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Seismic Exploration: Fundementals-2

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  • travel time for reflected and refracted seismic waves and formula
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  1. 1. Introduction to Geophysics Ali Oncel [email_address] Department of Earth Sciences KFUPM Seismic Exploration: Fundamentals 2
  2. 2. Previous Lecture Huygens's Principle Fermat's Principle Travel-time Graph Estimates of Seismic Velocity Reflected/Refracted waves Model Calculation for simple, horizontal, two layers Ray Paths Snell's law-Critically Refracted Arrival Seismic Refraction Behavior of refracted ray on velocity changes
  3. 3. A ray incident on surface results in 3 reflected and refracted rays. If the seismic velocities in medium 1 are α = 6.5 km/sec , β = 3.8 km/sec , what are the seismic velocities in medium 2? What type of material is medium2? Identify rays #1, #2, #3, #4 as P or S waves. Homework Due to Wednesday 12° 15° 20° 35° #1 #2 #3 medium 1 medium 2
  4. 4. Refracted Ray and Angle The angle of refraction increases as the angle of incidence increases.
  5. 5. Energy Return and Critical Angle A critically refracted wave, traveling at the top of the lower layer with velocity V 2 , leaks energy back into the upper layer at the critical angle ( θ 2 ) Lillie, Whole Earth Geophysics, Fig 3.25
  6. 6. Modified Table 2.3 after Berger, pp.29.
  7. 7. Total Time of Refraction T total = T 1 +T 2 +T 3
  8. 8. Travel time for Direct/Refracted Waves
  9. 9. Seismic Reflection Reflection occurs when Z 1 differs from Z 2 , where Z Acoustic impedance which is product of density and velocity V-shaped ray paths for a compressional wave from a source to 6 receivers, reflected from a horizontal interface. Lillie, Whole Earth Geophysics, Fig 3.28 =Z 1 =Z 2 Lillie, Whole Earth Geophysics, Fig 3.28
  10. 10. Reflection equation for a reflection hyperbolae:
  11. 11. Time Distance ?
  12. 12. Time Distance Direct
  13. 13. Time Distance ?
  14. 14. Time Distance Reflected
  15. 15. Time Distance ?
  16. 16. Time Distance Refracted or Head Wave
  17. 17. Time Distance Direct Reflected Refracted or Head Wave
  18. 18. <ul><li>? </li></ul>Time Distance Direct Refracted or Head Wave
  19. 19. <ul><li>Crossover distance </li></ul>Time Distance Direct Reflected Refracted or Head Wave
  20. 20. <ul><li>Crossover distance? </li></ul>Time Distance Direct Reflected Refracted or Head Wave t i
  21. 21. Miller et al. 1995 Records of Ground Motion and Travel-Time Curves Miller et al. 1995
  22. 22. Miller et al. 1995
  23. 23. These exercises are designed to illustrates some of the basic characteristics of wave propagation in a single layer model use ray-tracing concepts to determine the arrival times of particular events. These exercises require that you construct the time- distance plot for the given model. In addition to constructing the time-distance plots, Due to Next Week INTRODUCTORY RAY TRACING EXERCISES GENERAL INSTRUCTIONS
  24. 24. Single-Layer Model Equations
  25. 26. Be sure to do the following 1) label all plotted curves, 2) label all relevant points, and 3) in a paragraph or so discuss the significance and origins of the interrelationships portrayed in the resultant time-distance plots