Numerical Study Of Acoustic Field Of Heterogeneous Poroelastic Media Fully Saturated By Single Fluid

Study of elastic wave propagation in porous media fully saturated by one fluid is of great importance in many areas, such as geophysical exploration, sonic logging and reservoir engineering. Investigation and analysis of elastic wave propagation in complex media by means of numerical simulation methods also play very important role. Combination of elastic wave theory and numerical modeling study can further offer basic information for understanding and interpreting the seismic wave characteristics.On basis of Biot's poroelastic theory, the velocity-stress stagger-grid finite difference scheme was established. The perfectly matched layer (PML) technology which is efficient and stable method is applied as absorbing boundary condition to eliminate or diminish spurious the reflections from the artifact boundaries. The numerical results were validated by using analytical solutions in a homogeneous poroelastic model. The efficiency of the PML is highly satisfactory. Good numerical results can be obtained only if the depth of the PML is greater than the half of the wavelength of the fast velocity. The PML was compared with the damping factor absorption boundary condition, which shows that the former is better that the latter. During the course of writing codes, generally speaking, there are two different approaches to realize PML absorbing boundary condition. One is referred to as the non-uniform method, while another as the uniform method. From the view of point of the efficiency of damping the outgoing waves, the two methods are investigated. The numerical results show that effectiveness of the uniform method is slightly better than that of the non-uniform method, and the uniform method need more computer memory and time.Some numerical experiments are performed with the scheme developed in this paper, and some significant conclusions are acquired. In the higher frequency range, the attenuation of the Biot's slow wave is small, and the velocity of the slow wave is less than the velocity of fluid in pores. At low frequencies, the slow wave appears as static mode as its low velocity and strong attenuation, so it can not be observed in geophysical exploration. What is more, when heterogeneous media are taken into account, fast wave or shear waves can be converted into slow waves at interfaces, and the slow waves are rapidly attenuated. These waves should be taken into account in evaluating wave attenuation. The velocities of fast and slow waves increase and decrease, respectively, with decreasing porosity. And the attenuation of the slow wave increases as porosity decreases. Fluid viscosity effects dominate over inertial effects. The dissipation of the slow wave increases with increasing fluid viscosity. The fully saturated porous material behaves as a single medium when the saturating fluid is oil. The variation of the permeability mainly influences the slow wave. The effect is the inverse of that of the viscosity, i.e., lower permeability implies stronger dissipation of the slow wave.