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Introduction to Seismic Method

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Introduction to Seismic Method

  1. 1. Using Geophysics to Characterize the Subsurface: “The Principles” BERAN GÜRLEME OCAK/2011
  2. 2. Using Geophysics to Characterize the Subsurface: The Principles
  3. 3. Using Geophysics to Characterize the Subsurface: The Principles
  4. 4. We determine subsurface conditions by remotely sensing physical properties of materials in situ.
  5. 5. There are basically four modes (or “phases”) used in seismic and radar investigations.
  6. 6. a) Vertical reflection b) Wide angle reflection c) Critical refraction d) Direct wave
  7. 7. The seismic method records these signals and analyses their relationship.
  8. 8. To do so, we need to precisely determine their arrival times, or “traveltimes”.
  9. 9. To do so, we need to precisely determine their arrival times, or “traveltimes”.
  10. 10. To do so, we need to precisely determine their arrival times, or “traveltimes”.
  11. 11. These relative traveltimes are the basis for interpreting seismic data.
  12. 12. An example of 4 traces recorded at adjacent position offsets. “Picking” traveltimes (or “first breaks”) is sometimes a subjective art form.
  13. 13. An example of 4 traces recorded at adjacent position offsets. How do these look ?
  14. 14. An example of 4 traces recorded at adjacent position offsets. The traveltime of a phase corresponds to the relative time between its “first break” and the launch of the signal.
  15. 15. Seismic Sources !!
  16. 16. The transmitted chirp is cross-correlated with the composite signal received at the geophones to detect specific reflections, refractions, etc. Size varies: • One person vibrators or compactors; • Articulated earth movers.
  17. 17. (U British Columbia: (University of Bergen.) Lithoprobe Project.) (Network for Earthquake Engineering Simulation; U Texas.)
  18. 18. Vibrating type sources
  19. 19. Implementing the Seismic Method.
  20. 20. Consider a two layered earth model.
  21. 21. Principal instruments.
  22. 22. Add an operator.
  23. 23. A “Shot”.
  24. 24. A sound pulse is generated.
  25. 25. And recorded.
  26. 26. And . . .
  27. 27. . . . a reflection is generated.
  28. 28. And recorded.
  29. 29. To summarize: An impulsive source (a sledge hammer blow to a steel plate) generates a sound wave that travels through the subsurface. . . .
  30. 30. . . . If one knows the distance (x) between the “shot” and the sensor, and the time (T) it takes the wave to travel this distance, . . .
  31. 31. . . . one can determine the velocity (V) of the material, V = x / T and tell, for example, . . .
  32. 32. . . . whether the medium is bedrock, dry soil, or saturated soil, among other possibilities.
  33. 33. In this way, we determine the material properties of the subsurface.
  34. 34. Alternatively, if one knows the velocity (V) of the material and the time (T) it takes the wave to get to a sensor, then rearranging V = x / T . . .
  35. 35. . . . to the form x = V / T , one can determine the distance (x) from the shot to the sensor.
  36. 36. While not particularly useful for the case above, . . .
  37. 37. . . . the latter concept is critical for determining the nature of structures at depth below the surface. For example, when there are layers at depth.
  38. 38. Determining the depth when V and T are known is the principle of the reflection method.
  39. 39. Theory: Behavior of Waves in the Subsurface In order to understand how to extract more detailed subsurface information from geophysical measurements at the surface, we first analyze the behavior of waves (seismic or radar) in the subsurface.
  40. 40. This is the field situation to be considered.
  41. 41. Please review the animation sequence for Reflected Phases at this time. Please minimize this application, the animation sequence is found on the index page. Maximize this application when ready to continue.
  42. 42. Essential points for discussion. 1) The relative difference in arrival times of the ‘direct’ and ‘reflected’ phases as offset increases.
  43. 43. Essential points for discussion. 1) The relative difference in arrival times of the ‘direct’ and ‘reflected’ phases as offset increases. 2) The synchrony of the two phases along the lower interface.
  44. 44. Essential points for discussion. 1) The relative difference in arrival times of the ‘direct’ and ‘reflected’ phases as offset increases. 2) The synchrony of the two phases along the lower interface. 3) The difference in the ‘apparent’ velocity of the two phases along the surface a) The direct (primary) wave travels @ v1. b) The reflected wave @ v1 / sin θ i (where θ i is the incident angle).
  45. 45. Essential points for discussion. 1) The relative difference in arrival times of the ‘direct’ and ‘reflected’ phases as offset increases. 2) The synchrony of the two phases along the lower interface. 3) The difference in the ‘apparent’ velocity of the two phases along the surface a) The direct (primary) wave travels @ v1. b) The reflected wave @ v1 / sin θ i (where θ i is the incident angle).
