Geology Reference
In-Depth Information
2.6
Structure Contours
Structure contours provide an efficient and effective representation of the attitude of a
surface. The contours are strike lines and the dip direction is perpendicular to the con-
tours (Fig. 2.23). The simplest way to construct structure contours from a geologic map
is to connect points on a contact that lie at the same elevation. If two points on a contact
have the same elevation, for example a and b in Fig. 2.23, then a line joining them is a
strike line or structure contour. Finished maps normally show contours at even incre-
ments of elevation.
Structure contours are the primary method for illustrating the shape of structures in the
subsurface. Generated from outcrop maps, structure contours provide an effective method
for validating the outcrop pattern and for smoothing out local dip variations caused by bed
roughness. If a map horizon is planar, the contours on it are parallel and uniformly spaced.
Folding produces curved contours that usually are not far from parallel at a local scale.
Abrupt changes in direction indicate fold hinges, faults, or mistakes. Unless radical thickness
changes occur, the structure contours on adjacent horizons are approximately parallel.
The following sections describe the creation of straight-line structure contours from
the elevations of three points and the interpretation of contours in the vicinity of a dip
measurement. The generation of curved structure contours from multiple control points
is discussed in Chap. 3.
2.6.1
Structure Contours from Point Elevations
Structure contours are usually generated from point elevations, especially in subsur-
face work. Except for the unlikely circumstance where all the points happen to fall on
the selected contour elevations, contouring will require placing contours between con-
trol points that have different elevations. This means that contour values must
Fig. 2.24.
Structure contours on a dipping plane (from Fig. 2.20). The 650-ft and 750-ft contours are
interpolated between the four control points.
a
Plan view.
b
3-D oblique view to NE
