Sound propagation in shallow water with a soft bottom

Low-frequency (50 to 300 Hz) sound propagation in shallow water at the range r of 0.3 to 5 km from the source is investigated in numerical experiments. Sea bottom is assumed to be a liquid homogeneous medium. It is compared two cases: where the sound speed in the bottom is higher than the sound speed in the water column, and where it is lower (rigid and soft bottoms). Soft bottom is associated with gas-saturated sediments that are situated in the area of natural gas accumulation. Calculations are performed using the normal mode representation of the sound field. Both propagating and leaky modes (including quasi-modes) are taken into account. Averaged range dependencies of intensity I decay for different frequencies and sound speeds in the bottom are obtained. In the general case, significant deviation from the «3/2 law» (I ~ r^–3/2) is observed. Maximum deviation takes place for a soft bottom. By approximating the obtained intensity decay dependencies with straight lines on a logarithmic scale, waveguide sound attenuation coefficients \beta(tangents of the slopes) are estimated as functions of frequency and sound speed in the bottom. It is demonstrated that the value of \beta increases monotonically with sound speed in a soft bottom and decreases monotonically with sound speed in a rigid bottom. Maximum value of \beta depends on frequency, and it is reached when sound speed in the bottom is close to its value in water.