Primaries and multiple: stack
Stack
Stack: 7% wrong velocity
Zero-offset gather  (no stacking)
Effect velocity on stack/migration
Processing flow      shot data    CMP sorting                           velocity model   NMO correction        stack      ...
Migration (zero-offset)         Stacked section is assumed to be zero-offset(as if you shot with 1 geophone, and geophone ...
Diffractor: zero-offset            Modelling       (we know the Earth:       Position of diffractor       and seismic velo...
Diffractor: zero-offset           Migration      (we do NOT know        the position of       diffractor, but do      know...
Diffractor: zero-offset          Migration     (we do NOT know       the position of      diffractor, but do     know the ...
Diffractor: zero-offset stack
Diffractor: zero-offset stack           (60-fold stack)
Migration on dipping reflector               Modelling         (we know the Earth:          Position of reflector         ...
Migration on dipping reflector               Migration          (we do NOT know         position of reflector,            ...
Migration on piece of dipping             reflector:-  Moves piece up-dip-  Makes dip steeper-  Makes piece shorter
Migration on dipping reflector                   Migration              (we do NOT know           position of reflectors a...
Apparent dipsZero-offset section:              (time-) migrated section:          (depth-) migrated section = depth section:
Dip    x                  T = 2 R/c = 2 √(x2+z2)/cz       R         ∂T/∂x = (2/c) ½ (x2+z2)-½ 2 x                         ...
Double-dipZero-offset section:    After migrationbefore migration
Double-dip (2)                       Migration with tooMigration with too                       high velocitylow velocity
Syncline
Syncline  model  withray paths
Syncline: realdata example
Migration:HOW ??
1 Diffractor: zero-offset          “Wiggle plot”:     Signals next to each other
1 diffractor: zero-offset         “Image plot”:       Like a photograph
1 diffractor: zero-offset         Migration:     Add along hyperbola    (we do know velocity)
4 diffractors: zero-offset
4 diffractors: zero-offset
8 diffractors: zero-offset
16 diffractors: zero-offset
32 diffractors: zero-offset
Exploding-reflector modelZero-offset response can be simulated by putting sourcesat reflector and take half the medium vel...
9 diffractors: zero-offset
17 diffractors: zero-offset
33 diffractors: zero-offset
49 diffractors: zero-offset
ProcessingInput: Multi-offset shot recordsResults of processing (after migration):       1. Structural map of impedance co...
Effect migration
Conversion from time to depthMigration examples so far:       output section in timeThis is called time migrationStill, da...
Dix’ formula:from VRMS to VINTERVAL         (pdf-eqs)
Conversion from time to depth via         Dix’ formula       Theoretically: valid for plane horizontal layers          (Th...
Processing flow      shot data    CMP sorting                           velocity model   NMO correction        stack      ...
Strategy for seismic              migrationWhy ?In beginning: only RMS-velocity model known: very crude       à time migr...
Strategy for seismic          migration•  Simple structure: post-stack migration•  Complex structure: pre-stack migration ...
Complex structure:        simplest case of diffractorsdepth
Complex structure:             simplest case of diffractorsStack withno DMO
Complex structure:             simplest case of diffractorsMigrated stackWith no DMO
Complex structure:             simplest case of diffractorsStack withDMO
Complex structure:          simplest case of diffractorsMigrated stackWith DMO
Complex structure:          simplest case of diffractorsComplex case:      DMO = form of pre-stack time migration necessar...
Large lateral velocity variations:          time section
Large lateral velocity variations:          depth section
Large lateral velocity variationsNotice lateral shift of faults as well(so here time-stretching axis will not do !!!)
Kirchhoff Pre-stack Time Migration
Kirchhoff Pre-stack Depth Migration
Kirchhoff Pre-stack Time Migration
Wave Equation PSDM
Large lateral velocity variations    and complex structure
Large lateral velocity variations    and complex structure             (Interval-) velocity model
Velocities/Structures and        migration
Strategy for seismic          migration•  Simple structure, small lateral velocity variations:        post-stack time migr...
Migration: wave theory              Migration is:      ( X , Y , T ) à ( X , Y , Z )                or, better,( X , Y , ...
Migration: wave theory   Transform measurements at z=0 for all t     into measurements for t=0 for all z !               T...
Wavefield extrapolation
Wavefield extrapolation
Wavefield extrapolation (2)
Zero-offset migration schemeZero-offset migration can be applied by:•  Consider zero-offset data as exploding reflector me...
