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We obtain the amplitude and energy reflection coefficients of seismic waves in porous media with penny-shaped inclusions, based on the generalized Biot-Rayleigh model that takes into account the attenuation due to mesoscopic local fluid flow (LFF). We consider two cases, including a contact between two porous media having either different fluids (gas–water contact) or crack density and aspect ratio, as well as a water half-space overlying a porous medium, and study the frequency-dependent reflection-transmission (scattering) coefficients for open- and sealed-pore boundary conditions. Our examples show that the LFF mechanism mainly reduces the reflection coefficients (amplitude and energy) at the gas–water contact and at a water–porous medium interface for frequencies less than 10 kHz, due to the fact that the velocity in the lower medium decreases. For the latter case, if the fluid is gas, the LFF effect becomes only important at frequencies between 0.0001 and 10 Hz for the open-pore case. This is due to the fact that the acoustic impedance contrast between water and gas is high. At frequencies less than 0.0001 Hz, the interface is equivalent to a water–elastic medium one, and hence the results are the same as those of the sealed-pore case. Moreover, the crack density and aspect ratio affect the mesoscopic attenuation and relaxation frequency, and therefore the reflection coefficients. [ABSTRACT FROM AUTHOR]

The correct simulation of wave propagation with direct grid methods at fluid–solid interfaces requires a proper implementation of the boundary condition, which involves continuity of the normal component of the particle velocity and traction. The horizontal component of the particle velocities can differ (slip) and the tangential traction vanishes. The challenge is to model the interface waves, which are relevant when the source and receiver are located close to the interface, in particular the Scholte and leaky (or pseudo, or generalized) Rayleigh waves. The explicit modelling is based on a domain-decomposition technique (one mesh for the fluid and another mesh for the solid) and the Fourier and Chebyshev differential operators along the horizontal and vertical directions, respectively. On the other hand, two implicit modelling algorithms are based on a single mesh and the fluid is described by setting to zero the shear wave velocity above the interface. The modelling is verified by comparisons with the analytical solution for a fluid–solid interface in lossless media, with source and receiver away from and at the interface. To our knowledge, the latter comparison, mainly involving the interface waves (Scholte and leaky-Rayleigh), has not been performed with other grid methods so far. It is shown that the implicit simulation yields erroneous results. [ABSTRACT FROM AUTHOR]

Seismic reflections at the interface of heterogeneous porous media are significantly affected by the multi-scale fluid-flow losses. In this work, we study the seismic reflection dispersion from the ocean bottom separating seawater and heterogeneous porous seabed, described by the effective Biot theory where the mesoscopic loss is present. The analytical reflection coefficient is obtained based on the displacement potentials and suitable boundary conditions. Two reflection scenarios, including the water/water-bearing medium contact and water/gas-saturated medium interface, are considered. The shear-wave attenuation is also considered by using a Cole-Cole complex shear modulus. Numerical results showing the variation in reflection magnitude versus frequency and incidence angle are presented, revealing that the P-wave mesoscopic attenuation, complex shear modulus, boundary conditions and permeability can significantly affect the frequency and angle dependence of the reflection coefficients. Reflections at the water/gas-saturated medium interface exhibit quite different behaviors from those at the water/water-bearing medium contact. The open-pore interface gives frequency-dependent reflections, whereas in sealed-pore case, the reflection magnitudes remain unchanged for all frequencies, due to the fact that the interface degrades into a water/elastic solid one. The study can be relevant for reservoir prediction and fluid identification based on the reflected marine data. • Seismic reflection dispersion at the ocean bottom in the presence of mesoscopic loss is analyzed. • Impact of saturated fluid in porous seabed is investigated. • Effects of mesoscopic loss, complex shear modulus and interface conditions are discussed. [ABSTRACT FROM AUTHOR]