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.

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.

On September 25, 1998, SIAL Geosciences Inc. (SIAL) was awarded Department of Interior, contract 98CRCN1018, by the U.S. Geological Survey. This contract required SIAL to carry out a high-sensitivity magnetic and electromagnetic helicopter survey over one block located in the Eureka Graben Area. The primary goal of this project was to provide digitally recorded and processed magnetic and electromagnetic data in order to evaluate geologic and hydrologic resources of the Eureka Graben Area. The base of on-site field operations was installed at Silverton, Colorado. The survey was carried out by SIAL between May 27th and June 9th, 1999. This report, describing the parameters, data acquisition and processing procedures, was delivered at the end of August, 1999. This metadata file describes the flight line data and attributes, as well as gridded products generated from the flight line data that were delivered to the USGS by SIAL Geosciences Inc.

Time-domain electromagnetic (TEM) surveys were done northwest of the Air Force Research Laboratory (AFRL) at Edwards Air Force Base. TEM surveys were done at 33 locations between May and October 2018 to refine the understanding of the bedrock-alluvial aquifer transition zone downgradient from the AFRL. TEM surveys (also called transient electromagnetic surveys) provide 1D resistivity soundings of the subsurface, which can be related to lithology and hydrogeology. In the TEM method, electrical current is cycled through a transmitter loop (Tx) wire, which in turn produces a primary magnetic field. When the current is abruptly terminated, a secondary magnetic field is induced in the earth, and it moves downward and outward as the induced current decays with time. The decay is controlled by the resistivity of the earth. A receiver (Rx) at the center of the Tx loop measures the secondary magnetic field as a function of time (dB/dt). Decaying voltage measurements at the receiver are converted to apparent resistivity, which can be inverted to recover the depth-dependent vertical resistivity structure of the earth below the receiver location. Other geophysical techniques, including electrical resistivity tomography and horizontal-to-vertical spectral ratio passive seismic, co-located with the TEM data are made available in other child pages within this data release: https://doi.org/10.5066/P9ZGZTA4. This page contains the TEM data, spatial information for the TEM surveys, and preliminary processed TEM data.

Time-domain electromagnetic (TEM) surveys were done northwest of the Air Force Research Laboratory (AFRL) at Edwards Air Force Base. TEM surveys were done at 33 locations between May and October 2018 to refine the understanding of the bedrock-alluvial aquifer transition zone downgradient from the AFRL. TEM surveys (also called transient electromagnetic surveys) provide 1D resistivity soundings of the subsurface, which can be related to lithology and hydrogeology. In the TEM method, electrical current is cycled through a transmitter loop (Tx) wire, which in turn produces a primary magnetic field. When the current is abruptly terminated, a secondary magnetic field is induced in the earth, and it moves downward and outward as the induced current decays with time. The decay is controlled by the resistivity of the earth. A receiver (Rx) at the center of the Tx loop measures the secondary magnetic field as a function of time (dB/dt). Decaying voltage measurements at the receiver are converted to apparent resistivity, which can be inverted to recover the depth-dependent vertical resistivity structure of the earth below the receiver location. Other geophysical techniques, including electrical resistivity tomography and horizontal-to-vertical spectral ratio passive seismic, co-located with the TEM data are made available in other child pages within this data release: https://doi.org/10.5066/P9ZGZTA4. This page contains the TEM data, spatial information for the TEM surveys, and preliminary processed TEM data.

Surface electrical resistivity tomography, magnetic, and gravity surveys were conducted in July 2017 in the greater East River Watershed near Crested Butte Colorado with a focused effort in Redwell Basin as part of a broader study of the role of bedrock groundwater in the hydrogeology of mineralized mountain watersheds. Approximately ten kilometers of total field magnetics data were acquired on July 29, 2017 with a Geometrics G-858 cesium vapor magnetometer that detects changes in deep (tens of meters to kilometers) geologic structure based on variations in the magnetic properties of different formations. This data release includes the raw and processed magnetics data. They are provided as digital data, and data fields are defined in the data dictionary.

Surface electrical resistivity tomography (ERT), magnetic, and gravity surveys were conducted in July 2017 in the greater East River Watershed near Crested Butte Colorado with a focused effort in Redwell Basin as part of a broader study of the role of bedrock groundwater in the hydrogeology of mineralized mountain watersheds. Five electrical resistivity tomography profiles were acquired within Redwell Basin and Brush Creek to map geologic structure at depths up to 40 meters, depending on the subsurface resistivity, using the Advanced Geosciences, Inc. SuperSting R8 resistivity meter. This data release includes the raw and processed resistivity data as well as inverted resistivity models. All are provided as digital data, and data fields for each file type are defined in the respective data dictionary.

Surface electrical resistivity tomography (ERT), magnetic, and gravity surveys were conducted in July 2017 in the greater East River Watershed near Crested Butte Colorado with a focused effort in Redwell Basin as part of a broader study of the role of bedrock groundwater in the hydrogeology of mineralized mountain watersheds. Five electrical resistivity tomography profiles were acquired within Redwell Basin and Brush Creek to map geologic structure at depths up to 40 meters, depending on the subsurface resistivity, using the Advanced Geosciences, Inc. SuperSting R8 resistivity meter. This data release includes the raw and processed resistivity data as well as inverted resistivity models. All are provided as digital data, and data fields for each file type are defined in the respective data dictionary.