Sound attenuation in shallow water with the bottom in the form of a gas-filled sediment layer overlaying permafrost

Within the framework of numerical modeling the feasibility of mode description of the sound field is analyzed for shallow water with an Arctic type bottom: a liquid gas-filled sediment layer overlaying an elastic half-space (permafrost). It is determined that the optimum description of the field in the water layer at short distances from the sound source (order of 1÷10 depths of the waveguide) is achieved by using of the discrete spectrum of normal modes and quasi-modes calculated with use of the Pekeris cut. Computations of the transmission loss in the waveguide with the sediment layer thickness comparable to or higher than the acoustic wavelength in the sediments, shown that the seabed is acting as a uniform liquid half-space. If the thickness of the sedimentary layer is less than a quarter of the wavelength, then the layer can be neglected. The transmission loss abruptly increases with the sound speed in the sediments approaching the sound speed in the water. A method is proposed for estimating the sound speed in the sediment layer. The method based on the analysis of attenuation of the sound fields of higher modes.