Geology Reference
In-Depth Information
3.3.1
Equal Spacing
Equal-spaced contouring (Fig. 3.1a) is based on the assumption of constant dip mag-
nitude over as much of the map area as possible. In the traditional approach, the dip
selected is determined in an area of tightest control. The same dip magnitude is then
used over the entire map (Handley 1954; Dennison 1968). Find the dip from the con-
trol points with the three-point method (Sect. 2.4), or, if the dip is known, as from an
outcrop measurement or a dipmeter, find the contour spacing from the dip (Eq. 2.21).
This approach projects dips into areas of no control or areas of flat dip and so will
create large numbers of structures that may be artifacts (Dennison 1968). Because of
the great potential for producing nonexistent structural closures, this method is usu-
ally not preferred (Handley 1954).
Equal-spaced contouring of the Mtfp (Fig. 3.1a) produces multiple closures, two an-
ticlines and a syncline that is much lower than any of the data points. The anticlinal
nose defined by the 800-ft contour is reasonable but the closed 700-ft anticline to the
south and the syncline bounded by the 200-ft contour on the southwest are forced into
regions of low dip or low control.
3.3.2
Parallel
Parallel contouring is based on the assumption that the contours are parallel, in other
words, a strong linear trend is present. The contours are drawn to be as parallel as
possible and the spacing between contours (the dip) is varied as needed to maintain
the parallelism (Dennison 1968). The resulting map may contain cusps and sharp
changes in contour direction, but is good for areas with prominent fold trends (Tearpock
1992). The method tends to generate fewer closures between control points than equal-
spaced contouring and more than linear interpolation (Tearpock 1992).
The parallel contouring of the top of the Mtfp (Fig. 3.1b) is strongly influenced by
the parallelism of the contours that can be drawn through the data points on the north-
west limb and on the southeast limb of the major anticline. This method suggests an
elongate northeasterly trending anticline in the center of the map and a southwest-
plunging syncline on the southeast. The syncline is in the same position as the anti-
cline predicted by the equal-spaced method (Fig. 3.1a).
3.3.3
Interpretive
Interpretive contouring reflects the interpreter's understanding of the geology. The
preferred results are usually regular, smooth and consistent with the local structural
style and structural grain. Generally, the principle of simplicity is applied and the least
complex interpretation that satisfies the data is chosen.
Interpretive contouring (Fig. 3.2a) incorporates the knowledge that the structural
grain is northeast-southwest and that the regional plunge is very low. The main anti-
cline seen in each of the other techniques is present and is interpreted to be closed to
