ENHANCING 3-D VISUALIZATION OF STRUCTURAL GEOLOGY CONCEPTS

Structural geology students solve numerous 3-D spatial problems using 2-D projections, yet many have difficulty visualizing how the projections work. Thus, they may learn how to solve sample problems by a given set of operations, but they have difficulty extrapolating to novel problems. Well-drawn perspective diagrams can help some students understand how projections work, for example the stereographic projection, but their value is limited by their static nature.

3-D visualization can be dramatically enhanced with the aid of computer applications, which make it possible to construct accurate 3-D models that can be manipulated by the user to show how projections work. Google SketchUp, a free application, can be used to create a realistic model of the stereographic projection. The lower hemisphere, projection point, projection plane (complete with great and small circles), and planes or lines representing geologic features (all referenced to the center of the sphere) can be constructed accurately enough to measure angles within the 3-D model. The ability to rotate the model, and all of its elements, allows students to connect the 3-D spatial relationships with the 2-D stereographic projection. Once the model is rotated, it is easy for students to retain the 3-D perspective.

The three-point problem is a classic example of a 3-D problem that is solved by projecting ‘on the fly’ into a plane. ArcMap and ArcScene, commercially available GIS software, can be combined to produce precise 2-D and 3-D illustrations of the solution to this problem. Turning layers on sequentially permits an orderly demonstration of how the three initial points can be used to construct the plane and structure contours of a geologic contact. Rotation of the 3-D model shows how the intersections of the topographic and structure contours define the outcrop pattern, and how to determine the depth of a contact below the surface. These programs can also be used to show how to measure displacement across a fault and how to construct cross-sections from map data.

At present these programs can help show students how to use 2-D projections and geometric techniques to solve 3-D problems. With modification, however, the next generation of programs could actually solve such problems in a 3-D perspective without 2-D projections.