Listening for ambient bubbles in the marine boundary layer

Abstract

Bubbles created by breaking waves and raindrop splashes are responsible for much of the high-frequency (500 Hz–50<th>000 Hz) ambient sound in the ocean. Under high sea state and during heavy rainfall conditions clouds and layers of ambient bubbles in the marine boundary layer absorb sound from bubbles being newly created at the ocean surface. This absorption changes the shape of the ambient sound spectrum below the bubbly surface marine layer, allowing passive detection of the ambient bubbles. This change in the shape of the ambient sound spectrum is especially apparent in the situation of rainfall during high wind conditions, presumably because existing wind-generated turbulent from breaking waves is available to transport rainfall-generated bubbles downward. Estimates of vertically integrated ambient bubble populations can be made from the presumed frequency-dependent attenuation in the ambient sound spectrum. Ambient sound data from many months of deep ocean mooring measurements will be used to document this phenomenon. [Work sponsored by ONR Ocean Acoustics.]