Magnetotelluric Surveying and Monitoring at the Coso Geothermal Area, California, in Support of the Enhanced Geothermal Systems Concept: Survey Parameters, Initial Results

Abstract

Electrical resistivity may contribute to progress in enhanced geothermal systems (EGS) by imaging the geometry, bounds and controlling structures in existing production, and by monitoring changes in the underground resistivity properties in the vicinity of injection due to fracture porosity enhancement. To these ends, we are acquiring a dense grid of magnetotelluric (MT) stations plus contiguous bipole array profiling centered over the east flank of the Coso geothermal system. Acquiring good quality MT data in producing geothermal systems is a challenge due to production related electromagnetic (EM) noise and, in the case of Coso, due to proximity of a regional DC intertie power transmission line. To achieve good results, a remote reference completely outside the influence of the dominant source of EM noise must be established. Experimental results so far indicate that emplacing a reference a distance of 65 miles from the DC intertie in Amargosa Valley, NV, is still insufficient for noise cancellation much of the time. Even though the DC line EM fields are planar at this distance, they remain coherent with the non-planar fields in the Coso area so that remote referencing produces incorrect responses. We have successfully unwrapped and applied MT times series from the permanent observatory at Parkfield, CA, and these appear adequate to suppress the interference of the artificial EM noise. The efficacy of this observatory is confirmed by comparison to stations taken using an ultra-distant reference east of Socorro, NM. Operation of the latter reference was successful by using fast ftp internet communication between Coso Junction and the New Mexico Institute of Mining and Technology, using the University of Utah site as intermediary, and allowed referencing within a few hours of data downloading at Coso.