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Communication Dans Un Congrès Année : 2014

Quantitative measurements of coseismic seismoelectric fields for varying salinity and water content

Résumé

Resulting from an electrokinetic coupling, the seismoelectric (SE) effect was identified at field scale in the 1940s. But for long, observations remained strictly qualitative due to the weakness of the resulting electromagnetic fields and the incomplete understanding of the underlying physics. Finally, by combining poroelastodynamic formulation derived from Biot's theory to Maxwell's equations, Pride proposed a robust explanation for the SE effect in 1994. Within a decade, a dynamic formulation of the coseismic transfer function depending on a dozen parameters of the medium had been given. Our purpose is to confront this model to measurements carried out on a simple porous medium at lab scale. In this experiment, a seismic wave generated by a pneumatic source propagates within unconsolidated silica-sand under varying water content and salinity. The source shows a wide frequency spectrum [0.5 – 5kHz] enclosing the Biot frequency, which marks the transition of viscosity to inertia-driven flow and announces the dynamic dependence of the SE transfer function. To this end, a 120l sandbox was equipped with 20 accelerometers, 5 water-sensors and a 30 rods electrode array, all placed with a maximum offset of 30cm to the source. Electric dipoles were defined relatively to the last electrode of the array to enable the reconstruction of any dipole-length afterwards. This allowed checking the likeness of a given seismic acquisition to various electric records differing in dipole length and position to determine the best fit. By changing salinity in the range [2-8mS/m] at constant saturation, we observed a decrease in the transfer function proportional to the salinity increase, consistent with the computed transfer functions and published results as well. This proved quantitative measuring possible. After a relaxation time, seismic velocities attested that full saturation had been reached, leading to a whole new field of investigation regarding SE dependence on water content.
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Dates et versions

hal-01224790 , version 1 (09-11-2015)

Identifiants

  • HAL Id : hal-01224790 , version 1

Citer

J Holzhauer, Clarisse Bordes, Daniel Brito, Bertrand Guatarbes, Jean-Paul Callot. Quantitative measurements of coseismic seismoelectric fields for varying salinity and water content. Réunion des Sciences de la Terre, Oct 2014, Pau, France. ⟨hal-01224790⟩
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