APPROACHES TO CONSTRUCTING 3D PROPERTY MODELS FOR THE BASIN-FILL COMPONENT, ARID SOUTHWESTERN U.S

Ground-water flow models of basin-fill aquifers in the desert southwestern United States require the definition of the geometry and properties of surface and subsurface materials and structures. Models are improved with increasing amounts of data and knowledge of basin history; however, resource management decisions often require models to be built rapidly and with limited subsurface data. One approach to characterizing the spatial variability of the material properties is through basin-scale lithologic modeling. Data from driller’s logs can be reduced to a suite of consistent lithologic descriptions through the use of geologic reasoning and knowledge of the geologic setting of the basin. These lithologic units may then be distributed in 3D space through interpolation methods. The incorporation of gravity and magnetic data can provide limits to the basin depth, geometry and location of intra-basin structural features. The Amargosa and Pahrump basins, NV-CA, have been subdivided into zones that are based on sediment grain size, presence and type of volcanic rocks, and the potential for permeability-reducing zeolitic alteration. These zones provide insights into hydraulic property distributions because they are based upon material properties, rather than stratigraphy. A potentially more effective approach to basin modeling involves the definition and aggregation of hydrogeologic elements. Elements represent fundamental, modular pieces of the basin stratigraphic or structural architecture (e.g., alluvial fan complexes, ephemeral stream channels, playas, basin-bounding faults). The geometry and lithologic characteristics of each element are defined through detailed studies and forward lithologic modeling. Sequence stratigraphic rules are defined to govern how the elements stack in time and space. Finally, the elements are aggregated at the basin scale to form the hydrogeologic framework model. When an adequate toolbox of generic hydrogeologic elements has been studied and assembled, the geometry and properties of these elements can be applied to other basins where similar elements occur. Based on this approach, relatively detailed hydrogeologic framework models might be built even where subsurface data are limited.