Structural Geology Lab 7: Stereographic Analysis of Folds
I. Graphical Representation of Lines and Planes in Structural Analysis
A. Stereonets Revisited
1. Stereographic projection of lines and planes onto a circular grid or net
a. Essentially taking a 3-D sphere and projecting it to a 2-D piece of
paper (analogous to projecting the globe on world maps)
b. Schmidt Net or Equal Area Net
(1) Areas on the 3-D sphere are preserved as true on the 2-D
projection of the net
(a) Angles are not preserved, they become distorted
(b) Most commonly used since structural problems
require assessment of areal density distribution
c. Wulff Net or Stereographic Net
(1) Areas are not preserved, but angle are.
2. Schmidt Net Basics
a. Primitive Circle = outline of sphere
b. North-South and East-West Reference Lines
c. Plane Projection
(1) Lower Hemisphere Projections of Planes
(a) Great circles formed by intersection of inclined
plane with lower hemisphere of the reference
sphere
(b) Great circles are plotted on the stereonet
i) horizontal plane: dip = 0, plots as great
circle on primitive circle of net
ii) vertical plane: dip = 90, plots as straight line
passing through center
(2) Lower Hemisphere Projections of lines
(a) Lines plot as points of intersection between line
and lower hemisphere
(b) horizontal lines plot as points on outer primitive
circle
(c) vertical lines plot as points at center of net.
d. Poles to planes
(1) Imagine a line drawn perpendicular to plane, passing to
lower hemisphere of reference sphere
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