The U.S. Geological Survey, in cooperation with the U.S. Air Force Civil Engineer Center, used surface geophysical methods to delineate the top of the Cretaceous Pierre Shale along survey transects in selected areas east of Ellsworth Air Force Base, South Dakota. In 2019, the U.S. Geological Survey performed passive seismic and 2D electrical resistivity tomography (ERT) surveys along 21 co-located survey transects east of Ellsworth Air Force Base. 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 and the average shear wave velocity for unconsolidated deposits overlying the Pierre Shale was used to calculate the depth to Pierre Shale along survey transects. 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 that were interpreted to estimate the depth to the Cretaceous Pierre Shale. HVSR results were plotted with ERT profile results to delineate a continuous bedrock surface for each survey transect. The continuous bedrock surface results were converted to elevations using light detection and ranging (liDAR) elevation data and were extracted to electrodes locations that were part of ERT surveys for each survey transect. The unprocessed and processed data for each geophysical surveys and bedrock delineation are provided as either comma-separated values (.csv) files or zipped folders (.zip) and annotated accordingly in the metadata.

The U.S. Geological Survey, in cooperation with the U.S. Air Force Civil Engineer Center, used surface geophysical methods to delineate the top of the Cretaceous Pierre Shale along survey transects in selected areas east of Ellsworth Air Force Base, South Dakota. In 2019, the U.S. Geological Survey performed passive seismic and 2D electrical resistivity tomography (ERT) surveys along 21 co-located survey transects east of Ellsworth Air Force Base. 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 and the average shear wave velocity for unconsolidated deposits overlying the Pierre Shale was used to calculate the depth to Pierre Shale along survey transects. 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 that were interpreted to estimate the depth to the Cretaceous Pierre Shale. HVSR results were plotted with ERT profile results to delineate a continuous bedrock surface for each survey transect. The continuous bedrock surface results were converted to elevations using light detection and ranging (liDAR) elevation data and were extracted to electrodes locations that were part of ERT surveys for each survey transect. The unprocessed and processed data for each geophysical surveys and bedrock delineation are provided as either comma-separated values (.csv) files or zipped folders (.zip) and annotated accordingly in the metadata.

The U.S. Geological Survey, in cooperation with the Air Force Civil Engineering Center, investigated the use of surface geophysical surveys 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 electrical resistivity tomography surveys were performed at the Fuels Area C site on Ellsworth Air Force Base. In 2019, the U.S. Geological Survey performed passive seismic and 2D electrical resistivity tomography (ERT) surveys along 26 co-located survey transects within and near Ellsworth Air Force Base. Passive seismic data were analyzed using the horizontal-to-vertical spectral ratio (HVSR) method in Grilla version 9.6.3 software (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. ERT data were processed using EarthImager2D version 2.4.0 software from Advanced Geosciences, Inc. (https://www.agiusa.com/agi-earthimager-2d) to remove noisy measurements and produce subsurface resistivity profiles that were interpreted to estimate the depth to the Cretaceous Pierre Shale. HVSR results were plotted with ERT profile results to delineate a continuous bedrock surface for each survey transect. The continuous bedrock surface results were converted to elevations using light detection and ranging (liDAR) elevation data and were extracted to electrodes locations that were part of ERT surveys for each survey transect. The unprocessed and processed data for each geophysical surveys and bedrock delineation are provided as either comma-separated values (.csv) files or zipped files (.zip) and are annotated accordingly in the metadata. Zipped files (.zip) require extraction software, such as 7-zip, to unzip.

The U.S. Geological Survey, in cooperation with the Air Force Civil Engineering Center, investigated the use of surface geophysical surveys 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 electrical resistivity tomography surveys were performed at the Fuels Area C site on Ellsworth Air Force Base. In 2019, the U.S. Geological Survey performed passive seismic and 2D electrical resistivity tomography (ERT) surveys along 26 co-located survey transects within and near Ellsworth Air Force Base. Passive seismic data were analyzed using the horizontal-to-vertical spectral ratio (HVSR) method in Grilla version 9.6.3 software (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. ERT data were processed using EarthImager2D version 2.4.0 software from Advanced Geosciences, Inc. (https://www.agiusa.com/agi-earthimager-2d) to remove noisy measurements and produce subsurface resistivity profiles that were interpreted to estimate the depth to the Cretaceous Pierre Shale. HVSR results were plotted with ERT profile results to delineate a continuous bedrock surface for each survey transect. The continuous bedrock surface results were converted to elevations using light detection and ranging (liDAR) elevation data and were extracted to electrodes locations that were part of ERT surveys for each survey transect. The unprocessed and processed data for each geophysical surveys and bedrock delineation are provided as either comma-separated values (.csv) files or zipped files (.zip) and are annotated accordingly in the metadata. Zipped files (.zip) require extraction software, such as 7-zip, to unzip.

The U.S. Geological Survey, in cooperation with the City of Summerset, South Dakota, performed electrical resistivity tomography (ERT) surveys between July 28 and August 5, 2025. ERT data were collected using an Advanced Geosciences Incorporated SuperSting with 2 passive cables with 14 electrodes per cable. In total, ERT data were collected along 13 transects using 2 passive cables and electrodes spacings of either 1, 2, or 3 meters. Transect lengths varied from 27 to 81 meters. Real time kinematic (RTK) surveys were completed before ERT data collection to obtain latitude, longitude, and elevation of each electrode location. Elevation data collected during RTK surveys were uploaded to EarthImager2D software from Advanced Geosciences, Inc. (https://www.agiusa.com/agi-earthimager-2d) along with apparent resistivity data for inversion. EarthImager2D software allows users to remove noisy measurements either automatically or manually and invert apparent resistivity data to generate two-dimensional profiles of electrical resistivity. Inverted resistivity profiles were generated for all 13 transects. The unprocessed and processed data for each transect are provided as zipped folders (.zip) and annotated accordingly in the metadata. RTK data are provided in a comma separated values (.csv) file.

