Environmental and Exploration Geophysics II

1. Environmental and Exploration Geophysics II Moveout and Coincident Source-Receiver Concepts & 3D Seismic Interpretation 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. Just a reminder: pages 149 to 164 in Chapter 4 were on your reading list. Continue reading Chapter 4. • We will be addressing some issues in a different sequence than in the text. • Review TNMO relation on page 160 • Understand the t2-x2 transformation (p165-167) • We will come back to a discussion of determining velocities, thicknesses .. (p 170 – 180). Review for now. • You will be expected to understand how to apply relationships associated with the dipping interface problem (p 192- 199). You will encounter additional discussion of moveout in these pages. Tom Wilson, Department of Geology and Geography

3. 5) Read about multiple reflections and diffractions (p206-217). 6) Familiarize yourself with the common depth pointconcept (p 225-229). 7) Correcting for normal moveout (p232- 241). Tom Wilson, Department of Geology and Geography

4. Objectives for the day • Normal moveout (NMO) and its elimination (NMO Correction) • What do dipping layer reflections look like in the shot record? • Quantitative relationships for the dipping layer reflection • The problem posed by dipping layers • Common midpoint sorting & CMP gathers • Transformation of the dipping layer reflection in the CMP gather. Tom Wilson, Department of Geology and Geography

5. Here is some shot data collected in Marshall Co. WV Enhanced display We’d like to turn this into geology. Why do the amplitudes drop off below 200ms? How do we get here? Tom Wilson, Department of Geology and Geography

6. How do we get from the shot data to the data you’ve been interpreting in the Gulf – or Tom Wilson, Department of Geology and Geography

7. this data from the North Sea …. Tom Wilson, Department of Geology and Geography

8. or Appalachians The short story The effective source receiver geometry for the records shown at right across the east margin of the Rome Trough is corrected so that the source and receivers share the same surface location. Note that critical refractions point to individual source points. Tom Wilson, Department of Geology and Geography

9. st 1 Flatten in time At this point it is apparent that something has to be done to flatten out the hyperbolas to make them look more like continuous geologic horizons Tom Wilson, Department of Geology and Geography

10. Note that the reflection point coverage spans half the distance between the source and receiver Off-end Split spread The split spread provides symmetrical coverage about the source Tom Wilson, Department of Geology and Geography

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

12. 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

13. 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

14. 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

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

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

17. The normal moveout velocity - VNMO 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

18. 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

19. hyperbolas or ellipses Tom Wilson, Department of Geology and Geography

20. 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

21. And we have an ellipse Tom Wilson, Department of Geology and Geography

22. Roll-along split-spread shooting geometry Tom Wilson, Department of Geology and Geography

23. NMO corrected reflections NMO correction of the reflection events appearing in the shot records across relatively horizontal strata yields a more accurate image of subsurface geology. Tom Wilson, Department of Geology and Geography

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25. Dipping Layer Reflection Event has Offset Apex.How do you find depth h, velocity V and dip ? Tom Wilson, Department of Geology and Geography

26. Features of the reflection from a dipping interface as observed in the shot record. If you could not see the direct arrival then you could solve for V using either expressions for t0 or tapex. Tom Wilson, Department of Geology and Geography

27. The NMO correction is symmetrical about the zero offset or source point. The dipping layer reflection event is not. Tom Wilson, Department of Geology and Geography

28. Reflection points Tom Wilson, Department of Geology and Geography

29. Following the distribution of common reflection points Tom Wilson, Department of Geology and Geography

30. Different source receiver combinations provide information from the same reflection point This is referred to as a stacking chart. The significance of the name will become apparent later on. Tom Wilson, Department of Geology and Geography

31. For next week at this time construct a stacking chart for a symmetrical split spread consisting of 12 geophones arranged 6 on each side of the source. Bring questions to class on Tuesday Tom Wilson, Department of Geology and Geography

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34. Time-distance relationship for reflections in a CMP gather are identical to those in the shot record. Tom Wilson, Department of Geology and Geography

35. Definition - CMP Gather: A collection of traces sharing a common midpoint. Tom Wilson, Department of Geology and Geography

36. Raypaths in the CMP Gather don’t necessarily provide information from the same reflection point! Tom Wilson, Department of Geology and Geography

37. But reflection events in a CMP gather have a special property Tom Wilson, Department of Geology and Geography

38. Even when the layer dips they remain hyperbolic Tom Wilson, Department of Geology and Geography

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40. And they are symmetrical Tom Wilson, Department of Geology and Geography

41. Symmetrical hyperbola are easy to NMO correct! The effect of the moveout correction on the traces in the common midpoint (CMP) gather is to create a composite normal incidence trace that effectively shares a coincident source and receiver at the midpoint shared by all the traces in the gather. We’ll discuss CMP data in more detail in a couple lectures. Tom Wilson, Department of Geology and Geography

42. Homework • Construct a stacking chart for a symmetrical split spread consisting of 12 geophones arranged 6 on each side of the source (see handout). • Bring questions to class This Wednesday. The chart is due next Monday. • Complete your reading of Chapter 4. Dipping layer reflection events are covered on pages 183-186, with additional discussion on pages 186-196. The idea of common depth point sorting is discussed on pages 225 -229. We’ve talked tangentially about resolution (217-219) and velocity analysis (233-238). We will be talking about stacking of CDP gathers (238- 241) and migration (241-244). Discussions of migration will come later but it is helpful to be aware of the issues early on. • Look over problems 4.1, 4.4 and 4.8. Tom Wilson, Department of Geology and Geography