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