Finite-difference modeling of seismoelectric logs

In this study, a finite-difference algorithm is proposed for simulating the seismoelectric logs. Without the negligible feedback effect of the Electromagnetic (EM) field on poroelastic waves, the equations for the electrokinetic coupling waves in a fluid-saturated porous medium are solved by two steps. The poroelastic wave is calculated first and then the EM field is calculated. The acoustic waves in the borehole and the poroelastic waves in strata are computed by using the finite-difference time-domain method with a velocity-stress staggered grid in axisymmetric cylindrical coordinates. The EM field is considered as a quasi-static field and obtained by using the finite-difference method with five-point formula to solve the Poisson equation for electric potential. The results show that the presented algorithm can simulate the seismoelectric logs accurately when the source frequency is lower than 6.0 kHz. Within the whole frequency range of logging, the low-frequency limit approximations of the conductivity, electrokinetic coupling coefficient and dynamic permeability do not influence the results significantly. The horizontal interfaces cause an electric field accompanying reflected Stoneley waves and significant interface EM waves. The latter has a potential application in exploring stratum interfaces nearby boreholes.