Self-potential studies at the cerro prieto geothermal field

Abstract

A reconnaissance self-potential survey conducted at the Cerro Prieto geothermal field revealed a dipolar anomaly of about 160 mV maximum peak-to-peak amplitude and covering an area of more than 400 km 2. The anomaly is centered close to the present production zone, and its axis runs nearly due north-south, passing roughly through the location of the present 75-MW power plant. Preliminary theoretical analysis shows that a horizontally dipolar subsurface charge distribution, whose intensity increases with depth along a vertical plane, produces anomalies similar to that seen at Cerro Prieto. Such a charge distribution may be produced by the vertical flow of either heat or fluid parallel to a vertical discontinuity, if either the thermoelectric or electrokinetic coupling coefficients change across the discontinuity. Stratigraphic and geophysical evidence indicate that the trace of a fault, which extends to a depth of about 3 or 4 km and is thought to provide a major conduit for the geothermal fluids, coincides with the axis of the self-potential anomaly a few hundred meters north of the power plant. Thus, a vertical flow of heat and/or fluid along this fault could provide an explanation for the observed self-potential anomaly if the magnitudes of the flows and the coupling coefficient differences are large enough. We are presently measuring thermoelectric and electrokinetic coupling coefficients of core samples from Cerro Prieto, and will use these values in conjunction with temperature, fluid flow and electrical-resistivity data from the field to calculate anomaly amplitudes to be compared with the observed self-potential data.