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The article consider the application of the extended poroacoustic theory of the propagation of stress waves in marine sediments to the calculation of the coefficient of sound reflection from the seabed. It is proposed to calculate the reflection coefficient using a simple acoustic formula, replacing the equilibrium density of the porous medium in it with the effective density. The main assumptions are given and the dispersion relations of the extended poroacoustic model of unconsolidated marine sediments are derived (GSED). Model GSED allows calculating the frequency dependences of the propagation velocity and the attenuation coefficients of sound and shear waves in marine sediments. Internal friction between granules and viscous losses due to relative fluid movement are taken into account. The adequacy of GSED model is shown by comparison with experimental data taken from open sources. The results of calculating the reflection coefficient within the framework of the proposed method are compared with experimental data.

Purpose. Propagation of a shear wave in sandy marine sediments is considered. The acoustic properties of a shear wave are the phase velocity and the attenuation coefficient. It is known that in dry sandy sediments, the attenuation coefficient is directly proportional to frequency. In the saturated mediums, there are the deviations from this law that implies existence of two physical mechanisms of losses – the intergranular friction and viscous loss. The study is aimed at developing a two-phase theoretical model of the shear wave propagation in the unconsolidated marine sediments, and at identifying the dissipative effects caused by the fluid relative movement in the pore space. Methods and Results. The intergranular friction is modeled using a springpot, which represents an element combing conservative properties of a spring and dissipative ones of a dashpot. The equation of motion is applied, where a part of fluid is assumed to be associated with the media solid phase and another part is considered to be mobile. For a harmonic displacement, the equations of state and the equation of motion yield a new two-phase dispersion relation (the theory of Grain Shearing + + Effective Density, or GS + EDs, for short). The results of the GS + EDs theory are compared with the data of the velocity and attenuation measurements taken from the open sources. It is shown that during propagation of the compressional and shear waves, the mechanisms of interaction between the granules, and between the granules and fluid are not similar. Character of the changes in the grain-tograin friction parameters when the pore space is saturated with fluid, is analyzed. Conclusion. Manifestation of the dissipative effects resulting from the pore saturation with fluid depends on the density of the granules packing. In case of a dense packing, there are no conditions for the fluid relative movement, and the sandy sediments exhibit the property of a constant Q-factor. If the packing is loose, the viscous losses make a significant contribution, and the attenuation frequency dependence is nonlinear. The effective pore sizes for the compression and shear waves do not coincide.

The paper proposes an efficient wave method for simulating the propagation of impulsive signals in hydroacoustic waveguides of the sea shelf. The method of normal modes calculates the acoustic field in a wide frequency band. Then the inverse Fourier transform of the acoustic field is performed and the impulse response of the waveguide is restored. The signal replica is then calculated as a convolution of the impulse response and the signal. The advantages of this approach are as follows. Convolution is cyclical – there are no restrictions on the duration of the signal. Not only calculated, but also experimentally determined impulse response can be used. At the discretion of the researcher, the fields of individual modes can be excluded, add noise in the frequency or time domain, simulate the movement of the source, the impact of wind waves. Restriction - conditions for uniformity of the waveguide along the distance. A number of examples are considered, in which the possibility of determining the acoustic properties of the bottom is studied.