Magnetotelluric Investigation of Structures Related to a Geothermal Anomaly in the Buckman Well field in the Rio Grande Rift, New Mexico

High borehole temperature gradients have been measured over short spatial scales in the Buckman Well Field located within the Espanola Basin of the Rio Grande Rift, New Mexico. The proximity of the well field to the young Caja del Rio volcanic plateau prompted a study undertaken by the Summer of Applied Geophysical Experience (SAGE) program to uncover structure related to this geothermal anomaly. The localized nature of this geothermal anomaly is suggested to be indicative of a local controlling structure as opposed to a more regional structure. Two-dimensional (2-D) models were constructed using magnetotelluric (MT) and audiomagnetotelluric (AMT) data acquired during 2011-13 seasons of the SAGE field program. Geoelectric strike, being important in determining whether an optimal survey design was employed for 2-D MT inverse modeling, was determined from Swift';s formula, which is subject to galvanic distortion. The geoelectric strike direction obtained from a phase tensor analysis, unaffected by such distortion, generally agreed with the established geological strike of the region. The phase tensor analysis shows predominantly 2-D behavior, although some three-dimensional (3-D) character is observed in the low-frequency MT data. An independent statistical metric developed at SAGE confirms these findings. This observation could be reflected as a conductive anomaly found in the 2-D MT inverse model. Synthetic data were generated to test the sensitivity of the 2-D inversion method to different layer resistivity values and faulted structures in the AMT range. Using these synthetic results to understand the inversion of field data we identify conductive horizons at 100 m and 250-300 m depth. The MT models estimate basin depth at 3-4 km in accordance with independent constraints from geologic mapping, gravity models and seismic imaging. Variations in basement topography correlate to some degree with previously proposed structural features elsewhere beneath the Caja del Rio Plateau. Evidence of a horst block in the MT models is debatable; imaged basement topography may reflect fault-related down-dropping of the basement at the edge of the proposed horst. It may alternatively be a modeling artifact associated with statics shift distortion or 3-D effects in the low-frequency MT data. The localized nature of the anomalous geothermal gradients, however, suggests a local controlling structure as opposed to a more regional basement feature. Such a feature, if present, is not directly imaged by the MT and AMT models.