Geophysical measurements and related field data were collected by the U.S. Geological Survey (USGS) at the Alaska Peatland Experiment (APEX) site in Interior Alaska from 2018 to 2020 to characterize subsurface thermal and hydrologic conditions along a permafrost thaw gradient. The APEX site is managed by the Bonanza Creek LTER (Long Term Ecological Research). Nine instrument monitoring sites (APEX1-APEX9) were established in April 2018. To quantify permafrost and thaw zone characteristics along the instrumented gradient, electrical resistivity tomography (ERT) data were collected in August 2018 along four 82 meter (m)-long transects between select sites: APEX1-3, APEX5-3, APEX5-7, and APEX6-8. Data were collected for both dipole-dipole (DD) and inverse Schlumberger (IS) survey geometries. Inverted models of electrical resistivity were produced from the separate DD and IS datasets, as well as the combination of both datasets inverted jointly (labeled DDIS). The resulting models of electrical resistivity revealed the spatial variability in soil lithology and thermal state (frozen vs. thawed) to depths up to 10-15 m below the surface. Manual permafrost-probe measurements of thaw depths were collected at set intervals along each ERT transect and used for comparison to the resistivity models.

Geophysical measurements and related field data were collected by the U.S. Geological Survey (USGS) at the Alaska Peatland Experiment (APEX) site in Interior Alaska from 2018 to 2020 to characterize subsurface thermal and hydrologic conditions along a permafrost thaw gradient. The APEX site is managed by the Bonanza Creek LTER (Long Term Ecological Research). Nine instrument monitoring sites (APEX1-APEX9) were established in April 2018. To quantify permafrost and thaw zone characteristics along the instrumented gradient, electrical resistivity tomography (ERT) data were collected in August 2018 along four 82 meter (m)-long transects between select sites: APEX1-3, APEX5-3, APEX5-7, and APEX6-8. Data were collected for both dipole-dipole (DD) and inverse Schlumberger (IS) survey geometries. Inverted models of electrical resistivity were produced from the separate DD and IS datasets, as well as the combination of both datasets inverted jointly (labeled DDIS). The resulting models of electrical resistivity revealed the spatial variability in soil lithology and thermal state (frozen vs. thawed) to depths up to 10-15 m below the surface. Manual permafrost-probe measurements of thaw depths were collected at set intervals along each ERT transect and used for comparison to the resistivity models.

Electrical resistivity tomography (ERT) surveys were done northwest of the Air Force Research Laboratory (AFRL) at Edwards Air Force Base. ERT surveys were done at four locations in May through June of 2018 to refine the understanding of the bedrock-alluvial aquifer transition zone downgradient from the AFRL. The ERT technique injects direct-current electricity with known voltage and current into the earth using a series of electrodes and measures the resulting resistivity. This technique is generally limited to investigations of aquifer properties less than 100 meters below land surface. Data from other geophysical techniques co-located with the ERT data, including time-domain electromagnetics and horizontal-to-vertical spectral ratio passive seismic, are made available in other child pages within this data release: https://doi.org/10.5066/P9ZGZTA4. This page contains the ERT data, spatial information for the ERT transects, and preliminary processed ERT data.

Electrical resistivity tomography (ERT) surveys were done northwest of the Air Force Research Laboratory (AFRL) at Edwards Air Force Base. ERT surveys were done at four locations in May through June of 2018 to refine the understanding of the bedrock-alluvial aquifer transition zone downgradient from the AFRL. The ERT technique injects direct-current electricity with known voltage and current into the earth using a series of electrodes and measures the resulting resistivity. This technique is generally limited to investigations of aquifer properties less than 100 meters below land surface. Data from other geophysical techniques co-located with the ERT data, including time-domain electromagnetics and horizontal-to-vertical spectral ratio passive seismic, are made available in other child pages within this data release: https://doi.org/10.5066/P9ZGZTA4. This page contains the ERT data, spatial information for the ERT transects, and preliminary processed ERT data.

The U.S. Geological Survey (USGS) and Air Force Civil Engineering Center (AFCEC) have entered into a cooperative agreement to refine the hydrogeology in the Northeast AFRL and Arroyos groundwater areas of the Air Force Research Laboratory of Edwards Air Force Base. As part of these efforts, two electrical resistivity tomography (ERT) surveys- AFRL9 and AFRL10- were collected in the vicinity of the Mound Fault identified by Cyr and Miller (2022) to better determine the position of these faults. Electrical resistivity tomography is a direct current geophysical method that is used to estimate the subsurface distribution of the electrical resistivity (measured in ohm-meters; ohm-m) of a material, and is based on the assumption that measured electric potentials (voltages) near current carrying electrodes are influenced by the electrical resistivities of the underlying material (Zohdy and others, 1974; Loke, 2000). ERT is a popular technique for subsurface investigations because it is based on simple physical principles and for its efficient data acquisition (Dahlin and Zhou, 2004). A combination of the Dipole-Dipole and Strong Gradient arrays was used for this survey and combined to create an optimized dataset (Stummer and others, 2004). The Dipole-Dipole array type yields a high precision dataset, particularly of vertical structures, but can exhibit lower signal to noise ratios (Dahlin and Zhou, 2004; Binley and Kemna, 2005), while the Strong Gradient array provides more complete spatial coverage, and high signal to noise ratio with increased acquisition efficiency (Dahlin and Zhou, 2004; Dahlin and Zhou, 2006, Advanced Geosciences Inc., 2009).