Today we are going to increase our understanding of interpeting structures by looking at how dipping beds outcrop and are expressed in topography. We will also learn how to extract bedding attitudes from the high-resolution lidar data.
You will complete Task 1 in class and then we will review it together. There is nothing to turn. You can work indivually or in groups of 2. If no one wants to work by themselves, then there can be one group of 3.
You will likewise complete Task 2 in class. I want each of you to work on it individually, but by all means talk to each other and share thoughts on how to complete the work. For Task 2 there are items to email me, discused in Task 2 steps. Later we will make use of the data collected to use it to understand the structure in the area.
Task 1: Visualizing the way beds of different erosional resistance interact with topography.
Work on this individually on the workstations, but feel free to discuss with any other student.
Use the program visGeomMap for this Task.
As always first set your paths in Matlab. The easiest way to do this is to navigate to source directory on R
> cd 'R:\MATLAB\src'
then run the script
> startupPaths
Make sure that R:\MATLAB\src\geomvis; R:\MATLAB\src\shared; and R:\MATLAB\src\sample_data are in your path.
then type
>visGeomMap
Go over to the forth tab over entitled Map Geometry. Shown is a 3D surface rendering of the high-resolution topography (digital elevation model) from Franklin, WV from a lidar dataset that I have sub-sampled to 5m (originally 1m data). Use the check boxes to turn on and off items, and the navigation tools that show up above the figure once you click the surface. The Map View is an overhead view, which is what you see in a geologic map. Complete these steps and jot down the answers. We will go over them in class.
1) Turn the Surface, Contours and the Digitized Plane. For a bed with no dip, what should the map pattern be of the bed? That is, the intersection of the plane interacts with the topography to make what pattern? Be specific, there is a precise pattern it makes.
2) Now change the dip of the plane to ~5-10 deg to the NW. Describe the shape of the pattern of the plane intersecting the surface now. Use the drainage as the reference for your description.
3) As you steepen the dip to say ~50 deg, how does the map pattern change from the shallow dip plane? Spin the model around so you can see the map view.
4) When the dip = 90 deg, what is the map pattern? Again, there is a specific thing you can relate this to.
5) Now change the strike to 30 so that your plane will dip SE and repeat the above the steps. Are the patterns the same? Hint, they are not. In fact there is a very particular dip that makes a very particular map pattern not seen with the NW dip. What is that dip magnitude?
6) Turn off the Contours and the Digitized Plane, and only look at the surface. Spin the model around (illumination changes really help to see the underlying structure). Look at the river drainages on either side of the mountain; is there anything about the topography that makes you think you see the orientation of bedding near these? Describe qualitatively what you see that makes you think you see bedding. How do the features you see relate to the river drainages and the patterns that you have determined above?
7) Zoom in a little on a place where you think you see bedding . Between river drainages on the SE limb of the structure is best for this. Digitize the local contact that you have recognized defines bedding, such as across one drainage. You can go across more than one drainage if you like, but the plane that is created will be based on whatever you digitize so it will be more of an average over many ridges. In general, we want the local bedding orientation. Is your plane consistent with what you thought for bedding? If not tweak the attitude with the Strike and Dip sliders a little to make it better fit.
8)Look in map view, is this consistent with the rules you developed above for the pattern of contacts with topography? Look down the strike (true dip) with the plane on, as well as other oblique views. Toggle on and off the plane. Do you see the bedding plane in the topography?
9) Now instead of tweaking the Strike and Dip sliders, try digitizing the contact more carefully to get the representative bedding. Try it in other locations on the mountain. Are there places where you cannot get a representative plane even if you carefully digitize? If so, what is it about what you are digitizing that is not good? Hint: the algorithm is fitting a plane to the digitized contact.
10) Summarize the rules for how bedding contacts look in high-resolution topography in terms of varying dip of beds, and how the patterns change as the dip and dip direction change.
Now go to Task 2