A COMPUTER-MAPPING METHOD USED TO GENERATE BEDROCK GEOLOGIC MAPS IN INDIANA

Bedrock geologic maps recently published by the Indiana Geological Survey with funding from U.S. Geological Survey STATEMAP are implementations of a computer-mapping method that focuses on constructing digital three-dimensional (3-D) models of geologic boundary surfaces. Geologic maps and other geologic portrayals are derived from these surface models.

Geologic map units are volumes bounded by one or more geologic surfaces. In Indiana, bedrock units are bounded by conformable contacts, unconformities, the bedrock surface, the topographic surface, or faults. With the exception of faults, these surfaces can be modeled as 3-D functions using inexpensive surface-modeling software. Indiana's high-angle faults are essentially vertical surfaces, which can be represented by breaklines in the geologic model.

The process of modeling consists of building a database of point locations in 3-D space where the identity and spatial coordinates of geologic boundary surfaces are known from fieldwork, drilling, or historical data. A geologist interprets these data and uses surface-modeling software to construct a gridded model of each surface that expresses a geologic interpretation. The 3-D geologic model is the set of gridded boundary surfaces and fault breaklines that completely bound mappable geologic units in a project area. A geologic map is derived from the model by using grid math to compute the appropriate intersections between surface models and projecting these intersections to the map plane to form the polygons and polylines that make up a geologic map. Geologic portrayals that address special customer needs, such as isopach maps, overburden maps, cross sections, etc., are derived from the geologic model by computing different surface intersections, intervals between surfaces, or intersections with special surfaces such as the plane of a cross section.

Advantages of this computer-mapping method are that: 1) a variety of geologic portrayals, including geologic maps, can be derived from a single geologic model, 2) all geologic portrayals derived from the geologic model are consistent with one another, and 3) geologic maps derived from the geologic model are digital entities that can be transferred to a geographic information system for further analysis.