1. Land and Marine Seismic Acquisition from 2D to 3D
From chapters “Elements of 3D Seismology” by Chris Liner

2. Outline-1 CMP METHOD (Harry Mayne) Seismic sensors geophones
hydrophones
gimballed geophones and hydrophones
accelerometers
Sources
Explosives
Vibroseis
SEGY data

3. Outline-2 Acquisition Parameters Time Sample Rate Offset Range
Listen Time
Sample Rate and Temporal Aliasing
Geophone Spacing and Spatial Aliasing
Shooting geometry
inline
cross-line

4. Common Midpoint Method (CMP Method)
Please take a look at the powerpoint presentation for the radio-telemetry field trip at the following link:
This link has information to complement the explanation on the CMP method.

5. Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 1 #6 #5 #4 #3 #2 #1
Midpoints
Separation between midpoints is
1/2 separation between hydrophone groups

6. Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 2 #6 #5 #4 #3 #2 #1
Midpoints

7. Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 3 #1 #2 #3 #4 #5 #6
Midpoints

8. Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 4 #1 #2 #3 #4 #5 #6
Midpoints

9. Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 5 #1 #2 #3 #4 #5 #6
Midpoints

10. Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 6 #1 #2 #3 #4 #5 #6
Midpoints

11. Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 7 #1 #2 #3 #4 #5 #6
Midpoints

12. Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 8 #1 #2 #3 #4 #5 #6
Midpoints

13. Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 8 #1 #2 #3 #4 #5 #6
Midpoints

14. Common Midpoint Method (CMP Method)
Hydrophone groups #6 #5 #4 #3 #2 #1
Shotpoint # 1
Midpoints

15. Common Midpoint Method (CMP Method)
Hydrophone groups #6 #5 #4 #3 #2 #1
Shotpoint # 1
Shotpoint # 2
Shotpoint # 1
Shotpoint # 2
Midpoints

16. Common Midpoint Method (CMP Method)
Hydrophone groups #6 #5 #4 #3 #2 #1
Shotpoint # 1
Shotpoint # 2
Shotpoint # 3
Shotpoint # 1
Shotpoint # 2
Shotpoint # 3
Midpoints

17. Common Midpoint Method (CMP Method)
Hydrophone groups #6 #5 #4 #3 #2 #1
Shotpoint # 1
Shotpoint # 2
Shotpoint # 3
Shotpoint # 4
Shotpoint # 1
Shotpoint # 2
Shotpoint # 3
Shotpoint # 4
Midpoints

18. Common Midpoint Method (CMP Method)
Hydrophone groups #6 #5 #4 #3 #2 #1
Shotpoints # 1-8 1 2
Midpoints 3 4 5 6 7 8 8 13

19. Common Midpoint Method (CMP Method)
Fold or Multiplicity is the number of times that the same midpoint is sampled by different shots and different receivers
Signal-to-Noise increases as the square root of the fold
Fold 1 2
Midpoints 3 4 5 6 7 8 8 13

20. Common Midpoint Method (CMP Method)
Maximum Fold is achieved after the 6th shot
Fold 1 2
Midpoints 3 4 5 6 7 8 8 13

21. Common Midpoint Method (CMP Method)
When shotpoint spacing and group spacing are equal then
Maximum fold = number of geophones or hydrophones
Midpoint separation = 1/2 distance between geophones
In a more general case:
Maximum Fold = #recording groups * distance between groups
2 * distance between shots
Midpoint separation = 1/2 smaller of the two: receiver group spacing or shot spacing

22. Gather Types
A gather i.e. “a subset of the traces from the entire data set” can be of different types:
Shotpoint gather
Common source-receiver offset gather (COS)
Common midpoint gather

23. Shotpoint Gather
e.g. Shotpoint gather #3 #6 #5 #4 #3 #2 #1

24. A shotpoint gather samples various midpoints and a variety of angles#6 #5 #4 #3 #2 #1
Hydrophone groups #1 #2 #3 #4 #5 #6
A shotpoint gather samples various midpoints and a variety of angles

25. A shotpoint gather samples various midpoints and a variety of angles
What happens to the reflecting points in a shotpoint gather when the reflecting interrface dips?
Shotpoint #3 #6 #5 #4 #3 #2 #1
Hydrophone groups #1 #2 #3 #4 #5 #6
A shotpoint gather samples various midpoints and a variety of angles

26. What happens to the reflecting points in a shotpoint gather when the reflecting interface dips?#6 #5 #4 #3 #2 #1
Hydrophone groups #1 #2 #3 #4 #5 #6
Reflecting points
Midpoints
A shotpoint gather samples different reflecting points at a variety of angles

