Geology Reference
In-Depth Information
that the surface goes above or below the extreme values of the data points. It is impor-
tant to carefully label pseudopoints in the data base so that they will not be mistaken
for real data.
The relative spacing of grid nodes can be altered to produce a trend. Changing, for
example, from a 10
20 grid in the same area will alter the surface.
Rotating the grid directions will also have an effect.
A TIN network can be edited to change the nearest neighbors, which will then change
the resulting surface. Suppose that the map of the anticline in Fig. 3.9 would be better
interpreted without a saddle between separate closures. The control point that creates
the saddle should have nearest neighbors on the southeast limb, not the northwest
limb of the anticline. The desired result is obtained by re-defining the nearest neigh-
bor network (Fig. 3.13a), resulting in a new map (Fig. 3.13b).
×
10 grid to a 10
×
3.5
Mapping from Profiles
Frequently structure contour maps are derived from data distributed along linear
traverses, rather than from randomly spaced points. This is particularly true when
working with 2-D seismic-reflection profiles, ground penetrating radar profiles, or
predictive cross sections (Sect. 6.4). If the profile trend is not parallel or perpendicular
to the structural trend, the map may contain apparent structures related to the traverse
orientation. The effect is shown by obliquely sampling a cylindrical, sinusoidal fold
that has a horizontal axis with a north-south trend (Fig. 3.14). When the traverse data
are interpolated by the inverse-distance technique (Fig. 3.15), the correct general form
of the anticline is produced but smaller-scale NE and NW trends are superimposed.
The oblique trends are most evident in the low-dip region near the crest of the anti-
cline. Triangulation shows even more pronounced oblique trends near the crest
(Fig. 3.16a). The triangulation network (Fig. 3.16b) shows the reason for the oblique
trends. Nearest neighbors are controlled by the traverse spacing, not the underlying
Fig. 3.14.
Source data: non-plunging, sinusoidal anticline.
a
Oblique view showing section traces.
b
Structure contour map
