GIS TOOLS FOR THE CONSTRUCTION OF 3D GEOLOGIC MODELS

A set of prototype software tools has been developed to assist geologists in the construction of 3D geologic models from traditional geologic maps stored as GIS layers. The tools were built on top of the Arc 8.1 desktop GIS applications from ESRI, including 3D Analyst and Spatial Analyst. This effort is part of a new USGS project to create a 3D geologic model of the Santa Clara Valley, California.

Traditional geologic maps contain a wealth of information about the subsurface in the form of strikes, dips, fold axes, cross sections, fault traces, attitudes, and so on, as well as implicit information like the distribution and apparent thickness of units, and contacts on topography. This information generally is unevenly distributed and not suitable for direct input to a fully 3D model. We developed tools and techniques for interpolating and projecting this geologic information into the subsurface. Included are tools for cross-section construction, geologic map simplification, generation of structure contours and continuous dip surfaces, and strike and dip domain analysis. Important considerations include ensuring compatibility between adjacent cross-sections, using quantitative approaches whenever possible, and recording uncertainty and provenance. The goal of these techniques is the generation of elements suitable for input to the model (3D points, lines and surfaces), rather than for graphical portrayal. The map-derived data, along with geophysical, borehole and other data, are assembled into a 3D model in EarthVision (a modeling package from Dynamic Graphics), as a set of surfaces, their relationships, and the volumes defined by those surfaces. Model construction is an iterative process, so procedures for data import and export were developed. We extended the draft North American Geologic Map Data Model for this 3D data, for the history of the construction of each element (feature-level metadata), and for an uncertainty estimate for each object.