Electrokinetic conversion

Abstract

In their search for improved and new exploration tools, geophysicists have improved seismic and electromagnetic techniques. Since the 1930s it is known that there is a coupling between seismic and electromagnetic waves in the shallow subsurface of the earth. Electroseismic surveying and its reciprocal process, seismic-to-electromagnetic conversion, are methods for remotely identifying the presence of hydrocarbons in the subsurface of the earth. In this study we investigate electrokinetic coupling theoretically and experimentally. The origin of this effect lies in a very thin nano-layer which is conventionally present at all solid-fluid interfaces where an excess charge density with respect to the bulk charge density in the pore fluid exists. Any hydraulic disturbances of this nano-layer cause electric currents that are opposed by ionic counterflows generating electric fields. These effects become manifest when acoustic waves impinge upon the interface between two adjacent porous layers having different electro-mechanical properties. The theoretical basis for the coupling phenomena under investigation is conceptually imbedded in the framework of the combined Biot-Maxwell equations. An important aspect of the theory is the so-called dynamic (i.e., frequency-dependent) coupling factor. This coupling factor is studied in a dedicated experimental set-up where an oscillating flow through a porous material generates electric fields. We address the frequency-dependency of the coupling coefficient, the mathematical description and experimental detection results. We validate that the governing model is capable of modeling the coupling effect.