BUILDING HYDROGEOLOGIC FRAMEWORK MODELS OF BASINS: CONTRIBUTIONS FROM GEOPHYSICALLY-BASED ‘ELEMENTS’

Groundwater modelers often need to construct hydrogeologic framework and flow models in basins with sparse direct subsurface information. One approach to addressing this need is modular—creating hydrogeologic framework models through aggregation of fundamental geologic ‘elements’ known to be present in the basin, and then assigning hydrologic properties to the assembled 3D geologic framework models. Geologic elements represent fundamental portions of the stratigraphic or structural architecture of the system that are capable of being modeled quantitatively and are common to many basins. Gravity and magnetic data combined with geologic maps can define in 3D important elements including basin shape and geometry, location and geometry of basin-bounding faults, and the distribution and characteristics of basin-filling materials.

Basic 3D shapes of basins can be interpreted from gravity inversions, as already has been done on a regional scale for the Basin and Range and California Desert provinces, and many local basins (e.g. Timber Mountain caldera, Pahrump Valley, Frenchman Flat, NV). Faults that laterally bound basins or offset the bedrock floor can be inferred from basin geometry (e.g. Deadman basin, CA, Yucca Flat, NV), as can the geologic nature of non-fault boundaries. Intra-basin faults often can be characterized by high-definition aeromagnetic surveys in basins containing magnetic volcanic rocks (e.g. Yucca Flat) or fill eroded locally from magnetic crystalline basement. The distribution, thickness, and even the type of intra-basin volcanic rocks can be determined by combined interpretation of high-definition aeromagnetic data and surface geology (e.g. Verde Valley, AZ, Virgin Valley, NV). Increasingly, high definition aeromagnetic data reveal information about the sedimentary basin fill, such as the distribution and limits of fan deposits (e.g. Salinas Valley, CA), large-scale depositional anisotropy, folds (e.g. Livermore Valley, CA), and shallowly buried channels (e.g. Virgin Valley, NV).

Research now is focused on expanding the geologic aspects of the geophysically identified elements through process-based geologic inferences. Future research will be needed on linear and nonlinear methods for combining various elements into a single framework model of a basin.