FROM RESISTIVITY TO MODEL FRAMEWORK: DEVELOPING 3D HYDROSTRATIGRAPHY FROM AIRBORNE ELECTROMAGNETIC DATA IN THE NELSON BASIN OF THE NORTHERN MOJAVE DESERT

To support groundwater resource assessments at the Ft. Irwin National Training Center in the northern Mojave Desert of California, the USGS has conducted airborne electromagnetic (AEM) and magnetic surveys of multiple basins between 2010 and 2015. Results of these surveys highlight the strong geophysical contrasts across faults and between crystalline basement, volcanic flows and domes, and basin-fill deposits that are likely to place significant controls on groundwater flow in this geologically complex and tectonically active region. Airborne survey design varies significantly between basins, leading to different integration approaches with groundwater models and hydrogeologic studies. Of the basins currently targeted for numerical groundwater model development, Nelson basin has the highest density of AEM data, with regularly spaced flight lines every 400 m. These data, in conjunction with previously published ground-based gravity, transient electromagnetic, and borehole lithologic and geophysical data, have been used to develop a continuous 3D hydrogeologic realization of the subsurface to 500 m depth. A combination of resistivity thresholds and manual interpretation of inverted AEM sections were used to separate basin-fill deposits from both crystalline basement and volcanic flows and domes. Characteristic resistivity distributions associated with major hydrostratigraphic units were developed using correlations between AEM data, mapped geologic units at the surface, and stratigraphic descriptions from a limited number of boreholes. Several resistivity zones were selected from these distributions to define the geometry of model-parameter zones within the hydrologically active basin-fill deposits likely to have similar lithologic character and bulk hydraulic properties. This combination of resistivity thresholding, manual interpretation, and zonation facilitated the rapid integration of AEM data into the model hydrostratigraphic framework. By separating crystalline, volcanic, and sedimentary domains in the AEM interpretation, this approach permitted the translation of resistivity variations within the sedimentary deposits to major hydrogeologic units while accounting for the fundamental differences in mechanisms controlling resistivity between major rock types.