A high‐precision gravity survey in the Delaware Basin of southeastern New Mexico

Abstract

Since 1974, the Department of Energy has been studying bedded salt deposits in southeastern New Mexico as a possible location for disposing of defense‐generated transuranic and low‐level radioactive wastes. The program, known as the Waste Isolation Pilot Plant, includes intensive geologic characterization of about [Formula: see text] and construction of an underground test facility. The gravity survey reported here is part of the geologic site characterization. The gravity survey was conducted to delineate structural features near and at the proposed site. However, during the survey the gravity field was found to be dominated by effects of lateral density variations within relatively flat‐lying strata. Particularly distinctive is a pattern of elongate negative anomalies about one‐half mGal in amplitude. Boreholes in the anomalies encountered normal stratigraphy and no unusual geologic structures. However, borehole densilogs showed lower densities and uphole velocity surveys showed lower acoustic velocities than are measured outside of the anomalies. The low densities adequately account for the observed gravity anomalies. The regional stratigraphy contains water‐soluble minerals (halite, polyhalite, anhydrite‐gypsum, carbonates). Much of this material has dissolved and the region has been identified as a karstland. At the site, dissolution is slowly affecting the Rustler formation overlying the main salt‐bearing units. The low rock densities, associated with the negative gravity anomalies, are interpreted as due to alteration in the vicinity of solution conduits within the Rustler formation. This interpretation is supported by (1) partial coincidence between the negative gravity anomalies and closed topographic depressions (alluvial dolines); (2) greater anhydrite‐to‐gypsum conversion detected in boreholes within the anomalies; and (3) solution conduits encountered in one of the boreholes.