Anomalous low frequency sea surface reverberation

Low frequency sea surface sound backscattering from ̴100 Hz to a few kHz observed from the 1960s [A,B,C] to the 1990s [D,E] is substantially higher than explained by rough sea surface scattering theory. Alternative theories for explaining this difference range from scattering by bubble plumes/clouds formed by breaking waves to stochastic scattering from fluctuating bubble layers. Disparities remain between measurements and predictions [F] as a function of sound incidence angle, wind speed, and whitecap coverage. A first order, two dimensional approach for bubble plume scattering [G] that includes the rough sea surface shows that bubble plume shadowing dominates the backscattered signal strength at shallow grazing angles. To demonstrate the effect, scale model experiments were performed at the Technical University of Denmark [H]. The scattered signal strength fluctuations observed at shallow angles are characteristic of the predicted ocean environment. Shadowing has a first order impact on the scattering strength observed from bubble plumes and clouds.