The Subsurface Electrical Conductivity and the Attenuation of Coda Waves at Las tres Virgenes Geothermal Field in Baja California Sur, Mexico.

Abstract

The presence of highly fractured rocks with fluids at high temperature can substantially increase the attenuation of the seismic waves as well as the electrical conductivity. The data collected in Las Tres Virgenes geothermal field by a seismological network, as well as by multiple geophysical surveys including: magnetotellurics (MT), transient electromagnetic (TDEM) and d.-resistivity vertical soundings (Schlumberger), allowed to outline the zone where the electrical conductivity and the attenuation of elastic waves are unusually high. The seismic activity was registered during 23 days by a portable array consisting of thirteen digital stations. We used 26 local events with magnitude ranging between 0 and 3.0 to estimate the attenuation associated to underground volumes traversed by different ray paths. The quality factor Q was estimated in the frequency range from 6 to 24 Hz. using the coda waves and a single scattering model. We use the magnetotelluric responses measured in 90 sites to estimate the subsurface distribution of the electrical conductivity in a depth range fro 0 to 3 km. The results suggest the presence of a highly attenuating and conductive zone along El Azufre Canyon: the boundary between Las Tres Virgenes complex and El Ahuajito complex. In that zone we found an abnormally strong reduction of the quality factor Q as the frequency decreases, causing attenuation values 3 0r 4 times larger than the values estimated at the rest of the area. Likewise, the same region we found electrical resistivity lower that 5m at depths ranging fro 250 to 2500 m. Both results are in agreement with the geological knowledge that there are intense rock fracturing and high temperatures at depth (240oC at 1142 m in well LV-2).