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.

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 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).

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).

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).

Airborne electromagnetic (AEM) and magnetic survey data were collected during November 2017 over a total distance of 1,980 line kilometers in the southwestern San Joaquin Valley of California adjacent to the Elk Hills, North and South Coles Levee, and Buena Vista oil fields. These data were collected in support of groundwater salinity mapping and hydrogeologic framework development as part of the U.S. Geological Survey California Oil, Gas, and Groundwater program and the California State Water Resources Control Board’s Oil and Gas Regional Monitoring Program. Data were acquired by SkyTEM ApS with the SkyTEM 312 time-domain helicopter-borne electromagnetic system together with a Geometrics G822A cesium vapor magnetometer. The survey was flown at a nominal flight height of 35 m above terrain along block-style lines with a nominal spacing of 300 m. The AEM typical maximum depth of investigation is between 150 and 300 m. This data release includes minimally processed (raw) AEM and raw/processed magnetic data, fully processed AEM data used for resistivity model development, and laterally constrained inverted resistivity models. The complete data package received from the contractor is included in separate zip-file directory and described in the contractor's report.

Passive seismic data collection was done northwest of the Air Force Research Laboratory (AFRL) at Edwards Air Force Base using the horizontal-to-vertical spectral ratio (HVSR) technique. HVSR surveys were done at 43 locations between May and September 2018 to refine the understanding of the bedrock-alluvial aquifer transition zone downgradient from the AFRL. Specifically, the data were collected to help determine the depth to bedrock. The HVSR method is a passive seismic technique that uses a three-component seismometer to measure the vertical and horizontal components of ambient seismic noise. Seismic noise in the range of ~0.1 to 1 Hertz (Hz) is caused by ocean waves, large regional storms, and tectonic sources. A resonance frequency (f0) is induced in unconsolidated alluvium when there is a substantial contrast (greater than 2:1) in shear-wave acoustic impedance between the alluvial overburden and the bedrock. The f0 is determined from the analysis of the spectral ratio of the horizontal and vertical components of the seismic data. The thickness of the overburden can be related to the f0. In general, lower f0 relates to thicker sediments, and higher f0 relates to relatively thinner overburden. Other geophysical techniques–including time-domain electromagnetics and electrical resistivity tomography–co-located with the HVSR data are made available in other child pages within this data release: https://doi.org/10.5066/P9ZGZTA4HVSR. This page contains the raw HVSR data.

Passive seismic data collection was done northwest of the Air Force Research Laboratory (AFRL) at Edwards Air Force Base using the horizontal-to-vertical spectral ratio (HVSR) technique. HVSR surveys were done at 43 locations between May and September 2018 to refine the understanding of the bedrock-alluvial aquifer transition zone downgradient from the AFRL. Specifically, the data were collected to help determine the depth to bedrock. The HVSR method is a passive seismic technique that uses a three-component seismometer to measure the vertical and horizontal components of ambient seismic noise. Seismic noise in the range of ~0.1 to 1 Hertz (Hz) is caused by ocean waves, large regional storms, and tectonic sources. A resonance frequency (f0) is induced in unconsolidated alluvium when there is a substantial contrast (greater than 2:1) in shear-wave acoustic impedance between the alluvial overburden and the bedrock. The f0 is determined from the analysis of the spectral ratio of the horizontal and vertical components of the seismic data. The thickness of the overburden can be related to the f0. In general, lower f0 relates to thicker sediments, and higher f0 relates to relatively thinner overburden. Other geophysical techniques–including time-domain electromagnetics and electrical resistivity tomography–co-located with the HVSR data are made available in other child pages within this data release: https://doi.org/10.5066/P9ZGZTA4HVSR. This page contains the raw HVSR data.