Electrical Resistivity Tomography (ERT) and Horizontal-to-Vertical Spectral Ratio (HVSR) Data Collected Within and Near Ellsworth Air Force Base, South Dakota, from 2014 to 2019

This data release contains geophysical data from electrical resistivity tomography (ERT) and passive seismic surveys within and near Ellsworth Air Force Base, South Dakota, collected June 2014?September 2019. The data accompanies a USGS scientific invesgations map from Medler and Anderson (2021; LINK HERE) and a USGS scientific investigations report by Bender and others (2021; LINK HERE). The U.S. Geological Survey, in cooperation with the U.S. Air Force Civil Engineering Center, investigated the use of surface geophysical methods to delineate the top of the Cretaceous Pierre Shale along survey transects in selected areas within and near Ellsworth Air Force Base, South Dakota. In 2014, four ERT surveys were performed at the Fuels Area C site on Ellsworth Air Force Base. In 2019, ERT and passive seismic surveys were performed along 26 co-located survey transects within and near Ellsworth Air Force Base for a total of 12.7 line-kilometers. ERT and passive seismic surveys were co-located for direct comparison and to improve interpretation of the Pierre Shale bedrock surface. Electrical resistivity data were collected along lines to produce a two-dimensional (2D) image of subsurface resistivity, whereas passive seismic data were collected as soundings to provide a depth to Pierre Shale bedrock at a single point. 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 resistivity profiles to estimate the depth to the Pierre Shale bedrock. Passive seismic data were analyzed using the horizontal-to-vertical spectral ratio (HVSR) method in the Grilla software suite (https://moho.world/en/) to determine the fundamental resonance frequency peak at each site. Passive seismic data were also collected at existing well sites to develop a local regression equation that was used to calculate the depth to Pierre Shale along survey transects. HVSR results were plotted with ERT results to delineate a bedrock surface for each survey transect. Bedrock surface results were converted to elevations by subtracting delineated bedrock depths from elevation data from real-time kinematic surveys and a light detection and ranging (liDAR) elevation dataset. Bedrock surface results were then extracted to the location of electrodes part of ERT surveys for each survey transect. The unprocessed and processed data for each geophysical surveys, as well as bedrock depths and elevations, are provided as either comma-separated values (.csv) files or zipped folders (.zip) and are annotated accordingly.