A GIS-BASED APPROACH FOR THE CREATION OF 3-D GEOLOGIC MODELS

Three-dimensional (3-D) geologic models can be readily developed with some of the existing commercial Geographic Information Systems (GIS). The speed with which these models can be built depends upon the set of available software tools, the complexity of the geology to be modeled, and the programming and/or scripting abilities that the model maker possesses. Geologic scenarios of multiple faults and complex folding may require piecewise building of a composite model. Proper models for depositional geometries of units may benefit from specific surface interpolation tools. Generally, the modeler can augment scattered “exact” data such as contact information from outcrops or drill holes, with conceptual data controls to enforce a reasonable model from the tools at hand.

The 3-D geologic model which comprises the hydrogeologic framework model for the Pajarito Plateau in northern New Mexico was created using the ArcInfo GIS. The model spans four geologic quadrangles and is comprised of 28 geologic units and subunits that reflect the complex structural and depositional history of the sedimentary and volcanic rocks in this region of the Rio Grande rift system. This model is presently used in groundwater flow and contaminant transport studies at Los Alamos National Laboratory.

The techniques used to transfer geologic information into a usable framework model are varied and can be complex. The global availability of digital elevation model (DEM) data allows two-dimensional measurements of “hard” contact locations from outcrop mapping, well logging, and/or cross-sections, to be processed into sets of contact surfaces which fully define the geologic volumes of the model extent with high-fidelity at the control points. The true accuracy of the model can be enhanced by the judicious application of conceptual contours, developed from geologic intuition and “soft” constraints such as geophysical measurements, or the recorded absence of units within a drill hole.