International audienceUnder certain circumstances, seismic propagation within porous media may be associated toa conversion of mechanical into electromagnetical energy known as a seismoelectromagneticphenomemon. The propagation of fast compressional P-waves is more specifically associatedto manifestations of a seismoelectric field linked to fluid flow within the pores. The analysisof seismoelectric phenomena, which requires combining the theory of electrokinetics to Biot’stheory of poroelasticity, provides us with a transfer function noted E/ü that links the coseismicseismoelectric field E to the seismic acceleration u. In order to measure the transfer function,we have developed an experimental set-up enabling seismoelectric laboratory observation inunconsolidated quartz sand within the kilohertz range. The investigation focused on the impactof fluid conductivity and water saturation over the coseismic seismoelectric field. Duringthe experiment, special attention was given to the accuracy of electric field measurements. Weconcluded that, in order to obtain a reliable estimate of the electric field amplitude, the dipolefrom which the potential differences are measured should be of much smaller length than thewavelength of the propagating seismic field. Time-lapse monitoring of the seismic velocitiesand seismoelectric transfer functions were performed during imbibition and drainage experiments.In all cases, the quantitative analysis of the seismoelectric transfer function E=u was ingood agreement with theoretical predictions. While investigating saturation variations from theresidual water saturation to full saturation, we showed that the E/ü ratio undergoes a switchin polarity at a particular saturation S, also implying a sign change of the filtration, traducinga reversal of the relative fluid displacement with respect to the frame. This sign change atcritical saturation S stresses a particular behaviour of the poroelastic medium: the droppingof the coseismic electric field to zero traduces the absence of relative pore/fluid displacementsrepresentative of a Biot dynamically compatible medium.We concluded from our experimentalstudy in loose sand that measurements of the coseismic seismoelectric coupling may provideinformation on fluid distribution within the pores, and that the reversal of the seismoelectricfield may be used as an indicator of the dynamically compatible state of the medium

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