GeoDAWN: Airborne magnetic and radiometric surveys of the northwestern Great Basin, Nevada and California

The U.S. Geological Survey (USGS) and the Department of Energy (DOE) have collaborated to acquire high-resolution airborne magnetic and radiometric data, over northern and western Nevada and eastern California, to support geologic and geophysical mapping and modeling that will assist geothermal and critical mineral studies. The surveys, referred to as GeoDAWN (Geoscience Data Acquisition for Western Nevada), span areas of major resource potential associated with the Walker Lane and western Great Basin. They were conducted under the USGS’s Earth Mapping Resource Initiative (EarthMRI), with support from the DOE’s Geothermal Technologies Office (GTO), and involved acquisition of aeroradiometric and aeromagnetic data that provide key information on surface geology and soil composition, and subsurface structure and geology, respectively. Coordinated with this effort was the collection of airborne lidar (light detection and ranging) data (conducted through the USGS 3DEP Program) that yield detailed surface topographic models of the terrain over a similar extent spanned by the geophysical surveys. The GeoDAWN surveys were performed by EDCON-PRJ, Inc., under contract with the USGS from November 1, 2021 to November 20, 2022, and consisted of two different, overlapping surveys with different flight specifications (Area 1 and Area 2; Figure 1). Area 1, centered over Clayton Valley in western Nevada was selected primarily with a focus on the region’s Li-clay and brine resources. It was flown with rank 1 specifications (following criteria outlined by Drenth and Grauch, 2019) that met EarthMRI survey requirements. Area 2, consisting of the remainder of the GeoDAWN extent, was selected primarily with a focus on geothermal resources. Lower resolution flight specifications designated for Area 2 (falling between rank 1 and 2) enabled data collection across a substantially larger area (spanning numerous known, prospective, and undiscovered geothermal and mineral systems) than would have been possible with rank 1 specifications. The combined GeoDAWN area (consisting of a total of 149,030 line-km spanning an area of 51,857 sq km), was divided into four separate acquisition blocks (from north to south: Winnemucca, Fallon, Hawthorne, and Tonopah; Figure 1). The Tonopah block, which includes Area 1 and the southern part of Area 2 surveys, was flown by Precision GeoSurveys Inc. (under subcontract to EDCON-PRJ, Inc.), with a Bell Jet Ranger helicopter. Area 1 was flown with a nominal flight height targeted at 100 m above terrain over low-relief areas and 150 m over mountainous areas. Flight lines were spaced 200 m apart at an azimuth of 90 degrees, and tie lines were spaced 2000 m apart at an azimuth of 180 degrees. Area 2 was flown at a nominal flight height targeted at 150 m above terrain over low-relief areas and 200 m over mountain ranges. The survey was flown with flight lines spaced 400 m apart at an azimuth of 90 degrees, and tie lines spaced 4000 m apart at an azimuth of 180 degrees. The portion of Area 2 contained within the Tonopah acquisition block was flown with the Precision GeoSurveys’ Bell Jet Ranger, while the remainder was collected by Cloudstreet Flying Service (under subcontract to EDCON-PRJ, Inc.) and flown with a Cessna 180 and Turbo 206 fixed-wing aircraft. Nominal flight heights for both surveys were based on a best fit, pre-planned, three-dimensional draped surface designed with a maximum 22-degree climb/descent angle to follow terrain as closely as possible while maintaining a safe survey. Actual flight heights were subject to aircraft climb and descent limitations. In areas of steep terrain, the aircraft may have required deviating from the planned drape surface, and therefore variable terrain clearance should be considered when modeling and interpreting these data. Magnetic data (Figure 2) were processed by EDCON-PRJ, Inc. and include corrections for diurnal variations of the Earth’s magnetic field, magnetic field of the aircraft, tie-line leveling, micro-leveling, and an International Geomagnetic Reference of the Earth for the time of the survey. Radiometric data (Figure 3) were processed by the contractor and include corrections for aircraft and cosmic background radiation, radon background, Compton scattering effects, and variations in altitude. Included with this publication are: PDF files of the contractor's report and readme file (describing the surveys, field operations, equipment, data, and processing procedures), a .csv file of the contractor’s metadata, and compressed .zip files containing deliverable products [consisting of binary grid (.grd), map (.map), and database (.gdb) files of magnetic and radiometric grids and line data (and associated projection, metadata, and scaling files, .gi, .xml, and .mdf, respectively), that are readable with commercial "Oasis Montaj” software, or with the free downloadable "Geosoft Viewer” (available at https://www.seequent.com/); and Esri shapefiles (.shp) and associated projection (.prj), index (.shx) and dBASE (.dbf) files of the flight paths and survey outlines]. Also included in this report are compressed .zip files containing .csv files of flight line data for magnetic and radiometric surveys, a PDF of the radiometric ternary map, and geoTIFF images of geophysical grids.
Figure 1Figure 2Figure 3