Measurements of ambient bubble populations with a multibeam sonar

A 240 kHz multibeam sonar was deployed on the research platform FLIP off the coast of San Diego for two weeks in July, 2002. The sonar was oriented to measure backscattering from the ambient bubble field near the ocean surface. A bubble population density spectrum shape described by Novarini et al. [IEEE J. Ocean. Eng. 23, 423–438 (1998)] is assumed, and the spectrum level is shifted up or down to fit the backscatter measurements. This allows the bubble population to be inferred from the single frequency measurement. The wide field of view for the sonar (1.5° beams distributed over a 150° swath) allowed the spatial structure of the bubble field to be observed over the time period, and compared with environmental measurements including wind/wave conditions and the spatial variation of temperature and salinity. [Work supported by ONR Code 321US.]