  46. 46. Traveltime Relations for Direct and Reflected Phases
  47. 47. The field situation.
  48. 48. The field situation showing cutaway.
  49. 49. The field situation with geophone.
  50. 50. Direct and reflected ray paths.
  51. 51. How can we use the ‘reflected’ phase to determine the depth to the respective horizon (or layer) ?
  52. 52. Geometry for a reflection.
  53. 53. Traveltime for a reflection.
  54. 54. If we “know” x and v, we can determine d, the depth to the reflector.
  55. 55. Direct and reflected ray paths with traveltimes.
  56. 56. Please review tutorial on Analyzing Direct and Reflected Phases at this time. Please minimize this application, the tutorial is found on the index page. Maximize this application when ready to continue.
  57. 57. Next, consider the ‘refracted’ phase.
  58. 58. Snell’s Law for Reflection and Refraction.
  59. 59. Refraction at the Critical Angle.
  60. 60. Please review the animation sequence for Refracted Phases at this time. Please minimize this application, the animation sequence is found on the index page. Maximize this application when ready to continue.
  61. 61. Essential points for discussion. 1) The relative difference in arrival times of the ‘direct’, ‘reflected’ & ‘refracted’ phases as offset increases.
  62. 62. Essential points for discussion. 1) The relative difference in arrival times of the ‘direct’, ‘reflected’ & ‘refracted’ phases as offset increases. 2) The synchrony of the direct and reflected phases along the lower interface.
  63. 63. Essential points for discussion. 1) The relative difference in arrival times of the ‘direct’, ‘reflected’ & ‘refracted’ phases as offset increases. 2) The synchrony of the direct and reflected phases along the lower interface. 3) The refracted wavefront is tangential to the reflected wavefront at the critical angle.
  64. 64. Essential points for discussion. 1) The relative difference in arrival times of the ‘direct’, ‘reflected’ & ‘refracted’ phases as offset increases. 2) The synchrony of the direct and reflected phases along the lower interface. 3) The refracted wavefront is tangential to the reflected wavefront at the critical angle. 4) The difference in the ‘apparent’ velocity of the three phases along the surface a) The direct (primary) wave travels @ v1. b) The reflected wave @ v1 / sin θ i (where θ i is the incident angle). c) The refracted wave @ v2 = v1 / sin θ c (where θ c is the ’critical’ angle).
  65. 65. Essential points for discussion. 1) The relative difference in arrival times of the ‘direct’, ‘reflected’ & ‘refracted’ phases as offset increases. 2) The synchrony of the direct and reflected phases along the lower interface. 3) The refracted wavefront is tangential to the reflected wavefront at the critical angle. 4) The difference in the ‘apparent’ velocity of the three phases along the surface a) The direct (primary) wave travels @ v1. b) The reflected wave @ v1 / sin θ i (where θ i is the incident angle). c) The refracted wave @ v2 = v1 / sin θ c (where θ c is the ’critical’ angle).
  66. 66. Refraction at the Critical Angle.
  67. 67. In summary: A seismic (or radar) signal generates a number of modes.
  68. 68. How can we use the ‘refracted’ phase to determine structure in the earth ?
  69. 69. Recall the direct and reflected ray paths.
  70. 70. The direct and refracted ray paths.
  71. 71. Composite of direct, reflected and refracted ray paths.
  72. 72. Set of synthetic seismograms.
  73. 73. Details on “Picking” and Interpreting Seismic Data
  74. 74. Set of synthetic seismograms. (Direct phase.)
  75. 75. (Direct wave “picks” are best in here.)
  76. 76. Analysis of direct ground wave.
  77. 77. Direct ground wave traveltime should (?) go through origin.
  78. 78. Set of synthetic seismograms. (Reflected phase.)
  79. 79. Analyzing reflected phases: An alternative expression for the traveltime.
  80. 80. Analyzing reflected phases: An alternative expression for the traveltime.
  81. 81. Analyzing reflected phases: An alternative expression for the traveltime. The slope m, and the intercept b, provide the essential parameters for interpretation.
  82. 82. Analysis of reflected phase using T2 - X2 method.
  83. 83. Set of synthetic seismograms. (Refracted phase.)
  84. 84. Analysis of critically refracted phase.
  85. 85. Traveltime relations.
  86. 86. Actual seismogram showing various phases.
  87. 87. Picking “first breaks” (Continue).
