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# Presentation Chapter 4: Interpretation of Raw Seismic Records

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### Transcript

• 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
• 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