1. Environmental and Exploration Geophysics II
Moveout and Coincident Source-Receiver Concepts & 3D Seismic Interpretation
tom.h.wilson
Department of Geology and Geography
West Virginia University
Morgantown, WV
Tom Wilson, Department of Geology and Geography

2. Review TNMO relation on page 160
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 (p )
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 ). 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 point concept (p ).
7) Correcting for normal moveout (p ).
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. Why do the amplitudes drop off below 200ms?
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. Note that critical refractions point to individual source points.
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.
We wzat to continue on and put the kid of data you are using over Granny Creek and that you see most often in exploration applications into perspective
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. The split spread provides symmetrical coverage about the source
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. Square both sides of this equation
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. Normal Moveout Velocity, NMO velocity, or, just VNMO.
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

24. Tom Wilson, Department of Geology and Geography

25. Dipping Layer Reflection Event has Offset Apex
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. Bring questions to class on Tuesday
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

32. Tom Wilson, Department of Geology and Geography

33. Tom Wilson, Department of Geology and Geography

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

39. Tom Wilson, Department of Geology and Geography

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 , with additional discussion on pages The idea of common depth point sorting is discussed on pages We’ve talked tangentially about resolution ( ) and velocity analysis ( ). We will be talking about stacking of CDP gathers ( ) and migration ( ). 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