Seismo-electromagnetic conversions in porous media – TRANSEK

Transient seismo-electromagnetic phenomena are especially appealing to hydrogeophysics because of their potential to characterize the fluids contained in the reservoir rocks with the resolution of seismic methods. Indeed seismo-electromagnetic tomography is expected to combine the sensitivity of electrical properties to water-content and permeability, to the high spatial resolution of seismic surveys, which can not be achieved with other geophysical methods. To be able to develop the potential of this innovative method, a better understanding of the physical processes and a reliable quantification of the conversion between seismic energy and electric energy are necessary. Moreover a suitable interpretation of the observations, especially in the shallow subsurface, needs to take into account the water-content, the rock conductivity, and the water conductivity. Electrokinetic coupling phenomena can be created at the microscopic scale when there is a relative motion of electrolyte ions with respect to the mineral surface (grains). Seismic wave propagation in fluid-filled porous media generates conversions from seismic to electromagnetic energy which can be observed at the macroscopic scale, due to this electrokinetic coupling at the pore scale. The main objective of this project is to perform detailed studies of these seismoelectric coupling effects, both experimentally and theoretically, to develop an analysis of the observed signals, in order to improve our understanding of these phenomena and to assess their potential use for the characterization of the fluid contents of porous media. We propose the following specific aims: 1) develop specific instrumentation to be able to detect the exact magnitude of the signals: preamplifiers, that are not currently manufactured, have to be developed. After testing different ways to improve the signal to noise ratio and the electrical impedance, we should be able to report for the geophysical community the most appropriate development. These developments should be performed for field measurements, when we use a seismic source at frequencies below hundreds Hz. This is a key point in order to be able to compare the exact magnitude of the seismoelectric conversions between field and laboratory observations, to the results described in the literature. 2) provide a robust law for the evolution of the seismoelectric coupling with the water content through laboratory experiments, both with steady-state measurements and direct seismoelectric measurements. In light of the results of these two approaches, we should be able to provide a macroscopic law for the seismoelectric coupling. Finding the behavior of the seismoelectric coupling with water-content will clear the current ambiguity on this subject. This is of prime importance for modeling and interpretation of the field observations dealing with the uppermost layers of the earth, since they are generally not fully saturated. 3) improve the signal processing (multiscale signal processing), not only to remove the anthropogenic noise, but also to remove the coseismic signal when looking for a seismoelectric conversion linked to an interface. 4) use combined geophysical methods for seismoelectric field studies by: - performing regular experiments on two sites, to follow the evolution of these seismoelectric couplings with the evolution of rock resistivity, water resistivity, water content, and water table level, studying both the coseismic part of the seismoelectric phenomena and the converted part from an interface. This aim is to point out the most pertinent parameters to which the seismoelectric coupling is sensitive, and to test the current theory. - and interpreting the observations performed on site 'La Soutte' and site 'Larreule' using direct modeling and joint inversion with electrical resistivity tomography and piezometric data, using Ground Penetrating Radar (GPR), and water-content dependence of the seismoelectric couplings deduced from laboratory studies. We expect to characterize the fluids contained in these two field sites with the resolution of seismic methods.