  88. 88. To get truly good “first break” picks, you need to a) Turn up the gain; b) Adjust “events” to common amplitude. (This is because low amplitude picks tend to be biased to later times.) Picking “first breaks” (Continue).
  89. 89. Using the refraction method for more complicated field situations.
  90. 90. Consider “dipping” interfaces.
  91. 91. Consider “dipping” interfaces. We employ “reversed” refraction profiling.
  92. 92. Procedure: Step 1; The “Forward” Shot.
  93. 93. Procedure: Step 2; The “Reverse” Shot.
  94. 94. We use the theoretical traveltime of the respective refracted phases. September 1, 2002
  95. 95. The Theoretical Traveltime for a Refracted Phase on a Dipping Interface.
  96. 96. Summary of apparent velocities and intercept times for a dipping interface.
  97. 97. How do we gather and interpret field data ?
  98. 98. Procedure: Step 1; The “Forward” Shot.
  99. 99. Step 1: Shoot in Forward Direction.
  100. 100. Procedure: Step 2; The “Reverse” Shot.
  101. 101. Step 2: Shoot in Reverse Direction.
  102. 102. Step 3: Inspect Data.
  103. 103. Step 4: Determine Forward Velocity.
  104. 104. Step 5: Determine Reverse Velocity.
  105. 105. Step 6: Determine Forward Intercept.
  106. 106. Step 7: Determine Reverse Intercept.
  107. 107. Step 7: Determine Reverse Intercept. It is critical to remember that the “reciprocal traveltimes” are identical.
  108. 108. Traveltime relations: Dipping refractor.
  109. 109. Some Examples.
  110. 110. An actual interpretation of dipping plane interfaces (J. Sullivan; Seekonk, MA).
  111. 111. Characteristics of Field Area 1: Vertical GPR Time Section Ground Penetrating Radar Image from Field Site Freq:100 MHz Tx-Rx Offset: 2 m
  112. 112. Example of a refined interpretation using a combination of seismic refraction methods and ground penetrating radar.
  113. 113. Subsurface structure above bedrock at field site. [Seismic interpretation from Jeff Sullivan (personal communication.).] Composite interpretation using seismic refraction, DC resistivity, EM, GPR and gravity.
  114. 114. Example of refraction study: Palmer River Basin.
  115. 115. In summary, a seismic interpretation depends on properly identifying and time-picking appropriate phases.
  116. 116. Field Procedure for Seismic Refraction Surveys (A checklist for a "typical" seismic refraction sounding.) • Begin by deploying a 12 channel recording system w/ 40 Hz geophones at predetermined (1 m?) spacing.
  117. 117. Field Procedure for Seismic Refraction Surveys (A checklist for a "typical" seismic refraction sounding.) • Begin by deploying a 12 channel recording system w/ 40 Hz geophones at predetermined (1 m?) spacing. • Perform a walkaway calibration experiment w/ shot points (hammer blows) at offset distances of 1, 5, 10, 15, 20, 25 & 30 meters from the first geophone. This procedure provides 100% redundancy for any set of shot point-geophone offsets.
  118. 118. Field Procedure for Seismic Refraction Surveys (A checklist for a "typical" seismic refraction sounding.) • Begin by deploying a 12 channel recording system w/ 40 Hz geophones at predetermined (1 m?) spacing. • Perform a walkaway calibration experiment w/ shot points (hammer blows) at offset distances of 1, 5, 10, 15, 20, 25 & 30 meters from the first geophone. This procedure provides 100% redundancy for any set of shot point-geophone offsets. • Identify direct wave and refracted wave "first breaks".
  119. 119. Field Procedure for Seismic Refraction Surveys (A checklist for a "typical" seismic refraction sounding.) • Begin by deploying a 12 channel recording system w/ 40 Hz geophones at predetermined (1 m?) spacing. • Perform a walkaway calibration experiment w/ shot points (hammer blows) at offset distances of 1, 5, 10, 15, 20, 25 & 30 meters from the first geophone. This procedure provides 100% redundancy for any set of shot point-geophone offsets. • Identify direct wave and refracted wave "first breaks". • Reverse profile to identify dip on refractor.
  120. 120. Field Procedure for Seismic Refraction Surveys (A checklist for a "typical" seismic refraction sounding.) • Begin by deploying a 12 channel recording system w/ 40 Hz geophones at predetermined (1 m?) spacing. • Perform a walkaway calibration experiment w/ shot points (hammer blows) at offset distances of 1, 5, 10, 15, 20, 25 & 30 meters from the first geophone. This procedure provides 100% redundancy for any set of shot point-geophone offsets. • Identify direct wave and refracted wave "first breaks". • Reverse profile to identify dip on refractor. • Based on these “calibration” runs, design an optimal field plan.
