Low-frequency sound scattering from microbubble distributions.

Abstract

Backscatter from the sea surface is governed by the roughness of the surface and subsurface micro bubble distributions. At low frequencies, due to the paucity of large bubbles, scattering results primarily from coherent and/or collective scatter from bubbles entrained by the subsurface vorticity or carried to depth by the Langmuir circulation and thermal convection. Theoretical treatments using the work of Mallock and Wood show that scattering from compact regions is a function of the volume fraction of air and to first order can be described by a Minnaert formula modified with the volume fraction. Experiments to test this theory were conducted in a lake and large tank facility. Back scattering from a submerged bubble cloud produced results consistent with our theory when parameters independently measured, such as bubble size distribution, volume fraction, and the cloud size, were used to calculate the resonant frequency with the modified Minnaert formula. This theory was further tested by backscattering experiments on bubbly liquid filled tubes, bubbly gel filled cylinders, and voided polyurethane spheres. Again the resonant frequency and backscatter measurements were found consistent with theoretical expectations; however, the effective dampening was found to depend on the nature of the material containing the microbubbles.