SEA SURFACE REVERBERATION SCALE MODELING

摘要

Low frequency sound backscattering data from the sea surface in the range of 100 Hz to a few kHz as reported by Chapman and Harris and others is substantially higher than can be explained by Bragg scattering from rough sea surfaces. Alternative models have been proposed to explain the differences including scattering from bubble plumes and bubble clouds formed by breaking waves under different approximations and stochastic scattering from the fluctuating bubble layer. However, there remain a number of disparities between experimental measurements and model predictions as a function of sound incidence angle, wind speed, and whitecap coverage. The authors have developed an integrated, two-dimensional approach for bubble plume scattering which, to first order, includes the impact of a rough sea surface with multiple scattering and shadowing effects. At shallow grazing angles bubble plume shadowing appears to dominate the backscattered signal strength. To understand the magnitude of shadowing effects under controlled and repeatable conditions, scale model experiments have been performed in a 3 m x 1.5 m x 1.5 m tank at the Technical University of Denmark. The experiments used a 1 MHz transducer as the source and receiver, a computer controlled data acquisition system, a scale model target, and a surface wave generator. The fluctuations in scattered signal strength observed in the tank at shallow angles are characteristic of those predicted for the ocean environment. Thus one concludes that shadowing has a first order impact on the scattering strength observed from bubble plumes or bubble clouds.