While the growing abundance of remote sensing platforms has dramatically improved the ability to measure the Earth’s surface, research into the subsurface remains hindered by data scarcity. Borehole and well data, where available, represent invaluable but spatially limited windows into geologic and hydrogeologic conditions, and it can be difficult to assess variability between these points. Airborne electromagnetic (AEM) methods are being used by the USGS to fill in subsurface data gaps in research across the nation. AEM data are used to develop spatially comprehensive maps of electrical resistivity that, in turn, can be related to hydrostratigraphy and other subsurface properties such as lithology, mineralogy, groundwater salinity, and thermal conditions. While analogous to ground-based electromagnetic geophysical methods, AEM surveys encompass far larger study areas while producing high-resolution data, leading to new regional-scale insights into concealed geologic structure, groundwater movement and occurrence, salinity distributions, contaminant and nutrient fluxes, and permafrost dynamics.
AEM research is actively being used in several projects and assessments at the USGS. More than 43,000 kilometers of AEM data have been recently acquired over an area of nearly 140,000 square-kilometers as part of the Mississippi Alluvial Plain Regional Groundwater Availability Study. This effort is the largest continuous AEM survey undertaken in the United States and provides an unprecedented high-resolution and regionally extensive view into the subsurface hydrogeology of a prominent agricultural region (Fig 1, a) (https://www2.usgs.gov/water/lowermississippigulf/map/map.html). In the San Joaquin Valley of California (b), an AEM-based groundwater salinity mapping approach has been developed that estimates geophysical and interpretational uncertainty and is being applied over additional regions of the Valley to locate fresh and brackish groundwater near areas of energy development (http://dx.doi.org/10.1029/2019WR026273). In the southwestern Grand Canyon region (c), AEM data were used to refine the concealed hydrostratigraphy as part of a groundwater modeling effort (https://doi.org/10.3133/sir20205017D). A number of additional studies are actively underway spanning a wide range of applications, including studies of bedrock aquifers in mountain watersheds of the Upper Colorado River Basin (d), the exploration of Yellowstone National Park’s geothermal systems (e), groundwater contaminant transport (f), volcanic hazards (g), mineral resource assessment (h), and permafrost studies (i). Recently released airborne geophysical data inventories (see Airborne Geophysical Survey Inventory of the conterminous US, Alaska, Hawaii, and Puerto RicoAEM Survey Inventory ) make these data more discoverable to the public, researchers, and stakeholders.
USGS-supported airborne electromagnetic survey locations classified by their primary supporting USGS Mission Area. Letters indicate studies highlighted in the text.