Refraction Seismic

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Refraction Seismic

  1. 1. RefractionseismicJacques JENNY Geo2X Genève 1
  2. 2. Seismic refraction principes R e fle x io n a V1 R e fr a c tio n V2 aS in = V ite s s e 1S in β V ite s s e 2 b V3 V ite s s e 1 R e fr a c tio n to ta le R é f r a c t io n t o ta le S in α = 2 V ite s s e 2
  3. 3. Applications• Oil in 1920 th• Geotechnic• Water prospecting• Mineral research• Landslide study• Wheathering zone determination (for reflection seismic statics) 3
  4. 4. Unfavourable geological setting with refraction seismic Seisimic line S iclin eisim e A B Red ray pathes are always hidden by shorter black rays 4
  5. 5. Record example V1 V2 V3Time Distances 5
  6. 6. Record example Refraction Reflexion Air wave Surface wave 6
  7. 7. Refraction data acquisitionFor a efficient processing, you need at least:• 2 Offset shots (half spread distance)• 2 End shots• 1 Center shot 7
  8. 8. Geophone Shot 1/1 Shot 1/2 Shot 1/3 Shot 1/4 Shot 1/5 Example Shot 2/1 Shot 2/2 Shot 2/3 Shot 2/4 Shot 2/5 Shot 3/1 Shot 3/2 Shot 3/3 Shot 3/4 Shot 3/5 1 2 3 4 5 of a End shot 12 complex 6 7 8 Centre shot End shot 1 9 acquisition Offset 12 Offset 1 10 11 12 with 12 13 14 15 16 17 18 19 channels 20 End shot 12 Centre shot End shot 1 21 Offset 12 Offset 1 22 23 24 25 26 27 28 29 30 31 32 End shot 12 Centre shot End shot 1 33 Offset 12 Offset 1 34 35 36 8 Shot points
  9. 9. Equipment• Geophones• Seismograph• Battery• Cables• (Blasting box)• Radio• Portable drill 9
  10. 10. Energy sources• Sledge hammer (Easy to use, cheap)• Buffalo gun (More energy)• Explosives (Much more energy, legal problems)• Drop weight (Need a flat area)• Vibrator (Uncommun use for refraction)• Air gun (For lake / marine prospection) 10
  11. 11. Sledge hammerProduce a good energy with high frequencies,Possible investigation depth 10-50 m You can add (stack) few shots to improve signal/noise ratio 11
  12. 12. To avoid aerial projection and improve energy, explosives can be burried into a small drilling (1-1.5 m) using a portable mechanic drill or a jumper.ExplosivesTo buy and useexplosive is oftendifficult, impossiblein some countries.A miner licence isrequired 12
  13. 13. Processing• Few specific softwares are found to process seismic refraction• Most of them use conventional methods like Intercept Time (IT), ABC, GRM• New inversion softwares can produce tomography interpretation 13
  14. 14. First Break Picking• This is the most important operation, good picking on good data !!!!• A commun problem is the lack of energy, for far offset geophones• Seismographs produce SEG2, SEGY or special file format, generaly they must be converted to another file format, like Seismic Unix. 14
  15. 15. Picking FBP on good data (Wiggles are clipped) Noise 15
  16. 16. Picking FBP on poor data ? 16
  17. 17. Travel time assembly Offset 1 End Shot 1 Offset 24 Center shot End Shot 24Steep slope= low velocity gentle slope = high velocityX axis = distance Y axis = time Slope=1/velocity 17
  18. 18. Bedrock velocityUsing only offset shots, we can compute the true velocity of the bedrock,even if the bedrock is dipping. If the points are not straight, it means thatbedrock is not homogenous (fault, lithological boundary) Save the Velocity as layer 3 velocity 18
  19. 19. Bedrock velocity A E Pente= 1 / Vitesse réfracteur B F C G D H 19
  20. 20. PhantomingMove offset shot to end shot to determine which part corresponds to bedrock arrivals Intercept time 2 20
  21. 21. Velocity determinationVelocity = 1 / slope 21
  22. 22. Seismic velocity of some rocks 22
  23. 23. Rock velocity examples 23
  24. 24. Rock rippability• Rippability (bulldozer) is related with rock velocities.• A refraction profil can be used to determine if explosives use will be necessary instead bulldozer• Refraction survey can also be used to caracterize soils: soft, hard to set excavation rates 24
  25. 25. If you know all velocities and Intercept times, thickness can be computed below all shots, except offset shots Intercept time (IT) method 25
  26. 26. What is an Intercept time Distance = 0 m 26
  27. 27. IT FormulaZ1= thickness layer 1T1= intercept 1V1=layer 1 velocityV2=ayer 2 velocity 27
  28. 28. Plus minus Method Principle T o ta l tim e S in a = V 1 /V 2 T im e C D E = T im e A B C D + D E F G - T im e A B C E F G A G D a V1 B C E F V2 28
  29. 29. Plus Minus 1 Total time 29
  30. 30. A B C m e t h o d d e p t h c o m p u t a t io n Plus Minus 2Velocity meters/s 5000 4800 4600 4400 4200 4000 3600 3200 2800 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400 D E M O 2 .WS 4 Depth is computed below all receivers 30
  31. 31. GRM principle• Generalized Reciprocal Method (see PALMER papers) is a variant of the ABC method. It takes in account noncoincidence of the stations used for calculating plus values• GRM requires more receivers than IT or ABC• Different distances are used to compute time to bedrock, geophysicist must select the optimal distance (XY) 31
  32. 32. Generalized Reciprocal Method XY = Optimal distanceRed = plus-minus path Blue = plus-minus path 32
  33. 33. GRM 1 XY optimal distance selection 33
  34. 34. GRM 2: Depth with differents XY 34
  35. 35. GRM 3 35
  36. 36. GRM Seismic profil G R M m e t h o d d e p t h c o m p u t a t io nVelocity meters/s 3600 3200 2800 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400 D E M O 2 .WS 4 36
  37. 37. TomographyIf you input all geometrical data and first break picks, computercan build a theoretical model as close as possible to field datausing different algorithm.A very precise picking and closer shots are required to giveaccurate results. 37
  38. 38. Example 1 Tomography ona gallery 38
  39. 39. Example 1 39
  40. 40. Example 2 Landslide inSwiss Alps 40
  41. 41. Example 2 T11300 5000 T2 R o u te P r o fil 31280 4500 T3 40001260 T4 3500 T51240 3000 T61220 T7 2500 T81200 2000 S e n tie r T9 15001180 1000 T101160 500 0 20 40 60 80 100 120 140 160 180 41
  42. 42. FIN END 42

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