DEFINING SUBSURFACE HYDROGEOLOGIC FRAMEWORKS OF GLACIAL AQUIFERS USING AIRBORNE ELECTROMAGNETIC SURVEYS, GEOLOGIC MAPPING, AND HYDROGEOLOGIC CHARACTERIZATION

Over the last decade, the USGS has performed airborne electromagnetic (AEM) electrical resistivity surveys over selected areas of the glaciated central part of the United States, including areas of Nebraska, Montana, and Michigan. These surveys were conducted to map geology, contaminants, and hydrogeology in order to enhance subsurface mapping capabilities aiding development of hydrogeologic frameworks of buried glacial valleys and other local and regional aquifers. Buried glacial valleys and subglacial tunnel valleys form important and productive aquifer elements that have been juxtaposed by glacial erosional and depositional processes with other aquifer and aquitard elements in a complex 3-D glacial hydrogeologic framework that is commonly poorly understood.

AEM surveys are flown in frequency domain or time domain with both helicopter and fixed-wing aircraft. AEM surveys can provide characterization of electrical properties from the near surface 1–3 meters down to depths of 300–400 meters. AEM systems transmit an electromagnetic (radio) signal from an airborne platform that interacts with the earth, generating secondary currents. These secondary currents are a function of the subsurface electrical resistivity, which is controlled by the amount of mineralogical clay, gravel, water content (including total dissolved solids), metallic mineralization, and void space. Using numerical imaging and inversion, depth sections of estimated electrical resistivity can be created along flight lines. Interpolations between flight lines provide an estimation of the 3-D distribution of electrical resistivity. When the 3-D electrical resistivity distributions are coupled with hydrological and geological models it allows the characterization of the 3-D geometry and potential connectivity of glacial aquifer systems. The approach of AEM combined with hydrogeologic and geologic modeling improves our understanding of hydrogeologic frameworks by defining the location of sand and gravel deposits, paleochannels, and buried bedrock topography, which is needed to assess groundwater availability and sustainability. These improved hydrogeological models represent the hydrogeology at an accuracy not achievable using previous data sets.