| Abstract |
Changes in fluid pathways in the subsurface of a geothermal project during stimulation and operation are mainly recovered from micro-‐seismic monitoring. Micro-‐seismicity provides the location of fractures shear and open, but neither on fracture connectivity nor on the fluid content. Electromagnetic methods however are sensitive to conductivity contrasts and are typically used as a complementary tool to delineate reservoir boundaries (E.G. Geiermann et schill, 2010). In this respect, in july 2011, an injection test for a ~3.6km deep egs at paralana, south australia, was monitored by both micro-‐seismic and magnetotellurics (Peacock et al., 2012). First results from continuous magnetotelluric (Mt) Measurement suggest transient variations in subsurface conductivity structure generated from the introduction of fluids at depth. Furthermore, phase tensor representation of the time dependent mt response suggests fluids migrated in northeast direction from the injection well. Results from this experiment support the extension of mt to a monitoring tool for not only enhanced geothermal system (Egs) But also for other hydraulic stimulations. Magnetotellurics is developing as a monitoring technique able to enhance changes in underground fluids or pore structures (See e.G. Peacock et al, 2012). In first attempts to use mt monitoring, classical parameters such as mt impedance or apparent resistivities have been used over volcanoes in order to show the relationship to volcanic activity (E.G. Wawrzyniak, 2011). However in some cases (Eg. Time changes of local shallow conductivity heterogeneities, I.E. Well known galvanic problem), these classical quantities may yield misinterpretations because of their sensitivity to distortions; So we follow geothermal monitoring approaches based on phase tensors (Thiel et al. 2011; Peacock et al. 2013) Rather than resistivity and phase analysis. We consider the methodology of thiel & peacock and add uncertainty estimates, with tests on our data sets collected in 2014 at the end of the first drilling experiment. Especially, we consider the possibility of mt in the monitoring of a new enhanced geothermal site, at rittershoffen geothermal area apart from micro seismicity, this may contribute to the hydro – thermo – mechanical modelling and in hazard assessment. Test productions of the double wells are conducted in july-‐october at mean depth of 2.5-‐2.8 km. In this paper, we present the results obtained from continuous mt monitoring at ritt site (Fig. 1) Around rittershoffen geothermal site, northern alsace before and after main hydraulic/Chemical stimulations, which held in july-‐october. |