Geology 554 Interpretation Project Big Injun Sand, Trenton/Black River Plays, Central Appalachian Basin, WV

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1 Geology 554 Interpretation Project Big Injun Sand, Trenton/Black River Plays, Central Appalachian Basin, WV Lab Exercise- Horizon Interpretation and Correlation Wilson (2005) 1 Team effort on these interpretation exercises is encouraged. This will allow you to interpret a greater number of horizons and share insights into the interpretation of the area.

2 In today s exercise, we will digitize two prominent reflectors observed in the seismic data from the project area. 1. We will start by digitizing the base of the Greenbrier Limestone/Big Injun reflection event. The base of the Greenbrier is associated with the negative cycle appearing around 0.3 seconds along Line 6 (Figure 1) and throughout the area. This reflection event is also associated with the top of the Big Injun Sandstone. Figure 1: Line 6: the basal Greenbrier reflection event appears at approximately 0.3 seconds in the display. 2. Horizons are created in much the same way as faults. Anywhere on the seismic line, right click and select Horizon Management. Select the Create tab and then enter Greenbrier for the horizon name and then select a color (Greenbrier). Hit OK. The Greenbrier horizon is now active. 2

3 3. Horizon Picking: Right click on a seismic line and select Picking Parameters. Make sure that Stop at Displayed Fault Surface Intersections is enabled. This feature, when enabled, works with the Autopick-2D Hunt mode. Picking will stop either when an event terminates or the horizon encounters a fault surface. Also make sure that trough is selected. Figure 2: Picking parameters window 4. Display the Horizon Toolbar by left clicking on View, Toolbars, and Horizon bar. Note that the active horizon is highlighted in the toolbar. Hot keys are available, M = manual picking. F = Fill mode, H = 2d Hunt. E = Erase. P = Peak, and T = Trough. Hot keys are not available for zero crossings. 5. Observe that the shape of the cursor and the status bar. The cursor is now represented by a '+' with an E, M, F, or H next to it. Change the picking mode to either F or H. Pick the event as 3

4 far as you can. Jump faults when present. If you keep the map view window opened, note that the map display is updated immediately after picking. Figure 3: Close-up view of the basal Greenbrier reflector. 6. Using the 2D Hunt mode, left click once on the tick mark and a segment of the reflector will be digitized. You may have to click at several points along the reflector before the entire reflector is digitized, and then you will probably notice that you have some problems with the digitization (Figure 4). Figure 4: Greenbrier reflector on Line 6 digitized using the 2D Hunt mode. 4

5 The best thing to do at this point is to manually carry your interpretation through jagged areas in the interpretation. The result of manual adjustments is shown below (Figure 5). Figure 5: Manually adjusted Greenbrier reflection times are shown by the black line. 7. Complete your picks for the remaining lines in the data set. In Figure 6, I ve used a variable area wiggle trace display of a small area along Line 1 to illustrate in close-up some of the failures of the Hunt mode. Figure 6: Note that the red line doesn t always pass directly through a trough. In general they come pretty close. 5

6 Figure 7 is another, color raster, view of the picks. Figure 7: Close-up view of autohunt picks on the Greenbrier reflector. Don t worry too much about the subtle details. For the most part these differences are pretty small and will not have noticeable impact on your maps. The autohunt picks on lines 1, 2, 3, and 4 need very minor to no adjustments. Line 5 is fairly noisy and you may have to manually guide it in places. In some areas the data are so poor that I just erased the digitized segment entirely over small gaps. When done, double click the Greenbrier reflector in the Project Tree to bring up a time map. Your time map should look something like that shown in Figure 8. 6

7 Figure 8: Reflection travel times shown for the basal Greenbrier/Big Injun reflection event. 8) Also examine the reflection amplitudes associated with the Basal Greenbrier/Big Injun reflection event. 7

8 9) Next, we ll contour the basement reflector. Bring up Line 6 and set the scales or zoom in until you get a view similar to that shown below (Figure 8) which portrays the extent of the basement reflector across the entire length of the line. Figure 8: Closeup view of basement reflector along seismic line 6. Get into the horizon management window again and create a new reflector Basement. Under picking parameters, select Peak. You can also directly select the type of event peak or trough that you want to digitize simply by pressing the P or T key. Make sure you display your faults, then take a close look at the Picking Parameters Window (Figure 9). Make sure 2D Hunt is on. Also make sure the stop at displayed faults option is checked. 8

9 Figure 9: Picking parameters window. 10) The basement interpretation along Line 6 should look something like that shown below in Figure 10. I ve interpreted the basement reflection as a positive amplitude (blue) event. The next positive cycle beneath it is weaker and less coherent. This event is fairly consistent from line-to-line. 9

10 Figure 10: Basement horizon interpretation. 11) Complete interpretation of the basement reflector on all lines. Double click the Basement Horizon in the project tree to get the following display (Figure 11). Figure 11: Two-way basement reflection travel times shown along individual lines in the survey. 10

11 Interpolating horizon reflection times between lines Bring up the Greenbrier/Big Injun reflection time map (Figure 12). Figure 12: Greenbrier two-way reflection travel times displayed along individual lines. We d like to construct a travel time contour map across the entire area. One could easily do this by hand contouring the colors. However, the computer will also do this for you. First you have to construct a grid from the reflection travel time data along each line. On the top menu bar select Grid an then Create Grid. The following window should come up (Figure 13). 11

12 Figure 13: Create Grid window Select the horizon you wish to grid and give the output grid file a name. Indicate whether you want to grid times or amplitudes. I ve named the output grid file Big Injun. Next Click on Gridding Parameters The following window will open (Figure 14). 12

13 Figure 14: Grid Parameters window. Set the increment to 500 and the smoothing to high. Then click OK. and then OK again Your contoured grid will appear similar to that shown in Figure 15. Figure 15: Time contour map of the Greenbrier/Big Injun event. 13

14 In the remainder of the class, decide which reflectors you would like to digitize. Work in pairs if desired. Look at the interpreted reflection events in the labeled section provided to you in class last week. Coherent reflectors will be easiest to pick and contour. You should interpret a minimum of 3 reflection events including the two we went through today. Next time we will talk about gridding across the basement reflection event. We have to surround the faults by fault polygons so that the computer will honor the faults and not contour across them. Assignment: 1) Individuals should complete the interpretation and picking of at least 2 prominent reflection events in addition to the Greenbrier/Big Injun and basement event picked in class today. 2) Grid each horizon and contour it. Again, I encourage you to work as teams and to discuss your interpretations. Teams should coordinate event interpretations so that together they cover the stratigraphic section in greater detail. This is not a requirement but the team effort and discussion will allow for a sharing of ideas and perspectives. The varied backgrounds of members in any group always provides for an interesting exchange of ideas. 14