1. Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics II tom.h.wilson tom.wilson@mail.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV Dipping Layer Refraction Problem, Moveout and Coincident Source-Receiver Concepts

2. Tom Wilson, Department of Geology and Geography The time distance (t-x) plot

3. Tom Wilson, Department of Geology and Geography Determination of layer properties in the dipping layer case requires shots in the down-dip and up-dip directions Up-dip shot Down-dip shot

4. Tom Wilson, Department of Geology and Geography Repeat derivation for the up-dip direction

5. Tom Wilson, Department of Geology and Geography The t-x plot gets a little more complicated and includes the combining the responses in the up-dip and down-dip direction. Assuming there is no knowledge of dip these directions are simply referred to as “forward” and “reverse.”

6. Tom Wilson, Department of Geology and Geography

7. Note that the subscript d or u consistently refers to the location of the source as downdip or updip, respectively.

8. Tom Wilson, Department of Geology and Geography From an “intuitive” perspective - in which direction will the refraction arrivals come in earlier?

9. Tom Wilson, Department of Geology and Geography We can now determine  c and 

10. Tom Wilson, Department of Geology and Geography How do you determine V 2 ?

11. Tom Wilson, Department of Geology and Geography Questions about problems 1-4?

12. Tom Wilson, Department of Geology and Geography 1. A reversed seismic refraction survey indicates that a layer with velocity V 1 lies above another layer with velocity V 2 and that V 2 >V 1. We examine the travel times at a point located midway (at C) between the shotpoints (at A and B). The travel time of the refracted ray from end A to midpoint C is less than the travel time of the refracted wave from end B to midpoint C. Show that the apparent velocity determined from the slope of the travel time curve for refracted waves produced from the source at A is less that the apparent velocity for refracted waves produced from the source at B. Toward which end of the layout does the boundary between the V 1 and V 2 layers dip? i.e. where is down- dip? Explain! (Robinson and Coruh, 1988) Some problems to consider: Problem set 4

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

14. Tom Wilson, Department of Geology and Geography 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. But - this is not the way the data was collected. The short story

15. Tom Wilson, Department of Geology and Geography Note that the reflection point coverage spans half the distance between the source and receiver

16. Tom Wilson, Department of Geology and Geography The split spread provides symmetrical coverage about the source

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

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

19. Tom Wilson, Department of Geology and Geography  t is the normal moveout correction Assume  t 2 is small relative to other terms and can be ignored to approximate the moveout

20. Tom Wilson, Department of Geology and Geography Look at the reflection time distance relationship in terms of t 2 versus x 2 Square both sides of this equation

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

22. Tom Wilson, Department of Geology and Geography In the t 2 -x 2 form, the slope is 1/V 2

23. Tom Wilson, Department of Geology and Geography V is derived from the slope of the reflection event as portrayed in the t 2 -x 2 plot. The derived velocity is referred to as the Normal Moveout Velocity, NMO velocity, or, just V NMO. The moveout velocity

24. Tom Wilson, Department of Geology and Geography The V NMO is used as a correction velocity If the velocity is accurately determined the corrected time  equals t 0

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

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

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

28. Tom Wilson, Department of Geology and Geography

29. NMO correction of the reflection events appearing in the shot records across relatively horizontal strata yields a more accurate image of subsurface geology.

30. Tom Wilson, Department of Geology and Geography

31. These data sets are referred to as being single fold data. Single fold implies only one trace per mid-point. Single fold data are also sometimes referred to as 100% data

32. Tom Wilson, Department of Geology and Geography  Chapter 4, pages 165 to 199 and 206 to 229. Hand in Exercises I-III and the Attenuation problem today Continue your work on problem sets 3 and 4