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. 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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### Structural Geology (3443)Lab 2 – Contour Maps

Department of Geology University of Texas at Arlington

Elevation of the Earth

Thickness of sediment

Chemical species in groundwater

Depth to a geological formation

Etc.

### Lab 2 – Contour Maps

Earth’s 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.

### Lab 2 – Contour Maps

Identify data peaks, ridges, valleys, saddles, depressions.

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?

### Lab 2 – Contour Maps

If the contour interval is constant, what is the relationship between data gradient (slope angle) and contour spacing

### Lab 2 – Contour Maps

Where are the flatter areas and the steeper areas on the topo map?

What are the black squares?

### 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)

Interpreting topographic maps

The Earth’s 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.

### Lab 2 – Contour Maps

This area of the Appalachians in Pennsylvania is a classic example of topography showing the geology.

### Lab 2 – Contour Maps

Intersection of planes (strata) with topography: Rule of V’s

If the strata is folded (curved) and not planar, then intersection of the strata with topography is much more complicated

### 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.

### Lab 2 – Contour Maps

The datum is sea level; the blues are deepest and reds & violet are shallow.

Faults (discontinuities) are represented on structure contour maps as breaks in the contours.

### Lab 2 – Contour Maps

Faults are difficult to detect based on contours alone. Is it a fault, or just steeply dipping layers?

Other examples of structure contours

### 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.

### Lab 2 – Contour Maps

Uncorrected thicknesses from wells used in contour maps are called isochore maps

These are not reliable thickness maps.

### Lab 2 – Contour Maps

Faults can also affect thickness measurements.

Reverse faults thicken horizontal layers

Normal faults thin horizontal layers.

### 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.

### 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 interpreter’s judgment is used – his/her “feeling” of what the surface should like. Interpolation between points is qualitative.

### 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)

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.

### 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

### Lab 2 – Contour Maps

When all the contours have been interpolated, then the contours can be drawn in.

Where does the 80 contour go?

Example contours from data.