Cross Section Construction
• A big part of our task is to walk
around collecting surface data, then
we make a cross-section along lines
that show near-surface structures
and resources of interest
Our sections usually won’t run along true dip, so we must calculate
and plot apparent dip, sometimes with vertical exaggeration.
Planar orientations
Overturned
Area
• Over small distances,
many geologic features
are approximately planar
Source: Tom Bean
Apparent Dip
• True dip is
measured
perpendicular to
strike.
• On a vertical plane
that is not
perpendicular to
strike, we measure
apparent dip. It is
less than true dip.
Vertical Distance to Plane
• However, regardless
of where the angles
are measured, the
drop from a line
parallel to strike is
the same.
Three methods for Apparent Dip
• Geometric constructions
• Trig Calculations
• A Nomogram
Turn in your lab work
• Your constructions and calculations to be
turned in on the date specified.
• We will use apparent dip data to construct a
cross section.
Apparent Dip from Strike and True Dip
Problem 1
Given: A coal bed has a strike
and dip of N45E 35NW.
Find: The apparent dip of the bed
along a vertical section
trending N90W.
1. Draw the strike N45E, then a
perpendicular of arbitrary length.
2. Mark the far end o (origin).
Draw the dip angle back toward
the strike. Measure the length,
here 2.03 units, say centimeters.
3. Draw the trend of the vertical
wall N90W from point o till it hits
the strike line. Drop a
perpendicular with the same
length, here 2.03 units.
4. Measure the apparent dip.
It works because the central triangle is the surface,
and both verticals must sink the same distance to the
subsurface coal bed, because they start at the same
strike line. Fold the triangles down; you’ll see it.
True Dip from Strike and Apparent Dip
Problem 2
Given: A gold placer has a strike of E-W
on a hilltop, and an apparent dip of 40o
on the vertical hillside trending N35E.
Find: True dip angle and quadrant.
1. Draw the E-W strike line, then a
perpendicular N0oE.
2. Mark the far end o (origin). Draw
the trend of the vertical wall N35E ,
then the apparent dip angle 40o.
Measure the length, here 2.5697
units, say centimeters.
3. Measure the same distance away
along the strike line, in the other
direction shown.
4. Measure the angle, here 46o.
It works because the central triangle is the
surface, and both verticals must sink the same
distance to the placer bed, which has its true
dip perpendicular to the strike, and so must be
at identical depths from any particular strike
line. Cut out the figure, and fold the two outer
triangles down.; you’ll see it.
Given: 2 apparent dip angles
and trends of 25o, S50E, and
35o, S20W, formed by
excavating vertical cuts on the
upper contact of a planar
Copper Porphyry.
Find: Strike and true dip of this
valuable deposit.
1. Draw lines along the first
trend S50E, then draw the
apparent dip angle (here
25o) away from the center
as shown. At an arbitrary
distance draw a
perpendicular from the
trend to the angle line.
Measure the distance.
2. Draw similar lines for the
second trend and angle.
But draw the perpendicular
where the length is the
same. (this example 1.73
units)
Strike and true dip from two apparent dips
3. Connect the two trend lines. That is the Strike,
measure it. Here it is N 89W.
4. Drop a perpendicular from the origin to the strike,
then measure off the same depth (here 1.73 units)
to the planar deposit. Measure the angle
subtended, that is the true dip, here 36o 30’.
Line of Section X-X’
Tan app dip = Tan true dip x sine angle strike to section
The equation on the
previous slide can be
arranged in the
nomogram seen at left
X
X’
Slide courtesy Steve Dutch