DRESSING THE EMPEROR: THE ROLE OF GIS IN THE DEVELOPMENT OF THREE-DIMENSIONAL HYDROGEOLOGIC MODELS

Stone and others (2001) mapped structure contours for the tops of each of 20 individual units in intersecting and overlapping glacial morphosequences in Berrien County, Michigan (1,350 km²), as part of the mapping program of the Central Great Lake Geologic Mapping Coalition (CGLGMC). We have developed a methodology to translate this detailed morphostratigraphy first into a solid three-dimensional geologic model, and then into a three-dimensional block of data that can be used as input to a finite-difference groundwater-flow model. The technique involves a hybrid approach involving geographic information systems (GIS), three-dimensional information visualization software (3DIVS), and customized data-processing code. The methodology begins by converting Stone’s structure contours (they are attributed vector contours) for each individually mapped unit into a raster surface at a defined grid resolution (170 m x 170 m). The top of the geologic model is the surface topography (digital elevation model), which is also used to derive the drainage network that is an important boundary condition in the groundwater-flow model. The bottom of the geologic model is the bedrock topography, which was also mapped and contoured by Stone and others (2001). Stone constructed his structure contour model such that the bottom of each map unit is described by the surface contours of the unit that lies immediately below it. Complex interrelationships dictate that the tops of a number of individually mapped units are sometimes required to describe the bottom surfaces of laterally more extensive units. Once all of the requisite raster grids have been derived, they can be manipulated to provide input that is necessary for development of a detailed solid geologic model using 3DIVS. GIS software and custom code are also used to assign hydrogeologic attributes to the elements of the final three-dimensional finite-difference geologic model.