Optics programs.]