Model Studies of Scattering of Underwater Sound by the Sea Surface Caused by Waves and Air Bubble Screens in the Upper Layers

Experimental investigations of scattering by the sea surface are conducted on a model to separate the two scattering components, volume scattering by a bubble screen closely below the surface and scattering by the irregular sea surface. Volume scattering by the bubble screen may be described quantitatively by the Lommel-Seeliger illumination law of photometric optics valid for a secondary radiator, while known theories of sound scattering describe the surface scattering by periodic and randomly irregular model surfaces. The directional pattern of sound scattered by irregular resilient surfaces behind a layer containing air bubbles is determined almost exclusively by surface scattering for bubble concentrations of up to 1·10⁻³. Backscattering at angles larger than 60°, however, is dominated by volume scattering. For bubble concentrations of more than 4·10⁻³ backscattering and the directional pattern of the scattered sound for all angles show the influence of volume scattering only (masking of the irregular surfaces); the Lommel-Seeliger law again describes the directional dependence of the scattering.