HR: 0830h
AN: T51E-0204    [PDF]
TI: Observations of Seismoeletric Signals Induced by a Hydro-fracturing Experiment in a Deep Geothermal Reservoir
AU: * Marquis, G
EM: guy.marquis@eost.u-strasbg.fr
AF: EOST-IPGS, 5 rue Rene Descartes, Strasbourg, 67084 France
AU: Darnet, M
EM: mathieu.darnet@eost.u-strasbg.fr
AF: EOST-IPGS, 5 rue Rene Descartes, Strasbourg, 67084 France
AU: Michelet, S
EM: michelet@soultz.net
AF: EEIG Heat Mining, Route de Soultz, Soultz-sous-Forets, 67250 France
AU: Baria, R
EM: baria@soultz.net
AF: EEIG Heat Mining, Route de Soultz, Soultz-sous-Forets, 67250 France
AB: During a hydro-fracturing experiment at the Soultz-sous-For\^ets Hot Dry Rock site, more than 100,000 microseismic events of magnitude greater than -2.0 were induced by the continuous injection of 30,000 m$^3$ of fresh water at 5 km depth. At the same time, we carried out monitoring of surface electric fields at a sampling rate of 2 kHz with two pairs of unpolarizable electrodes. After removal of the man-made noise, we observed strong electric field perturbations associated to the 48 microseismic events of magnitude greater than 1.8. Their maximum amplitude is 20 mV for the largest event (M = 2.7) while the background electrical noise is roughly 70 mV. The start of these electric perturbations coincides with the P-arrival time of the seismic waves at a depth of 1.5 km i.e. roughly half a second before the surface arrival time and their duration is about one second. As the sediments - granite interface is located at the same depth, the source of these electromagnetic signals could be an electroseismic conversion at this high acoustic impedance contrast. Moreover, a detailed analysis of the electric waveform reveals that several electric phases are arriving on the surface after the first pulse which may be caused by electroseismic conversions within the sediment layers. We did not however observe any electric field perturbations prior to rupture and the alleged first pulse associated with piezoelectric effect. It seems therefore that the prevailing effect when monitoring high-frequency ($\geq$ 1 Hz) synearthquake EM phenomena is seismoelectric.
DE: 0925 Magnetic and electrical methods
DE: 7223 Seismic hazard assessment and prediction
SC: Tectonophysics [T]
MN: 2003 Fall Meeting