Three-Dimensional Delineation of the Pierre Shale Bedrock from Electrical Resistivity Surveys Conducted in Two Areas of Interest Within and Near Ellsworth Air Force Base, South Dakota, 2022

In 2022, the USGS, in cooperation with the U.S. Air Force Civil Engineer Center (USAF/CEC), performed geophysical surveys at two areas of interest within and near Ellsworth Air Force Base (EAFB) to delineate the Cretaceous Pierre Shale bedrock. Electrical resistivity tomography (ERT) data were collected in two grid patterns using the dipole-dipole array within and near EAFB and site information—including latitude, longitude, and elevation—were surveyed along geophysical transects comprising the grids using real-time kinematic (RTK) techniques. The purpose of collecting ERT data in grids rather than along individual transects was to create three-dimensional (3D) surfaces of the bedrock, which improve site characterization by providing higher resolution images of the bedrock compared to transects that provide only two-dimensional (2D) information. ERT data were processed using EarthImager2D software from Advanced Geosciences, Inc. (https://www.agiusa.com/agi-earthimager-2d) to remove noisy measurements and produce subsurface electrical resistivity profiles. Monitoring wells and soil borings with depth to bedrock information from the South Dakota Department of Agriculture and Natural Resources and Ellsworth Air Force Base were combined with subsurface electrical resistivity profiles to delineate the bedrock surface elevation along each transect in the grids. After bedrock delineation was completed, interpolation techniques were used to construct three-dimensional bedrock elevation surfaces for each grid.

Unprocessed and processed geophysical are provided in zipped folders (.zip) as machine- and user-readable files with accompanying text files that explains the content of each file. Site information from RTK surveys, bedrock delineation results along each transect in the two grids, and monitoring wells or soil borings used to aid delineation are included as comma-separated values (.csv) files.