A seismic and gravity study of the McGregor geothermal system, southern New Mexico

Abstract

Seismic and gravity studies have proven to be valuable tools in evaluating the geologic setting and economic potential of the McGregor geothermal system of southern New Mexico. An initial gravity study of the system demonstrated that a gravity high coincides with the heat‐flow high. A subsequent seismic reflection survey images a strong reflector, interpreted to be associated with a bedrock high that underlies the gravity and heat‐flow highs. A single reflection, which coincides with the water table, occurs within the Tertiary basin fill above bedrock. This reflector is subhorizontal except above structurally high bedrock, where it dips downward. This observation is consistent with well data that indicate a bedrock water table 30 m lower than water in the basin‐fill aquifer. Velocity models derived from seismic tomography show that the basin fill has velocities in the range of 800 to 4000 m/s and that the bedrock reflector coincides with high velocities of 5000 to 6000 m/s. Low‐velocity zones within the bedrock high are interpreted as karsted bedrock with solution‐collapse breccias and cavities filled with hot water. Higher velocity material that flanks the bedrock high may represent an earlier stage of basin fill or older alluvial‐fan deposits. The heat‐flow anomaly appears to be constrained to the region of shallowest bedrock that lacks these deposits, suggesting that they may act as an aquitard to cap underlying bedrock aquifers or geothermal reservoirs. Taken together, these observations suggest that the geothermal system is associated with karsted and fractured structurally high bedrock that serves as a window for upwelling and outflow of thermal waters. Thermal waters with a temperature as high as 89°C have the potential for space heating, geothermal desalinization, and small‐scale electrical production at McGregor Range.