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On a topographic map, contour lines connect points of equal elevation on the earth's surface. In contrast, structure contour is a line that connects points of equal elevation from sea level on a structural surface, such as the top of a bed or formation (Figure 2.8). This means that the orientation of a structure contour is parallel to the strike of the bed or other structural element. Dip direction is perpendicular to the structure contour and directly downslope. Structure contours are read and analyzed the same way as topographic contours, in most cases. For example, calculating dip angles is done the same way as calculating slope angles from topographic contours. Both use the formula:

Structure‐contour maps are used for depiction and recognition of features in the subsurface. They are used extensively in petroleum exploration to identify structural traps and in hydrology to characterize the subsurface configurations of water tables and aquifers. Although there are more sophisticated methods to model the hydrogeology of an area, structure contouring of the water table is an invaluable skill for all geologists working on environmental delineation/remediation projects.

Figure 2.8 Example of structure contours on a structural surface.

Structure‐contour maps are commonly constructed from drill‐hole data. See Figure 2.9, for example, which shows a faulted dome. Notice that, unlike topographic contours, structure contours can terminate abruptly. Gaps in the map indicate normal faults, and overlaps indicate reverse faults.

There are various techniques for contouring elevations. In the case of structure contours, there are usually not enough data to produce an unequivocal map, so experienced interpretation is valuable. Although there are computer programs that draw contour lines between data points, such programs cannot substitute for the judgment of an experienced geologist.

Figure 2.9 Block diagram (a) and structure‐contour map (b) of a subsurface faulted structural dome with contours on the top of shaded layer. D, down; U, up.