Presentation Chapter 4: Interpretation of Raw Seismic Records

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  • 1. Overview ta3520 Introduction to seismics
    • Fourier Analysis
    • Basic principles of the Seismic Method
    • Interpretation of Raw Seismic Records
    • Seismic Instrumentation
    • Processing of Seismic Reflection Data
    • Vertical Seismic Profiles
    • Practical:
    • Processing practical (with MATLAB)
  • 2. Signal and Noise Signal: desired Noise: not desired So for reflection seismology: - Primary reflections are signal - Everything else is noise!
  • 3. Signal and Noise (2) Direct wave: noise Refraction: noise Reflection: (desired) signal
  • 4. Signal and Noise (3) Direct wave: noise Refraction: noise Reflection: signal Multiply reflected : noise
  • 5. Signal and Noise for P-wave survey
    • Noise:
    • direct wave through first layer
    • direct air wave
    • direct surface wave
    • S-wave
    • Multiply reflected wave
    • Refraction / Head wave
    • Desired signal:
    • primary reflected P-waves
  • 6. Signal and Noise for P-wave survey Goal of Processing: Remove effects of All-but-Primary-Reflection Energy Signal Noise = Primary P-wave Reflected Energy All but Primary Reflection Energy
  • 7. Processing of Signal (Primary-reflected energy) Goal of processing: Focus energy to where it comes from
  • 8. Understanding signal and noise: wave theory Basic physics underlying signal is captured by wave equation Ray theory: approximation of wave equation (“high-frequency”) Resonances: modes expansion of wave equation S-waves, P-waves: elastic form of wave equation
  • 9. The seismic record
  • 10. Body waves: Surface waves:
  • 11. A seismic shot record on land Linear event: Direct/refraction Linear event: Slower, Direct Slightly hyperbolic event: Reflection Linear event: Very slow, Direct
  • 12. A seismic shot record on land Linear event: Direct/refraction Linear event: Slower, Direct Slightly hyperbolic event: Reflection Linear event: Very slow, Direct
  • 13. Interpretation of seismic land record thickness 200 m source detector offset air wave surface wave (velocity 1850 m/s) refraction velocity 340 m/s velocity 3500 m/s (P waves) velocity 4800 m/s
  • 14. (Old: Interpretation of seismic land record)
  • 15. Picture made within PowerPoint thickness 200 m source detector offset air wave surface wave (velocity 1850 m/s) refraction velocity 340 m/s velocity 3500 m/s (P waves) velocity 4800 m/s
  • 16. Modelling of seismic land record Observed data Modelled data (ray theory approx.)
  • 17. Modelling of seismic land record Observed data Modelled data (ray theory approx.) refraction direct P-wave surface wave reflection reflection direct air wave
  • 18. Wassenaar-beach data: Wednesday
  • 19. Wassenaar-beach data: Wednesday
  • 20. Wassenaar-beach data: array data (Thursday)
  • 21. A seismic shot record at Wassenaar beach Linear event: Direct/refraction Linear event: Slower, Direct Many slightly hyperbolic events: Reflections ! Linear event: Slow, Direct
  • 22. Interpretation of Wassenaar record
  • 23. Modelling of Wassenaar record Observed data Modelled data (ray theory approx.)
  • 24. A seismic shot record at sea Many Hyperbolic events: Reflections Linear event: Direct First hyperbolic event: primary reflection Hyperbolic event at twice time of primary reflection: multiple Linear event: Refraction
  • 25. Interpretation of marine record 300 m source detector offset direct P wave refraction velocity 240 m/s velocity 1500 m/s (P waves) velocity 1500 m/s (density different) 250 m reflection multiple reflection
  • 26. Picture made with PowerPoint 300 m source detector offset direct P wave refraction velocity 240 m/s velocity 1500 m/s (P waves) velocity 1500 m/s (density different) 250 m reflection multiple reflection
  • 27. Old: Modelling of marine record Observed data Modelled data (ray theory approx.)
  • 28. Modelling of marine record Observed data Modelled data (ray theory approx.) refraction direct P-wave (water) reflections multiple reflection multiple reflection