Structural Geology 3443 Lab 2 Contour Maps

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Structural Geology (3443)Lab 2 Contour Maps
Department of Geology University of Texas at
Arlington
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Lab 2 Contour Maps

• Any scalar value that changes with position can
  be contoured.
• Elevation of the Earth
• Thickness of sediment
• Chemical species in groundwater
• Depth to a geological formation
• Etc.

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Lab 2 Contour Maps
Earths topography is typical. A contour line
represents contiguous points that are of equal
distance above a reference plane. Mean sea level
is the usual reference. The contour interval is
constant and is distance between adjacent
contours. What is the contour interval of the
fig? Index Contours.
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Lab 2 Contour Maps
Reading Contour Maps Identify data peaks, ridges,
valleys, saddles, depressions.
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Lab 2 Contour Maps
Data Gradient (slope fraction) is defined as
dz/dl, or as rise/run in Fig. 2-4, which is a
vertical cross-section through the data. The
slope angle, f, is arctan(gradient) The grade is
just the gradient. What is the grade or a 45o
slope? A 90o slope?
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Lab 2 Contour Maps
If the contour interval is constant, what is the
relationship between data gradient (slope angle)
and contour spacing
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Lab 2 Contour Maps
Where are the flatter areas and the steeper areas
on the topo map? What are the black squares?
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Lab 2 Contour Maps

• Constraints
• Contour interval is constant
• Contour lines do not merge or cross (overhangs
  excepted)
• Contour lines form closed loops unless they hit a
  discontinuity (like the edge of the map)
• A Datum plane must be specified (e.g. mean sea
  level)

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Lab 2 Contour Maps
Interpreting topographic maps The Earths land
surface is produced by weathering and erosion of
rocks. Rocks and structural features have
different weathering and erosion characteristics,
so the topography reflects the underlying
geology. Elevated areas are resistant to erosion,
low areas less resistant.
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Lab 2 Contour Maps
This area of the Appalachians in Pennsylvania is
a classic example of topography showing the
geology.
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Lab 2 Contour Maps
Intersection of planes (strata) with topography
Rule of Vs
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Lab 2 Contour Maps
Intersection of planes (strata) with topography
Rule of Vs
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Lab 2 Contour Maps
Intersection of planes (strata) with topography
Rule of Vs
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Lab 2 Contour Maps
If the strata is folded (curved) and not planar,
then intersection of the strata with topography
is much more complicated
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Lab 2 Contour Maps
Instead of topography, we can contour the top of
a formation. This is called a structure contour
map because it shows the ups and downs of the
formation in the subsurface. The next slide shows
a structure contour maps of the Permian Wolfcamp
formation.
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Lab 2 Contour Maps
The datum is sea level the blues are deepest and
reds violet are shallow.
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Lab 2 Contour Maps
Faults (discontinuities) are represented on
structure contour maps as breaks in the contours.
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Lab 2 Contour Maps
Faults are difficult to detect based on contours
alone. Is it a fault, or just steeply dipping
layers?
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Lab 2 Contour Maps
Other examples of structure contours
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Lab 2 Contour Maps
In addition to structure maps, we can also
contour the thickness of a sedimentary formation.
These are called Isopach maps Remember, thickness
is the perpendicular distance between the layer
boundaries, so if the layer is tilted, the
thickness is not the same as the vertical
distance.
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Lab 2 Contour Maps
Thickness in the subsurface is often obtained
from vertical wells. Uncorrected thicknesses from
wells used in contour maps are called isochore
maps These are not reliable thickness maps.
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Lab 2 Contour Maps
Faults can also affect thickness
measurements. Reverse faults thicken horizontal
layers Normal faults thin horizontal layers.
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Lab 2 Contour Maps
Constructing contour maps Topographic maps are
usually constructed from stereo pairs of aerial
photos which provide a 3-D image of the ground.
These map are quite accurate because there are
almost an infinite number of data points. 3-D
seismic images, like stereo aerial photos, can
also provide accurate structural contour maps in
2-way travel time. (Seismic methods measure
travel time, not depth). The time can be
converted to depth knowing the acoustic velocity
of the rock, but that is not known very well, so
depths from seismic data are usually inaccurate.
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Lab 2 Contour Maps

• Constructing contour maps
• Usually, data for stratigraphic thickness and
  depth to a formation top comes from well
  information.
• Because well information is sparse and not
  uniformly distributed, this point data must be
  interpolated and extrapolated, so these contour
  maps are less reliable.
• Three methods are commonly used to construct
  contours
• Objective Strict interpolation used
• Parallel contours are kept parallel, strict
  interpolation is violated
• Interpretative only the interpreters judgment
  is used his/her feeling of what the surface
  should like. Interpolation between points is
  qualitative.

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Lab 2 Contour Maps

• In all methods of contouring, the rules of
  contours must be followed
• Contour interval is constant
• Different contour lines do not merge or cross
  (overhangs excepted).
• A contour line may join itself to form a closed
  loop
• Contour lines always form closed loops unless
  they hit a discontinuity (like the edge of the
  map or a fault)
• A Datum plane must be specified (e.g. mean sea
  level)

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Lab 2 Contour Maps
We will use both the objective method, which most
computer contouring programs use, and the
interpretative method. A contour interval is
selected that does not give more resolution than
the number of data points provides. Interpolation
lines are drawn between each point and its
nearest neighbor. The elevation of each contour
is drawn on each line assuming the slope is
constant along the line.
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Lab 2 Contour Maps

• We will use a cm ruler and calculator to
  interpolate.
• Imagine vertical triangle between points 40
  199.
• Measure map distance between the points
• Elevation change along baseline 3.975/mm
• Find location of 60 contour
• (60-40)/3.975 5.03mm from point 40
• Location of 180 contour (180-40)/3.975 35.22
  mm

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Lab 2 Contour Maps
When all the contours have been interpolated,
then the contours can be drawn in. Where does the
80 contour go?
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Lab 2 Contour Maps
Example contours from data.