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Understanding Normal Moveout in 3D Seismology: Concepts and Techniques

This article delves into Chapter 16 of "Elements of 3D Seismology" by Chris Liner, focusing on Normal Moveout (NMO) and its critical role in seismic data processing. It covers key topics such as overcorrection and undercorrection of NMO, the significance of chosen velocities, and the impacts of NMO stretching. Furthermore, it explores the principles of stacking, convolution, and deconvolution in enhancing signal-to-noise ratios, supplemented by practical examples and MATLAB code. This comprehensive overview is essential for understanding seismic events and improving seismic interpretation.

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Understanding Normal Moveout in 3D Seismology: Concepts and Techniques

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  1. Making CMP’s From chapter 16 “Elements of 3D Seismology” by Chris Liner

  2. Outline • Normal Moveout • Stacking

  3. Normal Moveout Hyperbola: x T

  4. Normal Moveout x T “Overcorrected” Normal Moveout is too large Chosenvelocity for NMO is too (a) large (b) small

  5. Normal Moveout x T “Overcorrected” Normal Moveout is too large Chosenvelocity for NMO is too (a)large (b) small

  6. Normal Moveout x T “Under corrected” Normal Moveout is too small Chosenvelocity for NMO is (a) too large (b) too small

  7. Normal Moveout x T “Under corrected” Normal Moveout is too small Chosenvelocity for NMO is (a) too large (b) too small

  8. Vinterval from Vrms Dix, 1955

  9. Vrms V1 V2 Vrms < Vinterval V3

  10. Vinterval from Vrms

  11. Primary seismic events x T

  12. Primary seismic events x T

  13. Primary seismic events x T

  14. Primary seismic events x T

  15. Multiples and Primaries x M1 T M2

  16. Conventional NMO before stacking x M1 NMO correction V=V(depth) e.g., V=mz + B T M2 “Properly corrected” Normal Moveout is just right Chosenvelocity for NMO is correct

  17. Over-correction (e.g. 80% Vnmo) x x M1 M1 NMO correction V=V(depth) e.g., V=0.8(mz + B) T T M2 M2

  18. f-k filtering before stacking (Ryu) x x M1 NMO correction V=V(depth) e.g., V=0.8(mz + B) T T M2 M2

  19. Correct back to 100% NMO x x M1 M1 NMO correction V=V(depth) e.g., V=(mz + B) T T M2 M2

  20. Outline • Convolution and Deconvolution • Normal Moveout • Stacking

  21. NMO stretching T0 V1 V2 “NMO Stretching”

  22. NMO stretching V1 T0 V2 “NMO Stretching” V1<V2

  23. NMO stretching V1 V1<V2 NMO “stretch” = “linear strain” V2 Linear strain (%) = final length-original length original length X 100 (%)

  24. NMO stretching original length = final length = V1 V1<V2 V2 X 100 (%) NMO “stretch” = X 100 (%)

  25. stretching for T=2s,V1=V2=1500 m/s Green line assumes V1=V2 Blue line is for general case, where V1, V2 can be different and delT0=0.1s (this case: V1=V2) Matlab code X 100 (%)

  26. Stacking + + =

  27. Stacking improves S/N ratio + + =

  28. Semblance Analysis X + + = Twtt (s) “Semblance”

  29. Semblance Analysis X V + + = V1 V2 Twtt (s) V3 Peak energy

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