Monitoring sismique et sismoélectrique d'un milieux poreux non-consolidé

Abstract : Seismic propagation within porous media is usually associated with wave attenuation and dispersion phenomena related to fluid flow. Under certain circumstances, it may also be correlated to a conversion of seismic into electromagnetic energy known as “seismoelectric”. The understanding of seismoelectric phenomena, combining the theory of electrokinetic to Biot’s poroelasticity, relies mainly on the formulation by Pride [1994]. On basis of these “theoretical” developments, Pride and Haartsen [1996] defined a transfer function expressing the link between the coseismic seismoelectric field and the seismic acceleration at its origin. We propose a quantitative analysis of coseismic seismoelectric couplings with the purpose of validating Pride’s theory and generalizing it to partially saturated media. With this aim in view we developed a laboratory experiment involving an adjustable device for electric acquisitions, conducted within the kilohertz range on unconsolidated quartz sand. Experimental data were subsequently processed in both time and frequency domains, enabling a full analysis that embraces phase velocities, attenuations and transfer functions. The conducted experiments focused on the impact of fluid conductivity and water saturation with regard to the coseismic seismoelectric phenomenon. Time-lapse monitoring were accordingly run under varying salinity or water content. In all scenarios, the quantitative analysis of the electric-to-seismic amplitude ratio E/u¨ appeared in good agreement with theoretical projections. Moreover, investigations of saturation variations, ranging from the residual water saturation (Sw = 0.3) to full saturation, showed that: i) the behavior of attenuations and transfer functions are directly related to fluid distribution, that greatly impacts the mechanical properties of the medium ; ii) in the very peculiar case of unconsolidated media, polarity inversion of the coseismic seismoelectric field may be experienced.
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Julia Holzhauer. Monitoring sismique et sismoélectrique d'un milieux poreux non-consolidé. Géophysique [physics.geo-ph]. Université de Pau et des Pays de l'Adour, 2015. Français. ⟨tel-02377486⟩

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