Streaming Potential and Permeability of Saturated Sandstones under Triaxial Stress : Consequences for Electrotelluric Anomalies prior to Earthquakes
Résumé
The streaming potential, due to fluid circulation in rock, was measured on saturated sediments (Fontainebleau sandstones). The electrokinetic coupling coefficient, which is the ratio of the streaming potential and the excess pore pressure, is proportional to the fluid resistivity. Additionally, for a fluid conductivity of 10-3 S/m, the electrokinetic coupling coefficient varies from 10 mV/0.1 MPa to 6642 mV/0.1 MPa for sample permeability in the range of permeabilities from 0.15x10-15 m2 to 1220x10-15 m2. The different values of the electrokinetic coupling coefficient have been explained by the effect of increasing surface conductivity which becomes non negligible compared to fluid conductivity for low permeability. When the sample is deformed under triaxial stress up to failure, the vertical permeability (along the principal stress) drops about by 0.20% per 0.1 MPa when failure occurs. The typical variation of the electrokinetic coupling coefficient is a large increase beginning with the onset of the localization of the shear band at about 75% of the yield stress, and stopping at the failure. This increase of the electrokinetic coupling coefficient is due to an increase of Ζ potential in the shear zone when new surfaces are created and connected. Possible consequences of our results are given concerning the electrical fields which could appear during the preparation of an earthquake. It is shown that in some cases self-potential anomalies reported in the deformed zone preceding an earthquake occurence, could be due to an increase of the electrokinetic coupling coefficient from 75% of the yield stress to rupture, in the vicinity of one of the electrodes. Any variation of fluid resistivity or permeability in the vicinity of one electrode could change the electrokinetic coupling coefficient, inducing a surface electrokinetic potential anomaly. As regards to the interpretation of the electrokinetic effect which occurs at large distance from the epicenter a larger electrokinetic potential anomaly could be measured between electrodes situated along a vertical fluid flow, for instance in a shallow borehole. An electrokinetic potential anomaly up to 30 mV, for a fluid conductivity of 0.01 S/m and a rock permeability of 10-12 m2, could be observed with a change of the underground water table level as slight as 50 cm (50 mbar). Moreover if the permeability between the electrodes is increased by a factor of 8 10+3, the electrokinetic coupling coefficient could be enhanced by a factor up to 650.
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