Reflection seismograms

1. Reflection seismograms Each reflection will follow a moveout trajectory according to the moveout equation

2. Reflection data processing A collection of shot gathers from around the world (from Yilmaz, 2001)

3. Reflection data processing • Even for simple models, reflection shot gathers can be very complex • Reflections, refractions, ground roll, direct waves are all present in the data • Mode conversions, diffractions, multiples also complicate the reflection record • Nevertheless, for many decades the reflection method has dominated • Part of this success is due to the development of a powerful approach for processing shot gathers and producing reflection sections

4. Reflection data processing A few shot gathers from a marine seismic survey

5. Reflection data processing The final stacked, migrated seismic section

6. Common Midpoint (CMP) method • The basic objective is to sample each subsurface point more than once • For horizontal reflectors, the reflection point is halfway between shot and receiver (at the “midpoint”) • Survey is organized to sample midpoints repeatedly

7. Common Midpoint (CMP) method • The first step in the processing is to re-sort the original data (the shot gathers) in “CMP” gathers, in which all traces in each gather share common midpoints • Note that each shot-receiver pair comes from a different physical shot point • The number of traces in a CMP gather is known as the “fold” of the survey: • N is the number of receivers, n is the “move-up” rate • The fold of modern surveys may be as large as several hundred

8. Common Midpoint (CMP) method • Each reflection will have a distinct “moveout” on the CMP gather, related to the RMS velocity of the overlying layers • The essence of CMP processing is: • Resorting into CMP gathers • Correction for moveout • Summation, or “stacking” • Result is the enhancement of reflected signal, and the discrimination against non-signal events

9. Reflection data processing • The essence of seismic data processing is simple: • Sort into CMP gathers • Correct for NMO • Stack • The chart on the right shows some of the details used in practice • In the following slides this flowchart will be described in more detail, with a sample dataset from the Caspian Sea, Turkey

10. Raw shot gathers

11. Low cut filtered data

12. Amplitude correction

13. “Deconvolution”

14. CMP sorting

15. CMP sorting

16. NMO correction

17. Reflection velocity analysis • In order to carry out the “Normal Moveout Correction”, we make use of • The rms velocity to each reflector is required • We generate the required velocities using “reflection velocity analysis” (see later)

18. Reflection velocity analysis

19. CMP sorting

20. NMO correction

21. Muting

22. Stacking

23. Amplitude scaling

24. Seismic “migration”

25. Next lecture: Reflection velocity analysis • In order to carry out the “Normal Moveout Correction”, we make use of • The rms velocity to each reflector is required • We generate the required velocities using “reflection velocity analysis”