27. Common Midpoint Method (CMP Method)
Hydrophone group #4
Common source-receiver offset and
common receiver, shotpoints 1-8 #6 #5 #4 #3 #2 #1

28. Common source-receiver offset and common receiver, shotpoints 1-8
Hydrophone group #4
Common source-receiver offset and
common receiver, shotpoints 1-8 #6 #5 #4 #3 #2 #1
COS means equal reflection angle
Midpoints

29. COS means equal reflection angle
In the case of a COS gather where are the true midpoints when the reflecting, geological interface has a dip? #6 #5 #4 #3 #2 #1
COS means equal reflection angle
Midpoints

30. COS NO LONGER implies equal reflection angles#6 #5 #4 #3 #2 #1
Actual reflecting points
Midpoints

31. Common Midpoint Method (CMP Method)
Hydrophone group #4
Common mid-points and
shotpoints 1-8 #6 #5 #4 #3 #2 #1
Midpoints

32. CMP gathers sample varying angles but a common geological midpoint
Hydrophone group #4
Common mid-point and
shotpoints 1-8 #6 #5 #4 #3 #2 #1
group 8 7 6 5 4 3 2 1
Midpoint #6
CMP gathers sample varying angles but a common geological midpoint

33. CMP gathers sample varying angles but a common geological midpoint
What happens to a common midpoint gather when the reflecting interface has a dip? #6 #5 #4 #3 #2 #1
group 8 7 6 5 4 3 2 1
Midpoint #6
CMP gathers sample varying angles but a common geological midpoint

34. CMP gathers SAMPLE varying angles but with
a relatively smaller spread of
reflecting points than the shotpoint and
common-offset gathers #6 #5 #4 #3 #2 #1
group 8 7 6 5 4 3 2 1
Midpoint #6
True Reflecting Points

35. A common midpoint gather minimizes the effect of dip while it helps increase the signal-to-noise ratio

36. Outline-1 CMP METHOD (Harry Mayne) Seismic sensors geophones
hydrophones
gimballed geophones and hydrophones
accelerometers
Sources
Explosives
Vibroseis
SEGY data

37. Convert ground motion into electricity
Geophones
Convert ground motion into electricity
at a rate of about 1 Volt/inch/sec
Natural Resonance Frequency 100 Hz
GS-100 from Geospace

38. Geophone layout

39. Geophone layout

40. Hydrophones convert changing pressure into Volts (Volts/bar)
Seismic Sensors
Hydrophones convert changing pressure into Volts
(Volts/bar)
e.g. Preseis 2517 from I/O 1V/microPascal

41. Gimballed Geophone-hydrophone combinations for sea-bottom work
Sea-Array 4 from Geospace

42. Streamer layout

43. Convert ground acceleration into Volts d(dx/dt) dt
Accelerometers
Convert ground acceleration into Volts
d(dx/dt) dt
E.g. VectorSeis from I/O
3-component digital accelerometer (requires battery)
full-scale at 3.3 m/s^2; noise level 0.44 microm/s^2
140db = 20 log (3.3/4*10^-7)

44. Outline-1 CMP METHOD (Harry Mayne) Seismic sensors geophones
hydrophones
gimballed geophones and hydrophones
accelerometers
Sources
Explosives
Vibroseis
SEGY data

45. Vibroseis Method (Liner, 2004; p.157, para. 4, )
An output sweep
(e.g., Hz)
enters the earth
…..and undergoes various reflections

46. + + = ...something too complicated to draw
Field correlation “unravels” the raw data into ….

47. Vibroseis images from the Lithoprobe Project, Canada www.lithoprobe.ca
A vibrator truck
“12 elephants dancing in unison” (LITHOPROBE, CANADA)

49. Noble Explochem Limited
Explosives
Noble Explochem Limited

50. NSF R/VIB NBPalmer- February/March 2003
GI Watergun Array
NSF R/VIB NBPalmer- February/March 2003

51. Sercel G. GUN 150 cu. In. firing at 2,000 p.s.i.
Link to movie of this G. Gun working in a pool

52. Outline-1 CMP METHOD (Harry Mayne) Seismic sensors geophones
hydrophones
gimballed geophones and hydrophones
accelerometers
Sources
Explosives
Vibroseis
SEGY data

53. SEGY data 3200 byte EBCDIC header 400 byte tape header
One line at a time
400 byte tape header
240 byte trace header
DATA
240 byte trace header
DATA
240 byte trace header
DATA

54. Outline-2 Acquisition Parameters Time Sample Rate Offset Range
Listen Time
Sample Rate and Temporal Aliasing
Geophone Spacing and Spatial Aliasing
Shooting geometry
inline
cross-line

55. Sample Rates What is the fewest number of times I need to sample
this waveform per second? ? ? ?

56. Sample Rates

57. Sample Rates

58. Sample Rates

59. Sample Rates What is the fewest number of times I need to sample
this waveform per second?
At least twice per wavelength or period!
OTHERWISE ….