  121. 121. Field Procedure for Seismic Refraction Surveys (A checklist for a "typical" seismic refraction sounding.) • Begin by deploying a 12 channel recording system w/ 40 Hz geophones at predetermined (1 m?) spacing. • Perform a walkaway calibration experiment w/ shot points (hammer blows) at offset distances of 1, 5, 10, 15, 20, 25 & 30 meters from the first geophone. This procedure provides 100% redundancy for any set of shot point-geophone offsets. • Identify direct wave and refracted wave "first breaks". • Reverse profile to identify dip on refractor. • Based on these “calibration” runs, design an optimal field plan. • Execute the optimized survey plan assuring adequate reciprocal shot point-geophone data for both conventional reversed profiling as well as a delay time analysis.
  122. 122. Field Procedure for Seismic Refraction Surveys (A checklist for a "typical" seismic refraction sounding.) • Begin by deploying a 12 channel recording system w/ 40 Hz geophones at predetermined (1 m?) spacing. • Perform a walkaway calibration experiment w/ shot points (hammer blows) at offset distances of 1, 5, 10, 15, 20, 25 & 30 meters from the first geophone. This procedure provides 100% redundancy for any set of shot point-geophone offsets. • Identify direct wave and refracted wave "first breaks". • Reverse profile to identify dip on refractor. • Based on these “calibration” runs, design an optimal field plan. • Execute the optimized survey plan assuring adequate reciprocal shot point-geophone data for both conventional reversed profiling as well as a delay time analysis. • Separate shot point time-terms from receiver time-terms.
  123. 123. Field Procedure for Seismic Refraction Surveys (A checklist for a "typical" seismic refraction sounding.) • Begin by deploying a 12 channel recording system w/ 40 Hz geophones at predetermined (1 m?) spacing. • Perform a walkaway calibration experiment w/ shot points (hammer blows) at offset distances of 1, 5, 10, 15, 20, 25 & 30 meters from the first geophone. This procedure provides 100% redundancy for any set of shot point-geophone offsets. • Identify direct wave and refracted wave "first breaks". • Reverse profile to identify dip on refractor. • Based on these “calibration” runs, design an optimal field plan. • Execute the optimized survey plan assuring adequate reciprocal shot point-geophone data for both conventional reversed profiling as well as a delay time analysis. • Separate shot point time-terms from receiver time-terms. • Shoot in orthogonal direction to determine dip and strike of refractor in three dimensions.
  124. 124. Field Procedure for Seismic Refraction Surveys (A checklist for a "typical" seismic refraction sounding.) • Begin by deploying a 12 channel recording system w/ 40 Hz geophones at predetermined (1 m?) spacing. • Perform a walkaway calibration experiment w/ shot points (hammer blows) at offset distances of 1, 5, 10, 15, 20, 25 & 30 meters from the first geophone. This procedure provides 100% redundancy for any set of shot point-geophone offsets. • Identify direct wave and refracted wave "first breaks". • Reverse profile to identify dip on refractor. • Based on these “calibration” runs, design an optimal field plan. • Execute the optimized survey plan assuring adequate reciprocal shot point-geophone data for both conventional reversed profiling as well as a delay time analysis. • Separate shot point time-terms from receiver time-terms. • Shoot in orthogonal direction to determine dip and strike of refractor in three dimensions.
  125. 125. Field Procedure for Seismic Refraction Surveys (A checklist for a "typical" seismic refraction sounding.) • Begin by deploying a 12 channel recording system w/ 40 Hz geophones at predetermined (1 m?) spacing. • Perform a walkaway calibration experiment w/ shot points (hammer blows) at offset distances of 1, 5, 10, 15, 20, 25 & 30 meters from the first geophone. This procedure provides 100% redundancy for any set of shot point-geophone offsets. • Identify direct wave and refracted wave "first breaks". • Reverse profile to identify dip on refractor. • Based on these “calibration” runs, design an optimal field plan. • Execute the optimized survey plan assuring adequate reciprocal shot point-geophone data for both conventional reversed profiling as well as a delay time analysis. • Separate shot point time-terms from receiver time-terms. • Shoot in orthogonal direction to determine dip and strike of refractor in three dimensions.
  126. 126. Each of these wave modes (or ‘phases’) provide useful, oftentimes essential, information on the subsurface In addition, strong analogies exist between • Seismic (acoustic or mechanical) phenomena and • Ground penetrating radar (electromagnetic) signals.
  127. 127. End of Presentation… Thanks for Listen...

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