Sound propagation in the shelf area with soft bottom

Abstract

Specific characteristics of offshore sound propagation are investigated in numerical simulations for soft bottom at a small range r ~ 3H to 50H from the source, where H is the water depth. The bottom is assumed to be a liquid homogeneous medium with sound speed c1 lower (soft bottom) than sound speed с in the water layer. It is demonstrated that the bottom sound speed can vary within wide limits for a gas-saturated bottom. It is shown that for a soft bottom, the sound attenuation coefficient β monotonically increases in the water layer with increasing sound speed in the bottom. The β maximum depends on the sound frequency and is obtained when c1 approaches ≈ c. This maximum can reach 10 dB/km for summer conditions when the vertical sound speed profile has a negative gradient in the water layer. A special procedure is proposed to estimate the averaged effective sound speed and gas concentration in the sea bottom. The procedure can be used for estimating oil-and-gas saturation and methane emission on the Arctic shelf.