Reflection And Transmission Of Elastic Waves By Sandwich Structure With Fluid-filled Porous Medium

Many resources,such as oil,gas and water,are stored in underground porous media.It has important theoretical and practical significance for the geophysical exploration of such resources to study the elastic wave propagation in fluid-saturated porous media.Biot first established the elastic wave dynamics theory in this kind of medium.However,in many cases,the dispersion and attenuation predicted by Biot theory are lower than the measured values.In order to accurately describe the elastic wave properties in fluid-saturated porous media,the influences of the viscoelasticity of the solid frame on the elastic wave in fluid-saturated/unsaturated porous medium,on the the seismoelectric waves in fluid-saturated porous medium,the influences of non-Darcy interporosity flow on the elastic wave in fluid-saturated double-porosity medium are investigated.The reflection and transmission problems of elastic waves through a porous medium sandwiched between two elastic solid half-spaces are investigated.The specific research contents are as follows:(1)The reflection and transmission problems of elastic waves through a liquid-saturated poroelastic interlayer are investigated.The reflection and transmission coefficients are derived based on the BISQ model,which incorporate both the Biot and squirt-flow mechanisms.Moreover,the viscoelasticity of the solid frame and the pore fluid are also considered.The fractional Zener model and the fractional dashpot model are adopted for the solid frame and the pore fluid,respectively.It is observed that the reflection and transmission coefficients and the attenuation due to the sandwiched interlayer are affected noticeably by the effect of the squirt flow,the viscoelasticity of the solid frame.(2)The reflection and transmission problems of elastic waves through an unsaturated porous interlayer are investigated.The solid frame is modelled by the fractional Zener model where the fractional order derivatives are used to describe the complex history-dependent viscoelastic behavior.It is observed that the reflection and transmission coefficients and the energy dissipation ratios are affected noticeably by the gas saturation,the interlayer thickness and the viscoelasticity of the solid frame.In particular,there exists the characteristic gas saturation where the energy dissipation increases drastically.(3)The reflection and transmission behaviors of elastic waves through a fluid-saturated double-porosity interlayer are studied.In order to simulate the interporosity flow,which cannot be appropriately described by the Darcy’s law,a modified transport equation using fractional order derivative is introduced to the double-porosity model for the wave propagation in fluid-saturated double-porosity solid.It is observed that the reflection,transmission and the energy dissipation behavior are affected noticeably by the interporosity flow.(4)The reflection and transmission of seismoelectric waves through a fluid-saturated interlayer are studied.In order to simulate the strong attenuation simultaneously at seismic and sonic frequencies,a new fractional viscoelastic model for the solid frame is introduced to the Pride’s seismoelectric coupling theory.The influences of the electrolyte concentration,the permeability and the relative electric permittivity of pore fluid as well as the tortuosity on the seismoelectric waves are mainly studied.It is observed that those parameters mainly affect the reflection and transmission coefficients of electromagnetic wave while have negligibly influence on the seismic waves.The porosity and the viscoelasticity of solid frame have evident influences not only on the electromagnetic waves but also on the seismic waves.The above studies show that in the theory of elastic waves propagating in porous medium,it is more flexible and precise to describe the attenuation and dispersion of elastic wave propagation by using the fractional model to describe the viscoelasticity of the solid frame,using the fractional interporosity flow equations to simulate the interporosity flow behavior that cannot be described by Darcy’s law,and using the fractional multiscale model to simulate the strong attenuation simultaneously at seismic and acoustic frequencies.Thus it can provide theoretical guidance for the geophysics exploration of oil and gas resources stored in porous medium.