Acoustic estimates of size distribution for bubbles released from the seabed.

Bubbles rising from the seabed evolve as they rise through the water column due to gas transfer and changes in hydrostatic pressure. Consequently, the bubble’s acoustic response can change quite dramatically as it rises. For depth integrated measurements of target strength from these bubble clouds, depth dependent changes can confound acoustic inversions for bubble size distribution, void fraction, and the amount of bubbles (and hence gas) reaching the ocean surface. One approach to solving this problem is to implement a model for the evolution of the bubble as it rises through the surface and to then compute an effective, depth integrated scattering response for the bubble plume as a function of bubble size at the source. This effective scattering strength can then be used for inversions of frequency dependent, depth integrated target strength measurements to find the source distribution of bubbles. Such an inversion procedure has been implemented using an incoherent forward model, and tested for a steady stream of bubbles pumped through an orifice at a depth of 11 m. Broadband (1–30 kHz) target strength estimates were made at a distance of approximately 10 m. Inversions for the source distributions of bubbles are compared with estimates made directly with underwater video. Results of this acoustic inversion, and possible coherent bubble cloud effects, will be discussed.