New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

As part of an extensive study of the groundwater resources in the Salt Basin in south-central New Mexico, geophysical surveys that measure both natural and induced electrical and magnetic signals were used to explore the subsurface of the region. Audio-magnetotelluric (AMT) data were collected at twenty eight sites in the study area. The AMT technique utilizes naturally-occurring electromagnetic waves generated by lightning (high frequency) and the interaction of solar winds with the Earth’s magnetosphere (low frequency). Transient electromagnetic (TEM) data was collected at twelve sites. The TEM method measures the electrical resistivity of the subsurface using an applied current. The subsurface resistivity measured by both methods is affected by rock type, porosity, and groundwater salinity. Saline water, clays, and mineralized rocks conduct electric currents well (i.e., are conductive), whereas fresh water, dry anhydrite, and bedrock with little permeability or porosity are poor conductors (i.e., are resistive). The study was focused on Crow Flats and Otero Mesa in New Mexico. Crow Flats is underlain by gypsum-rich playa deposits. The water table is close to the surface and both the AMT and TEM data indicate that the shallow groundwater is conductive, consistent with the high total dissolved solids (TDS) content of the groundwater measured during previous water-quality studies. The geologic and hydrologic interpretation of the AMT data from Otero Mesa was more challenging because, as mentioned above, many factors besides TDS affect the resistivity of the subsurface. The data indicate that limestone in the shallow subsurface (<300 m) is resistive and probably contains fresh water. The interpretation is trickier at depths below 300 m, where at many sites, a conductive zone is recorded in the AMT data. The conductive zone could be associated with a clay-rich rock type (i.e., the Abo Formation) or brackish water. Luckily, several oil wells have been drilled in the Salt Basin and geophysical well logs and rock cuttings from those wells are on file at the New Mexico Bureau of Geology and Mineral Resources. Careful examination of the logs and cuttings revealed that in some instances, the conductive zone correlates nicely with the Abo Formation. However, in other cases, the conductive zone is in the overlying Yeso Formation, a unit that contains anhydrite, which is not conductive unless the calcium sulfate goes into solution. Thus, using evidence derived from the oil wells and AMT data, we hypothesize that brackish aquifers can be identified in the Yeso Formation using this combined approach. This investigation was reconnaissance in nature; the AMT sites were scattered over an area of about 250 km². Given the encouraging results, the next step is to conduct more focused experiments near the oil wells to test the interpretation.