On bubble-mediated scattering from the sea surface: Modeling, and field and laboratory measurements

Abstract

Bubbles, located just below the air–sea interface, contribute significantly to the observed level of acoustic scattering originating from the sea surface, in contrast, for example, to microwave scattering. Typically, such bubbles are the result of breaking wind-generated waves, and thus their concentration is closely linked to wind speed. In this paper we discuss scattering from bubbles located within the proximity of the sea surface, with emphasis on the effects of the nearby reflecting surface, which gives rise to multiple paths from source to bubble to receiver. The effects of scattering geometry, e.g., as in bistatic forward scattering and monostatic backscattering, are illustrated using field data taken at a frequency of 30 kHz. An interpretive model for the contribution of near-surface bubbles to the apparent scattering cross section per unit area of sea surface is also discussed. This model has both a monostatic and a bistatic form, and, somewhat paradoxically, the bistatic form does not reduce to the monostatic form in the limit of source and receiver co-location. The issue is clarified, however, by examining the constituent pressures associated with the multiple paths, and by well-controlled laboratory measurements that include rough surface effects. These confirm both monostatic and bistatic forms of the model.