Geology Reference
In-Depth Information
Gridding algorithms typically extend the contours to the edges of the grid, regard-
less of whether or not data are present to support the extrapolation. Figure 10.4a is a
map produced by a computer kriging algorithm over a rectangular region. The region
without any data to control the contours is shaded in Fig. 10.4b. The closed highs and
lows within the shaded area are completely spurious.
A TIN network may contain extremely elongated triangles along the edges of the
data, implying a relationship between widely separated points. Such widely separated
points are not necessarily geologically related and probably should not influence the
shape of the surface between them. The TIN network should be examined for such
problems and such long-distance connections might be removed. Some computer
programs eliminate triangles for which one or two of the angles are smaller than a
threshold value. This tends to reduce the problem but does not eliminate it.
10.2.3
Excessive Detail
Excessively wiggly contours or areas containing multiple, small, structural closures may
be contouring artifacts (Krajeweski and Gibbs 1994). Closed contours that do not con-
tain control points should be viewed with suspicion as artifacts of the gridding algo-
rithm (Fig. 10.5). This type of artifact only occurs with gridding algorithms and is
more likely when using high-order surfaces (e.g., kriging with quadratic drift and grid-
node densities much greater than the control-point density).
Another type of artifact may arise if data from multiple sources, such as different
seismic surveys, are contoured together, because their datums may be different. Each
Fig. 10.5.
Structure contour map from
Fig. 3.11d. Control points are
small squares
.
Triangles
point
to local closures unjustified by
control points