60. Undersampled waveforms
Amplitude
Reconstructed frequency
(f -aliased)
True frequency (f -true)

61. Oversampled waveforms
Nyquist frequency
Amplitude
Reconstructed frequency
frequency is unaliased
= True frequency (f -true)
Nyquist frequency = 1 / twice the sampling rate
Minimum sampling rate must be at least twice the desired frequency
E.g., 1000 samples per second for 500Hz,
2000 samples per second for 1000 Hz

62. Oversampled waveforms
Nyquist frequency
Amplitude
In practice we are best oversampling by double the required minimum
i.e samples per second for a maximum of 500 Hz
i.e., 2000 samples per second for a maximum of 1000 Hz
Oversampling is relatively cheap.

63. Outline-2 Acquisition Parameters Sample Rate and Temporal Aliasing
Offset Range
Listen Time
Geophone Spacing and Spatial Aliasing

64. Offset Range Maximum shot-receiver offset Target depth
One-layer earth of a semi-infinite layer
Maximum shot-receiver offset >= target depth.
Near critical distance

65. Maximum shot-receiver
Offset Range
Maximum shot-receiver
offset
Target depth
Multi-layered earth

66. Outline-2 Acquisition Parameters Time Sample Rate Offset Range
Listen Time
Sample Rate and Temporal Aliasing
Geophone Spacing and Spatial Aliasing
Shooting geometry
inline
cross-line

67. ….Twice target time to be sage
Listen Time
….Twice target time to be sage

68. Outline-2 Acquisition Parameters Time Sample Rate Offset Range
Listen Time
Sample Rate and Temporal Aliasing
Geophone Spacing and Spatial Aliasing
Shooting geometry
inline
cross-line

69. Spatial aliasing
Spatial frequency, or wavenumber (k) is the number of cycles per unit distance.
One spatial cycle or wavenumber = frequency/velocity.
Each wavenumber must be sampled at least twice per wavelength
(two CMP’s per wavelength)
IN PRACTICE each wavenumber must be sampled at least four times per minimum
wavelength (two CMP’s per wavelength)

70. Spatial aliasing
However, dip (theta) as well as frequency and velocity event changes the number of cycles per distance, so
Liner, 9.7,p.192

71. For aliasing NOT to occur, delta(t) must be less than T/2
Spatial aliasing
For aliasing NOT to occur, delta(t) must be less than T/2

72. Spatial aliasing

73. Geophone Spacing and Spatial Aliasing
K=0

74. 1/4 wavelength shift per trace total shift across array=3/4 wavelength
K=+ or -ve?

75. 1/4 wavelength shift per trace total shift across array=3/4 wavelength
K=?

76. 1/2 wavelength shift per trace total shift across array=3/2 wavelength
K=0

77. 3/4 wavelength shift per trace
total shift across array=2 1/4 wavelength

78. Spatial aliasing
Degrades (“string of pearls”) stacked sections
Degrades migration

79. Signal-to-Noise
Improves with stacking:
greater fold
greater repetition of shots

80. Outline-2 Acquisition Parameters Time Sample Rate Offset Range
Listen Time
Sample Rate and Temporal Aliasing
Geophone Spacing and Spatial Aliasing
Shooting geometry
inline
cross-line

81. Fundamental Parameters for land 3D shooting

82. Common Midpoint

83. Source-Receiver Offset2D 3D

84. Azimuth (3D)

85. Inline geometry
Matlab code

86. Outline-2 Acquisition Parameters Time Sample Rate Offset Range
Listen Time
Sample Rate and Temporal Aliasing
Geophone Spacing and Spatial Aliasing
Shooting geometry
inline
cross-line

87. Cross-line geometry
Matlab code

88. Spatial aliasing
Degrades (“string of pearls”) stacked sections
Degrades migration

89. Outline Bins Calculated common midpoints “CMP bin center”
Length and width of bin <= spatial aliasing dimensions

90. Rule of Thumb: 12.5m by 12.5 m for > 2000 m
To prevent aliasing:
max dimension = V/4fmax
For GOM:
V = V x depth
Rule of Thumb: 12.5m by 12.5 m for > 2000 m
IDEAL BIN SIZE: 5m by 5m for seafloor and deeper

91. The “best” bin:
SMALL
ALL OFFSETS
ALL AZIMUTHS
LARGE FOLD

92. Outline
Justification for Processing
Processing Flow
Bins