The low-frequency radiation and scattering of sound from bubble distributions near the sea surface

Abstract

Microbubble plumes are produced when waves break and are convected to depth by the vortical wave motion. The question is, What role is played by these distributions in the production and scattering of sound near the sea surface at low frequency (LF)? Deep- and shallow-water ambient noise show dramatic increases with wave breaking. Individual wave breaking events show radiation down to 30 Hz. Tipping water-filled trough and bucket experiments show LF radiation associated with the entrainment of known bubble size distributions; suggesting that single bubbles cannot be the cause of this radiation. Microbubble distributions with void fractions greater than 0.01% act as collective monopole resonant oscillators excited by the energy of formation. Multipole expansions show the radiation is described by a modified Minnaert expression and a dipole radiation pattern (owing to the presence of the sea surface). Since the plume stays put, scattered sound would have a zero Doppler shift with ample Doppler spread. Acoustic images of the sea surface show discrete scattering events, while experiments show LF scattering from free bubble distributions. Results are shown which indicate that bubble plumes from breaking waves not only radiate but scatter LF sound. [Work sponsored by ONR.]