Geology Reference
In-Depth Information
be interpolated between the control points. Using linear interpolation (more complex
forms of interpolation are considered in Chap. 3), an interpolated even-elevation con-
tour lies at point c (Fig. 2.23) on the straight line between the two points at known
elevations
a
and
d
. Following the method of Eq. 2.10, the elevation of the upper control
point is
a
, the horizontal distance of the desired contour from the highest control point
is
D
, the elevation of the desired contour is
C
2
, the map distance between the two control
points is
h
', and the vertical distance between the two control points is
v
', giving
D
=
h
' (
a
-
C
2
)/
v
' .
(2.20)
As an example, the strike line constructed in Fig. 2.20b is itself a structure contour
line. Two additional structure contour lines are shown in Fig. 2.24.
2.6.2
Structure Contours from Attitude
If the dip is known, then the spacing between structure contours is found from the
method of Eq. 2.11 (Fig. 2.23) to be
H
=
I
/tan
δ
,
(2.21)
where
H
= horizontal spacing between structure contours measured perpendicular to
the contour strike,
I
= contour interval, and
δ
= dip. The trend of the structure con-
tours is perpendicular to the dip direction.
2.6.3
Dip from Structure Contours
If the attitude is given by structure contours, the dip is found by solving Eq. 2.21:
tan
δ
=
I
/
H
,
(2.22)
where
= dip,
I
= contour interval, and
H
= spacing between contours. The dip direc-
tion is perpendicular to the contour, in the direction toward the lower contour.
δ
2.7
Intersecting Contoured Surfaces
The trace of a contact on a geologic map represents the intersection of the topographic
surface with the boundaries of the geologic units. Finding the intersection between two
surfaces is a fundamental technique in three-dimensional interpretation. In Sect. 2.6.1,
three points along the outcrop trace of a planar geological contact were used to gen-
erate a structure-contour map. Conversely, the outcrop trace of a contact can be deter-
mined from a structure contour map. The following procedure works for both plane
and curved structure contour surfaces. The structure contour map is superimposed
on the topographic map (Fig. 2.25a). All intersection points where both surfaces have
