Self-Potential Expression of Hydrothermal Resources in the Southern Rio Grande Rift, New Mexico

Abstract

Self potential (SP) surveys appear to have important use as a primary tool in the exploration and delineation of hydrothermal upflow zones in the southern Rio Grande rift, New Mexico. SP surveys were recently completed over three distinct types of geothermal system in the rift. At Rincon, a well defined SP anomaly about 1 km long with a minimum of -122 mill volts (mV) was mapped over the highest temperature gradients and immediately adjacent to a radon anomaly. Two other SP minima of -57 and -74 mV at Rincon are untested with temperature gradient drilling but correlate with radon anomalies. At Radium Springs, several negative SP anomalies were mapped in an area north of a known upflow zone that currently produces fluids for greenhouse heating. The more significant anomalies had amplitudes of -92 and -146 mV. Six Radium Springs SP anomalies are immediately adjacent or over radon soil gas anomalies, and limited temperature gradient information indicates at least one shallow upflow zone near the SP anomalies. The SP expressions of the Tortugas Mountain part of the Las Cruces East Mesa geothermal area is quite complex. Several negative anomalies can be attributed to a "caliche effect" where stations of low ridges capped by well developed, late stage caliche produce large (-100 mV) anomalies over distances less that 20 m. This "caliches effect" presents a major noise source for SP surveys in arid regions. Elimination of caliche stations and filtering allowed interpretation of several -10 to -40 mV anomalies that may be a subdued expression of a deep (-200 m) and broadly dispersed upflow zone(s).