Experimental quantification of the seismoelectric transfer function and its dependence on conductivity and saturation in loose sand
Résumé
Under certain circumstances, seismic propagation within porous media may be associated to
a conversion of mechanical into electromagnetical energy known as a seismoelectromagnetic
phenomemon. The propagation of fast compressional P-waves is more specifically associated
to manifestations of a seismoelectric field linked to fluid flow within the pores. The analysis
of seismoelectric phenomena, which requires combining the theory of electrokinetics to Biot’s
theory of poroelasticity, provides us with a transfer function noted E/ü that links the coseismic
seismoelectric 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 in
unconsolidated quartz sand within the kilohertz range. The investigation focused on the impact
of fluid conductivity and water saturation over the coseismic seismoelectric field. During
the experiment, special attention was given to the accuracy of electric field measurements. We
concluded that, in order to obtain a reliable estimate of the electric field amplitude, the dipole
from which the potential differences are measured should be of much smaller length than the
wavelength of the propagating seismic field. Time-lapse monitoring of the seismic velocities
and seismoelectric transfer functions were performed during imbibition and drainage experiments.
In all cases, the quantitative analysis of the seismoelectric transfer function E=u was in
good agreement with theoretical predictions. While investigating saturation variations from the
residual water saturation to full saturation, we showed that the E/ü ratio undergoes a switch
in polarity at a particular saturation S, also implying a sign change of the filtration, traducing
a reversal of the relative fluid displacement with respect to the frame. This sign change at
critical saturation S stresses a particular behaviour of the poroelastic medium: the dropping
of the coseismic electric field to zero traduces the absence of relative pore/fluid displacements
representative of a Biot dynamically compatible medium.We concluded from our experimental
study in loose sand that measurements of the coseismic seismoelectric coupling may provide
information on fluid distribution within the pores, and that the reversal of the seismoelectric
field may be used as an indicator of the dynamically compatible state of the medium.
Domaines
Géophysique [physics.geo-ph]
Origine : Fichiers produits par l'(les) auteur(s)
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