In a porous material saturated by a fluid electrolyte, mechanical and electromagnetic disturbances are coupled. The coupling is due to an excess of electrolyte ions that exist in a fluid layer near the grain surfaces within the material; i.e., the coupling is electrokinetic in nature. The governing equations controlling the coupled electromagnetic-seismic (or ‘‘electroseismic’’) wave propagation are presented for a general anisotropic and heterogeneous porous material. Uniqueness is derived as well as the statements of energy conservation and reciprocity. Representation integrals for the various wave fields are derived that require, in general, nine different Green’s tensors. For the special case of an isotropic and homogeneous wholespace, both the plane-wave and the point-source responses are obtained. Finally, the boundary conditions that hold at interfaces in the porous material are derived.

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Electroseismic phenomena in porous media, first observed almost 60 years ago (Ivanov, 1939), were recently "rediscovered" due to their potential to detect zones of high fluid mobility and fluid chemistry contrasts in the subsurface (Thompson and Gist, 1993;Haartsen et al., 1995). However, a limited number of field studies of these phenomena reported in the literature were not able to support the results with an explicit comparison to theoretical predictions. In this paper, we demonstrate that electroseismic phenomena in porous media can be observed in the field, explained, and modeled numerically, yielding a good agreement between the field and the synthetic data.We first outline the design of our field experiment and describe the procedure used to reduce noise in the electroseismic data. Then, we present and interpret the field data, demonstrating how and where different electroseismic signals originated in the subsurface. Finally, we model our field experiment numerically and demonstrate that the numerical results correctly simulate arrival times, polarity, and the amplitude-versusoffset behavior of the electroseismic signals measured in the field.

Abstract. When seismic waves generate a relative fluid-solid motion in a fluidsaturated porous medium, the moving charges (streaming current) in the electric double layer induce an electroma. gnetic (EM) field. This paper first experimentally confirms that the coupling between the seismic wave and the electromagnetic field in the kilohertz range is electrokinetic in nature. Seismoelectric signals are measured in homogeneous cylindrical porous rock samples and multilayered models. The seismoelectric signals in homogeneous rock are electric fields that move along with the acoustic wave. The mechanism of the seismoelectric conversion is completely different from the piezoelectric effect of quartz grains. The seismoelectric sensitivity with respect to salinity of the saturant has been experimentally determined. The amplitude of seismoelectric signals increases as the saturant conductivity decreases. The seismoelectric effects are generated by two different mechanisms. Both the EM radiation and the electric potential generated at an interface and within a porous medium, respectively, were measured as the P wave, at ultrasonic frequencies, passes through the layered models. Our experimental results demonstrate that seismoelectric effects exist and are measurable in the kilohertz range. The paper concludes with a comparison of experimental data and modeled data in a threelayer porous model. Seismoelectric measurements could be an effective means of obtaining transport coefficients such as hydraulic permeability and other porous rock properties.

Experimental and theoretical studies show that there are electromagnetic (EM) fields generated by seismic waves with two kinds of conversion mechanisms in a fluid‐saturated, porous medium. Within a homogeneous formation, the seismic wave generates a seismoelectric field that exists only in the area disturbed by the seismic wave and whose apparent velocity is that of the seismic wave. At an interface between differing formation properties, the generated seismoelectric wave is a propagating EM wave that can be detected everywhere. An electrode, used as a receiver on the ground surface, can detect the propagating EM wave generated at an interface, but cannot detect the seismoelectric field generated in a homogeneous formation. When the electrode is in a borehole and close to a porous formation, it can detect both the EM waves and the seismoelectric field. In this paper, electrokinetic measurements are performed with borehole models made of natural rocks or artificial materials. Experimental results show that the Stoneley wave and other acoustic modes, excited by a monopole source in the borehole models, generate seismoelectric fields in fluid‐saturated formations. The electric components of the seismoelectric fields can be detected by an electrode in the borehole or on the borehole wall. The amplitude and frequency of the seismoelectric fields are related not only to the seismic wave, but also to formation properties such as permeability, conductivity, etc. Comparison between the waveforms of the seismoelectric signals and acoustic logging waves suggests that seismoelectric well logging may explore the different properties of the formation. Electroseismic measurements are also performed with these borehole models. The electric pulse through the electrode in the borehole or on the borehole wall induces Stoneley waves in fluid‐saturated models that can be received by a monopole transducer in the same borehole. These measurement methods (seismoelectric logging or electroseismic logging) might directly apply to well logging to investigate formation properties related to the pore fluid flow.