GeoFlight: Airborne magnetic and radiometric surveys of the Salton Trough, Southern 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 the Salton Trough in southern California, to support geologic and geophysical mapping and modeling that will assist geothermal and critical mineral studies. The surveys, referred to as GeoFlight spans an area that contains some of the largest and hottest known hydrothermal systems in the world, as well as a substantial lithium brine resource that could potentially meet the nation’s lithium demand for electric vehicles. 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 GeoFlight surveys were performed by EDCON-PRJ, Inc., under contract with the USGS from January 14, 2022 to March 26, 2023, and consisted of two different, overlapping surveys with different flight specifications (Area 1 and Area 2; Figure 1). The combined GeoFlight surveys included a total of 94,671 line-km spanning an area of 16,772 sq km. Area 1 is centered on the Imperial Valley, and characterized by little topographic relief. The Area 1 survey was conducted by Precision GeoSurveys Inc. (under subcontract to EDCON-PRJ, Inc.), with a standard Cessna 206 aircraft and flown with rank 1 specifications (Drenth and Grauch, 2019) that met EarthMRI survey requirements. Area 2, consisting of the remainder of the GeoFlight extent, was collected around the perimeter of Area 1 and included the most rugged topography across the entire region. Somewhat lower resolution flight specifications designated for Area 2 (falling between ranks 1 and 2) were necessary for flight safety considerations over steep terrain. These specifications enabled data collection across a substantially larger area than would have been possible with rank 1, that would have required acquisition by a helicopter. The Area 2 survey was conducted by Cloudstreet Flying Service (under subcontract to EDCON-PRJ, Inc.) with Cessna 180 and Turbo 206 aircraft. The surveys were flown with flight lines spaced 200 m apart at an azimuth of 45 degrees, and tie lines spaced 2000 m apart at an azimuth of 135 degrees. Nominal flight heights were based on a best fit, pre-planned, three-dimensional draped surface designed with a maximum 20° climb/descent angle to follow terrain as closely as possible while maintaining a safe survey. This incorporated a variable terrain clearance of 120 m over low relief, 200 m over steep terrain, and 150 m over population centers. 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 1
Figure 2
Figure 3