Multilayered model for the electro-kinetic effect

The electro-kinetic effect represents a class of processes in which there is a conversion from electromagnetic to kinetic energy and vice versa. In the case of this transfer taking place in a saturated porous medium we name the effect electro-seismic for the transfer from electromagnetic to kinetic energy, and seismo-electric for the transfer from kinetic to electromagnetic energy. There exist other effects sometimes called electro-kinetic like the piezo-electric effect or the modulation of rock resistivity by seismic waves, but we will not take those effects into account in this paper. This interaction between seismic and electromagnetic waves is due to the relative motion of the electrically charged ions in the pore fluid. When in equilibrium a porous medium saturated with an electrolyte is electrically neutral, but if a wave, seismic or electromagnetic, perturbs this equilibrium, the relative motion of the ions in the pore fluid will generate both seismic and electromagnetic waves. In the case of a passing seismic wave the flow of ions and the consequent electric imbalance generates electromagnetic waves. If the passing wave is electromagnetic there will be an induced flow of fluid in the pores that will be transmitted as a seismic wave. The existence of the seismo-electric conversion is known since the early 1930s, however in all the published papers the main topic is the generation of an electromagnetic wave as a fast P wave hits an interface (usually the water table), showing this method to be a useful tool to characterize parameters like fluid content and fluid geochemistry in the Earth's subsurface. Although the seismo-electric effect is the most known, in our model we deal with it as a particular case of the electro-kinetic effect, which contains all the interactions between seismic and electromagnetic waves in a layered porous medium. It is then very interesting to look at the possibilities offered by the use of shear waves as well as the not so well known electro-seismic effect, which could be a very promising prospecting tool. Possible applications include groundwater detection and monitoring of pollutant migration. This effect can possibly be also employed in borehole measurements as a way to determine permeable formations or monitoring multiphase flow through porous areas.