Presentation Chapter 5: Processing of Seismic Reflection Data (Part 2)

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  • 1. Primaries and multiple: stack
  • 2. Stack
  • 3. Stack: 7% wrong velocity
  • 4. Zero-offset gather (no stacking)
  • 5. Effect velocity on stack/migration
  • 6. Processing flow shot data CMP sorting velocity model NMO correction stack velocity model(zero-offset) migration image
  • 7. Migration (zero-offset) Stacked section is assumed to be zero-offset(as if you shot with 1 geophone, and geophone at shot position) A zero-offset section: energy is still not focussed !!!
  • 8. Diffractor: zero-offset Modelling (we know the Earth: Position of diffractor and seismic velocity)
  • 9. Diffractor: zero-offset Migration (we do NOT know the position of diffractor, but do know the velocity)
  • 10. Diffractor: zero-offset Migration (we do NOT know the position of diffractor, but do know the velocity)
  • 11. Diffractor: zero-offset stack
  • 12. Diffractor: zero-offset stack (60-fold stack)
  • 13. Migration on dipping reflector Modelling (we know the Earth: Position of reflector and seismic velocity)
  • 14. Migration on dipping reflector Migration (we do NOT know position of reflector, but do know seismic velocity)
  • 15. Migration on piece of dipping reflector:-  Moves piece up-dip-  Makes dip steeper-  Makes piece shorter
  • 16. Migration on dipping reflector Migration (we do NOT know position of reflectors and diffractors, but do know seismic velocity)
  • 17. Apparent dipsZero-offset section: (time-) migrated section: (depth-) migrated section = depth section:
  • 18. Dip x T = 2 R/c = 2 √(x2+z2)/cz R ∂T/∂x = (2/c) ½ (x2+z2)-½ 2 x = (2/c) x/(x2+z2)½ = (2/c) x/R θ sin θ = x/R So: ∂T/∂x = 2/c sin θ
  • 19. Double-dipZero-offset section: After migrationbefore migration
  • 20. Double-dip (2) Migration with tooMigration with too high velocitylow velocity
  • 21. Syncline
  • 22. Syncline model withray paths
  • 23. Syncline: realdata example
  • 24. Migration:HOW ??
  • 25. 1 Diffractor: zero-offset “Wiggle plot”: Signals next to each other
  • 26. 1 diffractor: zero-offset “Image plot”: Like a photograph
  • 27. 1 diffractor: zero-offset Migration: Add along hyperbola (we do know velocity)
  • 28. 4 diffractors: zero-offset
  • 29. 4 diffractors: zero-offset
  • 30. 8 diffractors: zero-offset
  • 31. 16 diffractors: zero-offset
  • 32. 32 diffractors: zero-offset
  • 33. Exploding-reflector modelZero-offset response can be simulated by putting sourcesat reflector and take half the medium velocity
  • 34. 9 diffractors: zero-offset
  • 35. 17 diffractors: zero-offset
  • 36. 33 diffractors: zero-offset
  • 37. 49 diffractors: zero-offset
  • 38. ProcessingInput: Multi-offset shot recordsResults of processing (after migration): 1. Structural map of impedance contrasts 2. Velocity model
  • 39. Effect migration
  • 40. Conversion from time to depthMigration examples so far: output section in timeThis is called time migrationStill, data needs to be converted to depth: true earthSimplest case: horizontal layersFrom root-mean-square velocities to interval velocities: Dix’ formula
  • 41. Dix’ formula:from VRMS to VINTERVAL (pdf-eqs)
  • 42. Conversion from time to depth via Dix’ formula Theoretically: valid for plane horizontal layers (Theoretically: no migration necessary) Practically: still works well for dipping reflectors and mild lateral velocity variations, but then: AFTER (time) migration
  • 43. Processing flow shot data CMP sorting velocity model NMO correction stack velocity model(zero-offset) migration image
  • 44. Strategy for seismic migrationWhy ?In beginning: only RMS-velocity model known: very crude à time migration (relatively fast/cheap)When one well available: better (interval-) velocity model à some depth migration possible (relatively slow/expensive)When more wells available: good (interval-) velocity model known à pre-stack depth migration possible (slow/expensive !$!)
  • 45. Strategy for seismic migration•  Simple structure: post-stack migration•  Complex structure: pre-stack migration (such as DMO)•  Small lateral velocity variations: time migration•  Large lateral velocity variations: depth migration (no hyperbolae any more)
  • 46. Complex structure: simplest case of diffractorsdepth
  • 47. Complex structure: simplest case of diffractorsStack withno DMO
  • 48. Complex structure: simplest case of diffractorsMigrated stackWith no DMO
  • 49. Complex structure: simplest case of diffractorsStack withDMO
  • 50. Complex structure: simplest case of diffractorsMigrated stackWith DMO
  • 51. Complex structure: simplest case of diffractorsComplex case: DMO = form of pre-stack time migration necessary (only root-mean-square-velocity model necessary)
  • 52. Large lateral velocity variations: time section
  • 53. Large lateral velocity variations: depth section
  • 54. Large lateral velocity variationsNotice lateral shift of faults as well(so here time-stretching axis will not do !!!)
  • 55. Kirchhoff Pre-stack Time Migration
  • 56. Kirchhoff Pre-stack Depth Migration
  • 57. Kirchhoff Pre-stack Time Migration
  • 58. Wave Equation PSDM
  • 59. Large lateral velocity variations and complex structure
  • 60. Large lateral velocity variations and complex structure (Interval-) velocity model
  • 61. Velocities/Structures and migration
  • 62. Strategy for seismic migration•  Simple structure, small lateral velocity variations: post-stack time migration•  Complex structure, small lateral velocity variations: pre-stack time migration (generalisation DMO)•  Simple structure, large lateral velocity variations: post-stack depth migration (pull-up effect)•  Complex structure, large lateral velocity variations : pre-stack depth migration (e.g. side of salt domes)
  • 63. Migration: wave theory Migration is: ( X , Y , T ) à ( X , Y , Z ) or, better,( X , Y , Z=0 , T ) à ( X , Y , Z , T=0 )
  • 64. Migration: wave theory Transform measurements at z=0 for all t into measurements for t=0 for all z ! Two-step procedure: 1: Extrapolation:Bring P ( x , y , z=0 , t ) to P ( x , y , z=zm , t ) 2: Imaging = Select only t=0: Pmigr ( x , y , z=zm , t ) = P ( x , y , z=zm , t=0 )
  • 65. Wavefield extrapolation
  • 66. Wavefield extrapolation
  • 67. Wavefield extrapolation (2)
  • 68. Zero-offset migration schemeZero-offset migration can be applied by:•  Consider zero-offset data as exploding reflector measurements in medium with half the true velocity•  Extrapolating surface data into subsurface•  At t=0, the imaged reflector should appear at correct depth ( Imaging condition )Often for geological interpretation, the output is calculatedas a function of time.
  • 69. Ideal migration when velocity model is known Pre-stack Depth Migration: •  Inverse wavefield extrapolation receivers into medium •  Forward wavefield extrapolation source into medium •  Correlate two resulting wavefield •  Extract t=0 component