Dynamic streaming currents from seismic point sources in homogeneous poroelastic media

Abstract

SUMMARY In a porous medium saturated with a £uid electrolyte, mechanical and electromagnetic disturbances are coupled. The coupling is electrokinetic in nature. The seismic waves generate relative £uid^solid motion that induces an electrical streaming current. Mechanically induced streaming currents generated by point sources in homogeneous, isotropic porous media are presented. The electrically induced streaming current is shown to be second order in electrokinetic coupling coe⁄cient and is neglected. This decouples the mechanical behaviour from the electromagnetic behaviour withrespect to the induced £uxes.We have used Biot theory to calculate the amount of induced relative £ow by Green’s function solution. The transport coe⁄cients, conductivity, dynamic permeability and electrokinetic coupling coe⁄cient, and their sensitivities with respect to porosity, DC permeability and frequency changes are evaluated. The conductivity decreases with increasing DC permeability. It has a k {1=2 0 dependence when grainsurface conductances dominate over the bulk-£uid-phase conductivity. Stationaryphase relative-£ow and streaming current solutions are calculated for point sources acting on both phases of the porous medium. The streaming currents are induced by both P and S waves.The streaming current decreases with increasing £uid conductivity. This is consistent with the decrease of the diiuse double-layer thickness and f-potential. The porosity aiects the bulk moduli of the solid. Its eiect combined with the frame bulk modulus and compressibility of the saturating £uid determines the streaming current amplitude induced by a P wave.The increase in streaming current amplitude induced by S waves with increasing porosity is due to the decrease of the shear frame modulus with increasing porosity.The streaming current behaviour with respect to DC permeability is found to diier for sources applied to the elastic frame and volume-injection sources.