Structural Geology 3443 Lab 3 Attitude Calculations

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Structural Geology (3443)Lab 3 Attitude
Calculations
Department of Geology University of Texas at
Arlington
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Projections
Geology is 3-D, so depiction of geological
features requires a projection of the 3-D image
onto a 2-D surface. All maps are projections.
There are a number of ways to project (Fig
3.1) The orthographic projection is common in
geology.
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Projections
To see different aspects of the 3-D body, it is
projected in different directions and a folding
line is used to rotate the projections onto a
flat plane.
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Projections
Several folding lines are possible to get picture
of the 3-D body from different directions.
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Projections
In this lab, we are finding the attitude (strike
and dip) of planes. This makes use of the fact
that the intersection of two planes makes a line.
The strike line is the intersection of what two
planes? The dip and dip direction is the
intersection of what two planes?
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3 Point Problems
The first type of problem to solve asks What is
the attitude of the plane (usually a bedding
plane or a fault) is we know the location and
depth of three points on that plane (usually from
drill holes)
For example if we know the location and elevation
of points A, A and B, what is the strike dip
of the plane A-A-B-B?
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3 Point Problems
In this problem, the rectangle is a map, and
points XY lie on the surface of the map which is
at elevation 100. Point Z is projected vertically
from a depth of 60 up to the map level at point Z
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3 Point Problems
What is the strike direction? What is the dip
direction? What is the dip? (Use a graphical
method and a trig method).
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3 Point Problems
This version of the problem is more complicated
all the points are at different elevations. The
map plane passes through point L at 200. M is
projected up from 160 to M and N projected up
from 100 to N
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3 Point Problems
The strike line is horizontal so it connects
points of equal elevation. There is a point
between L and N that is the same elevation as Q
(160). Construct a folding line along L-N to
show the vertical triangle L-N-N
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3 Point Problems
The depth of 160 along N-N is the same elevation
as point M, and point Q is at elevation 160 along
the line N-L Point Q is then projected up to the
map surface at Q which is the position of a
point that has an elevation of 160, the same as
M.
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3 Point Problems
The strike line is then M-Q The dip direction
is perpendicular to the strike D-N The Dip f
arctan(NN/DN)
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3 Point Problems
The same problem can be solved from the
intersection of a plane with a topographic
surface Draw the strike line, the dip direction,
and the vertical triangle to calculate dip.
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Drawing an outcrop trace
Knowing the attitude of a plane, it is possible
to predict where it would outcrop using a
topographic map. Knowing the strike and dip of
the plane at point A, construct a cross section
perpendicular to the strike along Fold Line F1.
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Drawing an outcrop trace
To draw the outcrop find each topographic
elevation on the cross section and project it
parallel to the strike until it crosses the same
contour line. That would be an outcrop. Connect
the dots on each contour line to make the
geologic map.
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True Apparent Dip
True dip is always measured perpendicular to
strike. Apparent dip is the angle measured on
any other vertical plane and is always less than
the true dip.
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True Apparent Dip

• Sometimes we can only measure apparent dips, so
  true dip must be calculated from apparent dips.
• The following types of problems are solved
• True dip from a strike an apparent dip.
• True dip from 2 apparent dips
• Apparent dip from true dip.

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True Dip from strike Apparent Dip
Problem 3-5 Graphical Solution Strike
330 Apparent dip 25 Apparent dip direction 260
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True Dip from strike Apparent Dip
Problem 3-5 Trig Solution Strike 330 Apparent
dip d 25 Apparent dip direction 260 True Dip
Direction ? Angle between true and apparent dip
directions b ? True Dip f arctan(tand/cosb)
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True Dip from two Apparent Dips

• Problem 3-6 Graphical Solution
• Apparent dip d 25 bearing 240
• Apparent dip m 20 bearing 170
• Find Strike True Dip
• angle between apparent dip dir. ?
• l angle between strike apparent dip

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True Dip from two Apparent Dips
Problem 3-6 Graphical Solution Fold up the two
apparent dip triangles ABB and ADD BB
DD Strike line is BD Dip direction is CA
perpendicular to BD
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True Dip from two Apparent Dips
Problem 3-6 Graphical Solution Construct CC
BB DD Measure true dip f CAC Measure
strike of BD using right hand rule.
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True Dip from two Apparent Dips

• Problem 3-6 Trig Solution
• Angle between strike and dip direction m
• Angle between the two dip directions
• x (90-l) - Angle between Apparent dip m and
  true dip
• 90 arctantanm/(sinD tand) cotD
• arctan(tand/cosb)

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Apparent Dip from True Dip

• Problem 3-7 Graphical Solution
• SD of bed 315 30
• Apparent Dip direction 280
• Step 1 Make map
• Step 2 draw strike line SR crossing dip line
  and apparent dip line at B C

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Apparent Dip from True Dip
Problem 3-7 Graphical Solution Step 3 Flip up
profile along true dip direction and make CAC
knowing dip f. Step 4 flip up profile along
apparent dip direction and construct ABB where
BB CC Step 5 measure apparent dip d
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Apparent Dip from True Dip
Problem 3-7 Trig Solution Step 1 Find angle b
(BAC) d ArctanTanf Cos b