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).

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.

Edwards Air Force Base (EAFB) is located about eight kilometers (km) northeast of the city of Lancaster, California. In 1990 EAFB was placed on the U.S. Environmental Protection Agency’s National priorities list due to the presence of contaminated soil and groundwater. The base was divided into Operable Units (OU) based on location and similar contaminant types (Lahontan Staff Report, 2010). The Air Force Research Laboratory (AFRL) was established in the 1950’s, and contains facilities for research, development, testing, and evaluation of rocket propulsion systems (Lahontan Staff Report, 2010; AECOM, 2014), and has been in operation under various names since. Past activities at rocket test stands, evaporation ponds, burn pits, catch basins and leaking waste collection tanks has contaminated the groundwater in the AFRL area (AECOM, 2014). The AFRL occupies roughly 125 square miles (mi^2) on Leuhman Ridge and surrounding areas and is part of OU 4 and 9, roughly 5 kilometers (km) south of the community of Boron. Electrical resistivity Tomography (ERT) 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). The U.S. Geological Survey (USGS) and Air Force Civil Engineering Center (AFCEC) have entered into a cooperative agreement to refine conceptual models of the geology in the NE AFRL. As part of these efforts, two electrical resistivity tomography (ERT) surveys were done in the vicinity of the Leuhman and Spring Faults identified by Dibblee (1960) to better determine the position of these faults. ERT profile AFRL7 was done in the vicinity of the Leuhman Fault, and profile AFRL8 was done in the vicinity of the Spring Fault.

Edwards Air Force Base (EAFB) is located about eight kilometers (km) northeast of the city of Lancaster, California. In 1990 EAFB was placed on the U.S. Environmental Protection Agency’s National priorities list due to the presence of contaminated soil and groundwater. The base was divided into Operable Units (OU) based on location and similar contaminant types (Lahontan Staff Report, 2010). The Air Force Research Laboratory (AFRL) was established in the 1950’s, and contains facilities for research, development, testing, and evaluation of rocket propulsion systems (Lahontan Staff Report, 2010; AECOM, 2014), and has been in operation under various names since. Past activities at rocket test stands, evaporation ponds, burn pits, catch basins and leaking waste collection tanks has contaminated the groundwater in the AFRL area (AECOM, 2014). The AFRL occupies roughly 125 square miles (mi^2) on Leuhman Ridge and surrounding areas and is part of OU 4 and 9, roughly 5 kilometers (km) south of the community of Boron. Electrical resistivity Tomography (ERT) 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). The U.S. Geological Survey (USGS) and Air Force Civil Engineering Center (AFCEC) have entered into a cooperative agreement to refine conceptual models of the geology in the NE AFRL. As part of these efforts, two electrical resistivity tomography (ERT) surveys were done in the vicinity of the Leuhman and Spring Faults identified by Dibblee (1960) to better determine the position of these faults. ERT profile AFRL7 was done in the vicinity of the Leuhman Fault, and profile AFRL8 was done in the vicinity of the Spring Fault.

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 (USGS), in cooperation with the U.S. Air Force Civil Engineer Center (USAF/CEC), conducted a pilot study at Malmstrom Air Force Base (MAFB) to evaluate the utility of two non-invasive geophysical techniques for detecting hydrocarbon contamination in subsurface materials. Electrical resistivity tomography (ERT) and electromagnetic induction (EMI) surveys were completed August 28–29, 2023, along 7 transects overtop known contaminated and uncontaminated sites for comparison. Additionally, the USGS completed ultraviolet optical scanning tool (UVOST) soundings and collected soil samples from temporary boreholes to validate geophysical survey results. A total of 11 UVOST soundings and 7 boreholes were completed August 28–31, 2023. Two soil samples from various depths were collected from each borehole for a total of 14 samples. Ten quality assurance/quality control samples were collected consisting of 2 field equipment blanks (1 per day), 4 replicate soil samples (2 per day), and 4 trip blanks (2 per day). Soils were tested for volatile organic compounds, semi-volatile organic compounds, gasoline range organics, diesel range organics, metals, and per- and polyfluorinated substances (PFAS/PFOA).

The Air Force Research Laboratory (AFRL) is about 7 kilometers southwest of Boron, California, and covers 320 square kilometers of Edwards Air Force Base. The AFRL consists of 12 facilities for testing full-size rocket engines, engine components, and liquid and solid propellants. The historical release of contaminants from rocket test stands, evaporation ponds, burn pits, catch basins, and leaking waste-collection tanks has contaminated groundwater in the AFRL. Groundwater aquifers near the AFRL are mostly restricted to fractured granitic bedrock, but previous studies indicate that groundwater and associated contaminants have moved into alluvium to the north and northwest. The U.S. Geological Survey (USGS) and the U.S. Air Force entered into a cooperative agreement to refine the understanding of the bedrock-alluvial aquifer transition zone downgradient from the AFRL. As part of that effort, surface geophysical data were collected to: (1) assess changes in the depth to bedrock with increasing distance from the AFRL; (2) to provide information on shallow geologic structures near the AFRL; and (3) to assess the presence of any faults that could present partial barriers to groundwater flow. The surface geophysical methods collected northwest of the AFRL in 2018 were electrical resistivity tomography (ERT), horizontal-to-vertical spectral ratio (HVSR) passive seismic, and time-domain electromagnetic (TEM).

The Air Force Research Laboratory (AFRL) is about 7 kilometers southwest of Boron, California, and covers 320 square kilometers of Edwards Air Force Base. The AFRL consists of 12 facilities for testing full-size rocket engines, engine components, and liquid and solid propellants. The historical release of contaminants from rocket test stands, evaporation ponds, burn pits, catch basins, and leaking waste-collection tanks has contaminated groundwater in the AFRL. Groundwater aquifers near the AFRL are mostly restricted to fractured granitic bedrock, but previous studies indicate that groundwater and associated contaminants have moved into alluvium to the north and northwest. The U.S. Geological Survey (USGS) and the U.S. Air Force entered into a cooperative agreement to refine the understanding of the bedrock-alluvial aquifer transition zone downgradient from the AFRL. As part of that effort, surface geophysical data were collected to: (1) assess changes in the depth to bedrock with increasing distance from the AFRL; (2) to provide information on shallow geologic structures near the AFRL; and (3) to assess the presence of any faults that could present partial barriers to groundwater flow. The surface geophysical methods collected northwest of the AFRL in 2018 were electrical resistivity tomography (ERT), horizontal-to-vertical spectral ratio (HVSR) passive seismic, and time-domain electromagnetic (TEM).