Three-Dimensional (3-D) sound propagation in a shallow-water waveguide with a constant depth and inhomogeneous bottom is studied through numerical simulations. As a model of inhomogeneity, a transitional region between an acoustically soft and hard bottom is considered. Depth-averaged transmission loss simulations using the “horizontal rays and vertical modes” approach and mode parabolic equations demonstrate the horizontal refraction of sound in this region, even if the water column is considered homogeneous. The observed wave effect is prominent at low frequencies, at which the water depth does not exceed a few acoustic wavelengths. The obtained results within the simplified model are verified by the simulations for a real seabed structure in the Kara Sea.

The seabed data from the Kara Sea (a part of the Arctic Shelf) are used to build a low-frequency (up to 250 Hz) acoustic waveguide model and study sound propagation in this region. A 30-m deep, well-mixed, and homogenous water layer over a flat seafloor is considered. The seabed's acoustic model is based on the spatial distribution of a sound speed recorded during a three-dimensional seismic survey in the Kara Sea, as well as density data from core sample analysis. One of the region's most distinctive features is the presence of large areas (up to several tens of square kilometers) where the bottom sound speed is close to that in water. In such a setting, the normal mode approach is applied to the sound propagation problem. The overall acoustic field is made up of propagating and leaky modes, which include quasi-modes. Numerical simulations show a high spatial variability of attenuation in the waveguide, 1.5 to 20 dB/km at 137 Hz. Even if the water depth is constant and the seafloor is uniform and smooth, mode coupling and horizontal refraction can occur owing to the bottom inhomogeneities. For higher modes, the predicted angle of refraction is up to 10°.

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