Structural geology 3443 lab 2 contour maps
Download
1 / 29

- PowerPoint PPT Presentation


  • 1389 Views
  • Updated On :

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.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about '' - LionelDale


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Structural geology 3443 lab 2 contour maps l.jpg

Structural Geology (3443)Lab 2 – Contour Maps

Department of Geology University of Texas at Arlington


Lab 2 contour maps l.jpg

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.

Lab 2 – Contour Maps


Lab 2 contour maps3 l.jpg

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


Lab 2 contour maps4 l.jpg

Lab 2 – Contour Maps contiguous points that are of equal distance above a reference plane. Mean sea level is the usual reference.

Reading Contour Maps

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


Lab 2 contour maps5 l.jpg

Data Gradient (slope fraction) is defined as contiguous points that are of equal distance above a reference plane. Mean sea level is the usual reference.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


Lab 2 contour maps6 l.jpg

Lab 2 – Contour Maps contiguous points that are of equal distance above a reference plane. Mean sea level is the usual reference.

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


Lab 2 contour maps7 l.jpg

Lab 2 – Contour Maps contiguous points that are of equal distance above a reference plane. Mean sea level is the usual reference.

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

What are the black squares?


Lab 2 contour maps8 l.jpg

Lab 2 – Contour Maps contiguous points that are of equal distance above a reference plane. Mean sea level is the usual reference.

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)


Lab 2 contour maps9 l.jpg

Interpreting topographic maps contiguous points that are of equal distance above a reference plane. Mean sea level is the usual reference.

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


Lab 2 contour maps10 l.jpg

Lab 2 – Contour Maps contiguous points that are of equal distance above a reference plane. Mean sea level is the usual reference.

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




Lab 2 contour maps13 l.jpg

Lab 2 – Contour Maps V’s

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


Lab 2 contour maps14 l.jpg

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

Lab 2 – Contour Maps


Lab 2 contour maps15 l.jpg

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


Lab 2 contour maps16 l.jpg

Lab 2 – Contour Maps formation. This is called a

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


Lab 2 contour maps17 l.jpg

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

Lab 2 – Contour Maps


Lab 2 contour maps18 l.jpg

Lab 2 – Contour Maps contour maps as breaks in the contours.

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


Lab 2 contour maps19 l.jpg

Other examples of structure contours contour maps as breaks in the contours.

Lab 2 – Contour Maps


Lab 2 contour maps20 l.jpg

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


Lab 2 contour maps21 l.jpg

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.

Lab 2 – Contour Maps


Lab 2 contour maps22 l.jpg

Faults can also affect thickness measurements. wells.

Reverse faults thicken horizontal layers

Normal faults thin horizontal layers.

Lab 2 – Contour Maps


Lab 2 contour maps23 l.jpg

Constructing contour maps wells.

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


Lab 2 contour maps24 l.jpg

Lab 2 – Contour Maps wells.

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 maps25 l.jpg

Lab 2 – Contour Maps wells.

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)


Lab 2 contour maps26 l.jpg

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


Lab 2 contour maps27 l.jpg

We will use a cm ruler and calculator to interpolate. contouring programs use, and the interpretative method.

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


Lab 2 contour maps28 l.jpg

Lab 2 – Contour Maps contouring programs use, and the interpretative method.

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

Where does the 80 contour go?


Lab 2 contour maps29 l.jpg

Example contours from data. contouring programs use, and the interpretative method.

Lab 2 – Contour Maps