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 Velocity Analysis and Pitfalls in Seismic Interpretation

2. Tom Wilson, Department of Geology and Geography Hand back problem 4.1 and discuss Exercise VI will is due this Wednesday: Questions? Problems 4.4 and 4.8 are also due this Wednesday: Questions? Hand in Exercises IV-V today. Hand in your Mid Term report today.

3. Tom Wilson, Department of Geology and Geography 4.1 Given that the near- offset trace is located 40 meters from the source. Successive traces are spaced at 3 meter intervals yielding offsets ranging from 40 meters to 73 meters on the outer trace. Shot record shows reflection event with near offset arrival time of approximately 80 ms.

4. Tom Wilson, Department of Geology and Geography You were asked to treat this as a single layer reflection problem. But - one might argue that there is a shallower reflection event that has been overlooked. Can you see it? Multilayer rather than single layer?

5. Tom Wilson, Department of Geology and Geography Anything else?? Of course it is tempting to simplify problems, but just as with the refraction analysis, if we miss a shallower layer, error is introduced into our result.

6. Tom Wilson, Department of Geology and Geography Start with definition of the RMS velocity The V i s are interval velocities and the t i s are the two-way interval transit times.

7. Tom Wilson, Department of Geology and Geography Let the two-way travel time of the n th reflector

8. Tom Wilson, Department of Geology and Geography hence

9. Tom Wilson, Department of Geology and Geography Since V n is the interval velocity of the n th layer t n in this case represents the two-way interval transit time through the n th layer

10. Tom Wilson, Department of Geology and Geography Hence, the interval velocities of individual layers can be determined from the RMS velocities, the 2-way zero - offset reflection arrival times and interval transit times.

11. Tom Wilson, Department of Geology and Geography Offset (m)Reflection1Reflection2Reflection3 xt1 (ms)t2 (ms)t3 (ms) 321.462.379.4 62562.479.5 930.162.679.6 1236.162.979.9 1542.563.280.1 1849.263.680.5 2156.264.180.9 2463.364.781.3 2770.465.481.8 3077.666.182.4 3384.966.983 3692.267.783.7 Table 1 (right) lists reflection arrival times for three reflection events observed in a common midpoint gather. The offsets range from 3 to 36 meters with a geophone spacing of 3 meters. Conduct velocity analysis of these three reflection events to determine their NMO velocity. Using that information, determine the interval velocities of each layer and their thickness.

12. Tom Wilson, Department of Geology and Geography In t-x space we see hyperbolic moveout of the three reflection events.

13. Tom Wilson, Department of Geology and Geography Recall - The variables t 2 and x 2 are linearly related.

14. Tom Wilson, Department of Geology and Geography Estimates of RMS velocities can be determined from the slopes of regression lines fitted to the t 2 -x 2 responses. Keep in mind that the fitted velocity is actually an NMO velocity! In t 2 -x 2 space we have nice straight lines

15. Tom Wilson, Department of Geology and Geography the two-way travel time to the n th reflector surface the two-way interval transit time between the n and n-1 reflectors The terms represent the velocities obtained from the best fit lines. Remember these velocities are actually NMO velocities. is the interval velocity for layer n, where layer n is the layer between reflectors n and n-1 See Berger et al. page 173

16. Tom Wilson, Department of Geology and Geography Layer 1 Layer 2 Layer 3 Reflector 1 2 3 Terminology review

17. Tom Wilson, Department of Geology and Geography The interval velocity that’s derived from the RMS velocities of the reflections from the top and base of a layer is referred to as the Dix interval velocity. However, keep in mind that we really don’t know what the RMS velocity is. The NMO velocity is estimated from the t 2 - x 2 regression line for each reflection event and that NMO velocity is assumed to “represent” an RMS velocity. You put these ideas into application when solving problems 4.4

18. Tom Wilson, Department of Geology and Geography The purpose of common midpoint is multi fold! It enhances signal It suppresses noise Both coherent and incoherent It provides useful information about subsurface physical properties

19. Tom Wilson, Department of Geology and Geography Recall that the relative amplitude of the noise is analogous to the distance traveled by the random walker. We actually get somewhere when we take steps at random! In the seismic analogue, if N traces are summed together, the amplitude of the resultant signal will be N times its original value. The signal always arrives at the same time and sums together constructively. The amplitude of random noise on the other hand increases as Hence, the ratio of signal to noise is or just where N is the number of traces summed together or the number of traces in the CMP gather.

20. Tom Wilson, Department of Geology and Geography Getting the velocities correct is critical to attenuating noise and enhancing signal. After NMO Correction

21. Tom Wilson, Department of Geology and Geography All the traces in the NMO corrected CDP gather are summed together to yield one trace in the stacked seismic data set. If the signal is summed together in phase we get nice looking reflcetion events in the stacked seismic section

22. Tom Wilson, Department of Geology and Geography Stacking also helps attenuate coherent noises Multiples Refractions Air waves Ground Roll Streamer cable motion Scattered waves from off line

23. Tom Wilson, Department of Geology and Geography Multiples are considered “coherent” noise or unwanted signal

24. Tom Wilson, Department of Geology and Geography Waterbottom and sub-bottom multiples

25. Tom Wilson, Department of Geology and Geography or

26. Tom Wilson, Department of Geology and Geography In theanalysis (see pages 197 to 199), you’ll run across the idea of “dip moveout.” This is just the difference at some value of x from the source of the arrival times at a distance x on either side of the source. In the text  is used for dip rather than  Read over this material (pages 197 to 199) to help guide you through the solution. Also see table 4.7 and related discussions (pages 182-199).

27. Tom Wilson, Department of Geology and Geography Pitfalls in seismic interpretation

28. Tom Wilson, Department of Geology and Geography Hand back problem 4.1 and discuss Exercise VI will is due this Wednesday: Questions? Problems 4.4 and 4.8 are also due this Wednesday: Questions? Hand in Exercises IV-V today. Hand in your Mid Term report today.