Field trip this Saturday! We will leave from the loading
dock at 7:30 AM sharp!
HYDRATE... or else! BRING >1 GALLON OF WATER!
Techniques for projecting and plotting structural
data (p. 684-716)
Structural measurements involve 2 angles:
(1) Compass direction (azimuth) in horizontal
plane
(2) Angle down from horizontal
Stereographic projection
plotting 3D structural data on a hemisphere (usually
the lower), which is projected onto a horizontal plane
bring tracing paper to lab
next week
All lines and planes are imagined to pass through
the center of the hemisphere
Lines intersect the lower hemisphere as points.
Shallow lines plot near the perimeter
Steep lines plot near the center
Let's plot the following lines:
(1) plunge = 70, trend = 60
(2) plunge = 10, trend = 340
• Planes intersect the lower hemisphere in
the form of great circles.
• Let's plot the following planes:
(1) strike = N-S, dip = 90
(2) strike = 30, dip = 60
(3) Strike = 120, dip = 10
A plane can also be represented by its pole,
which is a line perpendicular to the plane.
Let's plot the pole of the following plane:
(1) strike = 340, dip = 60
• Stereonets are useful for presenting a
large amount of structural data and for
statistical analysis.
• Stereographic projection is a powerful tool
to solve a number of interesting structural
problems
Example: Problem from lab
A fault plane is oriented 250, 68. Which of the
following lineations cannot lie within it?
a) plunge = 68, trend = 250
b) plunge = 68, trend = 340
c) plunge = 0, trend = 70
d) plunge = 80, trend = 310
Rake = The acute angle between the horizontal
(strike line) and a line in the plane,
MEASURED IN THE PLANE
Orthographic projection: 2D line drawings that are
used to determine angular and spatial relationships
in 3D
Determining the true thickness of a bed
1. Draw a structural profile (Xsection) perpendicular to strike
2. Plot the true dip of the beds and
project them to depth
3. Use trigonometry to calculate
the true thickness
For a dipping bed, the map-view thickness is an
"apparent" as opposed to "true" thickness!
Constructing structure contour maps
project surface data to depth
project points vertically to surface
plot points along bed at various depths
make structure contour map
"3-point" Problem
Any three points on a plane (not in a straight line) can be
used to determine the attitude of that plane
invtan
(200/350) = 30
degrees
350m
strike
Stereographic Projection (Summary)
Imagine planes or lines being projected onto a
hemisphere
Lower hemisphere projection (always look from
above) - convention