Velocity Extraction Concepts in Seismic Data Analysis

1. Environmental and Exploration Geophysics II Coincident Source receiver Concepts Extracting Velocities from Seismic Data tom.h.wilson tom.wilson@mail.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV Tom Wilson, Department of Geology and Geography

2. Common Midpoint (CMP) gather, also often referred to as Common Depth Point (CDP) Tom Wilson, Department of Geology and Geography

3. Common Midpoint Gather Doug Smith’s seismic Data Processing site http://www-geo.phys.ualberta.ca/~doug/G438/Assignments07/Lectures/Common_Midpoint.pdf Tom Wilson, Department of Geology and Geography

4. Moveout and the moveout correction Tom Wilson, Department of Geology and Geography

5. Redefine the reflection time equal to the 0-offset arrival time (t0) plus the t (drop from t0 or “moveout”). Tom Wilson, Department of Geology and Geography

6. Assume t2 is small relative to other terms and can be ignored to approximate the moveout t is the normal moveout correction Tom Wilson, Department of Geology and Geography

7. Look at the reflection time distance relationship in terms of t2 versus x2 Square both sides of this equation Tom Wilson, Department of Geology and Geography

8. The hyperbola becomes a straight line Tom Wilson, Department of Geology and Geography

9. In the t2-x2 form, the slope is 1/V2 Tom Wilson, Department of Geology and Geography

10. The moveout velocity V is derived from the slope of the reflection event as portrayed in the t2-x2 plot. The derived velocity is referred to as the Normal Moveout Velocity, NMO velocity, or, just VNMO. Tom Wilson, Department of Geology and Geography

11. The VNMO is used as a correction velocity If the velocity is accurately determined the corrected time  equals t0 Tom Wilson, Department of Geology and Geography

12. Fun with hyperbolas and ellipses Tom Wilson, Department of Geology and Geography

13. If the correction velocity (VNMO) is too high then the correction is too small and we still have a hyperbola Tom Wilson, Department of Geology and Geography

14. And we have Tom Wilson, Department of Geology and Geography

15. Velocity Analysis Tom Wilson, Department of Geology and Geography

16. Discussion of problems 4.1 Read over and think about how you are going to solve problems 4.5 and 4.8 Tom Wilson, Department of Geology and Geography

17. Dix Interval Velocities The Dix interval velocity assumes VNMO velocities are equivalent to RMS velocities. Starting with the basic definition of the RMS velocity we derive the interval velocity Vn. Read over discussions of Dix Interval Velocities (pages 170 -181) Tom Wilson, Department of Geology and Geography

18. Using Fault Polygons • General Steps • Create a fault polygon set: usually identified by horizon • Digitize your fault polygon, double click and assign a fault to it • Digitize fault polygons for all faults intersecting that surface and assign the appropriate fault to each polygon • Under Fault Polygon Management Specify “Fill Color Based on Associated Fault Surface” • Go to next horizon Tom Wilson, Department of Geology and Geography

19. See revisions (on line) to Parts 3 and 4 of the 3D seismic Interpretation procedural notes Tom Wilson, Department of Geology and Geography

20. Horizon > Polygon Smooth Another useful tool – smoothing can help fill gaps in your interpretations Tom Wilson, Department of Geology and Geography

21. Some structural views and potential traps should be emerging in your interpretations Tom Wilson, Department of Geology and Geography

22. To Do List • Work problem 4.1 (Bring questions in Monday). 4.1 is due next Wednesday. • Digitize additional horizons • Discuss divisions of labor with your team mates • Read over discussions of Dix Interval Velocities (pages 170- 181) Tom Wilson, Department of Geology and Geography