Ideal migration when velocity       model is known         Pre-stack Depth Migration:  •  Inverse wavefield extrapolation ...
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Presentation Chapter 5: Processing of Seismic Reflection Data (Part 2)

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

  1. 1. Primaries and multiple: stack
  2. 2. Stack
  3. 3. Stack: 7% wrong velocity
  4. 4. Zero-offset gather (no stacking)
  5. 5. Effect velocity on stack/migration
  6. 6. Processing flow shot data CMP sorting velocity model NMO correction stack velocity model(zero-offset) migration image
  7. 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. 8. Diffractor: zero-offset Modelling (we know the Earth: Position of diffractor and seismic velocity)
  9. 9. Diffractor: zero-offset Migration (we do NOT know the position of diffractor, but do know the velocity)
  10. 10. Diffractor: zero-offset Migration (we do NOT know the position of diffractor, but do know the velocity)
  11. 11. Diffractor: zero-offset stack
  12. 12. Diffractor: zero-offset stack (60-fold stack)
  13. 13. Migration on dipping reflector Modelling (we know the Earth: Position of reflector and seismic velocity)
  14. 14. Migration on dipping reflector Migration (we do NOT know position of reflector, but do know seismic velocity)
  15. 15. Migration on piece of dipping reflector:-  Moves piece up-dip-  Makes dip steeper-  Makes piece shorter
  16. 16. Migration on dipping reflector Migration (we do NOT know position of reflectors and diffractors, but do know seismic velocity)
  17. 17. Apparent dipsZero-offset section: (time-) migrated section: (depth-) migrated section = depth section:
  18. 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. 19. Double-dipZero-offset section: After migrationbefore migration
  20. 20. Double-dip (2) Migration with tooMigration with too high velocitylow velocity
  21. 21. Syncline
  22. 22. Syncline model withray paths
  23. 23. Syncline: realdata example
  24. 24. Migration:HOW ??
  25. 25. 1 Diffractor: zero-offset “Wiggle plot”: Signals next to each other
  26. 26. 1 diffractor: zero-offset “Image plot”: Like a photograph
  27. 27. 1 diffractor: zero-offset Migration: Add along hyperbola (we do know velocity)
  28. 28. 4 diffractors: zero-offset
  29. 29. 4 diffractors: zero-offset
  30. 30. 8 diffractors: zero-offset
  31. 31. 16 diffractors: zero-offset
  32. 32. 32 diffractors: zero-offset
  33. 33. Exploding-reflector modelZero-offset response can be simulated by putting sourcesat reflector and take half the medium velocity
  34. 34. 9 diffractors: zero-offset
  35. 35. 17 diffractors: zero-offset
  36. 36. 33 diffractors: zero-offset
  37. 37. 49 diffractors: zero-offset
  38. 38. ProcessingInput: Multi-offset shot recordsResults of processing (after migration): 1. Structural map of impedance contrasts 2. Velocity model
  39. 39. Effect migration
  40. 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. 41. Dix’ formula:from VRMS to VINTERVAL (pdf-eqs)
  42. 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. 43. Processing flow shot data CMP sorting velocity model NMO correction stack velocity model(zero-offset) migration image
  44. 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. 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. 46. Complex structure: simplest case of diffractorsdepth
  47. 47. Complex structure: simplest case of diffractorsStack withno DMO
  48. 48. Complex structure: simplest case of diffractorsMigrated stackWith no DMO
  49. 49. Complex structure: simplest case of diffractorsStack withDMO
  50. 50. Complex structure: simplest case of diffractorsMigrated stackWith DMO
  51. 51. Complex structure: simplest case of diffractorsComplex case: DMO = form of pre-stack time migration necessary (only root-mean-square-velocity model necessary)
  52. 52. Large lateral velocity variations: time section
  53. 53. Large lateral velocity variations: depth section
  54. 54. Large lateral velocity variationsNotice lateral shift of faults as well(so here time-stretching axis will not do !!!)
  55. 55. Kirchhoff Pre-stack Time Migration
  56. 56. Kirchhoff Pre-stack Depth Migration
  57. 57. Kirchhoff Pre-stack Time Migration
  58. 58. Wave Equation PSDM
  59. 59. Large lateral velocity variations and complex structure
  60. 60. Large lateral velocity variations and complex structure (Interval-) velocity model
  61. 61. Velocities/Structures and migration
  62. 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. 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. 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. 65. Wavefield extrapolation
  66. 66. Wavefield extrapolation
  67. 67. Wavefield extrapolation (2)
  68. 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. 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
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