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Land and Marine Seismic Acquisition from 2D to 3D

Land and Marine Seismic Acquisition from 2D to 3D. From chapters 7-12 “Elements of 3D Seismology” by Chris Liner. Outline-1. CMP METHOD (Harry Mayne) Seismic sensors geophones hydrophones gimballed geophones and hydrophones accelerometers Sources Explosives Vibroseis SEGY data.

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Land and Marine Seismic Acquisition from 2D to 3D

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  1. Land and Marine Seismic Acquisition from 2D to 3D From chapters 7-12 “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: http://www.geol.lsu.edu/Faculty/Juan/ReflectSeismol05/labs/ppt/Radio-telemetry.ppt 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. #6 #5 #4 #3 #2 #1 Common Midpoint Method (CMP Method) Hydrophone groups Shotpoint # 3 Midpoints

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

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

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

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

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

  13. #6 #5 #4 #3 #2 #1 Common Midpoint Method (CMP Method) Hydrophone groups Shotpoint # 8 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 6 7 8 8 13 5

  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 6 7 8 8 13 5

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

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

  25. #6 #5 #4 #3 #2 #1 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 A shotpoint gather samples various midpoints and a variety of angles

  26. #6 #5 #4 #3 #2 #1 What happens to the reflecting points in a shotpoint gather when the reflecting interface dips? Shotpoint #3 #6 #5 #4 #3 #2 #1 Hydrophone groups 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. 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. 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. Hydrophone group #4 Common mid-point and shotpoints 1-8 #6 #5 #4 #3 #2 #1 group 8 7 6 5 3 2 1 4 Midpoint #6 CMP gathers sample varying angles but a common geological midpoint

  33. 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 3 2 1 4 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 3 2 1 4 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. 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. 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. 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., 10-80 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)

  48. Explosives Noble Explochem Limited

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

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