Experimental Studies Of Electroseismic Conversion In A Fluid-Saturated Porous Medium

Abstract

The coupling between seismic and electromagnetic waves in a fluid-saturated porous medium is essentially controlled by the electrokinetic effect. The inverse effect of the seismoelectric conversion, the electroseismic conversion, is investigated experimentally in the laboratory. The electric field induces movement of the ions in a pore fluid relative to the solid matrix. The interaction between the pore fluid and the solid matrix generates acoustic waves known as electroseismic waves. Our studies confirm experimentally that the electroseismic conversion at ultrasonic frequencies is the electrokinetic effect in nature. In measurements with a homogeneous rock cylinder, P- and S-wave transducers receive, respectively, the P- and S- components of the extensional and flexural waves generated by an electric pulse with ring or parallel electrodes when the electrodes are on the surface or inside a porous medium. The electroseismic waves are measured in layered models, made of sandstone or artificial materials, to determine the area where the electroseismic waves are generated. Further experiments with the layered model investigate the relationship between electroseismic conversion and the conductivity of the fluid-saturated medium or the pore fluid. When fluid conductivity increases, the amplitude of the electroseismic wave increases. Experimental results show that electroseismic conversion is different from the piezoelectric effect of quartz grains in sandstone and is closely related to the relative motion between the fluid and the solid. The results also eliminate the possibility that the electroseismic wave is generated by a spark of a high-voltage pulse. Electroseismic waves can be generated at low voltage and increased continually with the voltage, without a big voltage jump similar to the spark in an isolated material. Our results confirm the existence and measurability of electroseismic conversion in porous formation at ultrasonic frequency ranges. Therefore, electroseismic measurements may be an effective means to investigate the pore fluid flow and rock properties.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumUnited States. Dept. of Energy (Grant DE-FG02-93ER14322