Conductivity Dependence of Seismoelectric Wave Phenomena in Fluid-Saturated Sediments

Seismoelectric phenomena in sediments arise from acoustic wave-induced fluid motion in the pore space, which perturbs the electrostatic equilibrium of the electric double layer on the grain surfaces. Experimental techniques and the apparatus built to study the conductivity dependence of the electrokinetic (EK) effect are described and outcomes for studies in loose glass microspheres and medium-grain sand are presented. By varying the NaCl concentration in the pore fluid, we measured the conductivity dependence of two kinds of EK behavior: (1) the electric fields generated within the samples by the passage of transmitted acoustic waves, and (2) the electromagnetic waves produced at the fluid–sediment interface by the incident acoustic wave. Both phenomena are caused by relative fluid motion in the sediment pores—this feature is characteristic of poroelastic (Biot) media, but not predicted by either viscoelastic fluid or solid models. A model of plane-wave reflection from a fluid–sediment interface using EK–Biot theory leads to theoretical predictions that compare well to the experimental data for both loose glass microspheres and medium-grain sand.