Geology Reference
In-Depth Information
Fig. 2.15.
Apparent dip in the plane given by the dip vector 32, 150 on an equal-area, lower-hemisphere
stereogram.
a
Plane plotted with apparent dips shown as
black squares
. The south-trending apparent
dip is 28, 180.
b
Overlay rotated 10° to bring the 080° trending apparent dip into measurement posi-
tion; apparent dip is 15, 080
Plotting poles and lines can be done more quickly on a stereogram using a
Biemsderfer plotter (Wise 2005), a calibrated dip scale that rotates on the center of the
net to allow plotting of points without rotating the overlay. The attitudes of lines such
as dip vectors and lineations are also quickly and easily plotted on a tangent diagram,
described in Sect. 2.3.3.
Apparent dips are quickly determined on a stereogram as the orientation of the
point of intersection between a line in the direction of the apparent dip and the great-
circle trace of a plane. For example, find the apparent dips along the azimuths 080
and 180 for the plane plotted previously (
= 32, 150). Plot lines from the center of the
graph in the azimuth directions (Fig. 2.15a). The intersections of the azimuth lines
with the great circle are the apparent dips. Dips can be read from the N-S axis as well
as the E-W and so the 180° azimuth is in measurement direction. The angle measured
inward from the primitive circle to the intersection is the apparent dip, 28°. The over-
lay is rotated into measurement position for the 080° azimuth (Fig. 2.15b) to find the
apparent dip of 15°.
Apparent dip problems can be worked backwards to find the true dip from two
apparent dips. Plot the points representing the apparent dips, then rotate the overlay
until both points fall on the same great circle. This great circle is the true dip plane.
δ
2.3.2
Natural Variation of Dip and Measurement Error
The effect of measurement error or of the natural irregularity of the measured surface
on the determination of the attitude of a plane is readily visualized on a stereogram.
The plane is represented by its pole (Fig. 2.16). Irregularities of the measured surface
and measurement errors should produce a circular distribution of error around this
pole (Cruden and Charlesworth 1976). An error of 4° around the true dip is probably
