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# Presentation Chapter 4: Interpretation of Raw Seismic Records

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### Presentation Chapter 4: Interpretation of Raw Seismic Records

1. 1. Overview ta3520 Introduction to seismics <ul><li>Fourier Analysis </li></ul><ul><li>Basic principles of the Seismic Method </li></ul><ul><li>Interpretation of Raw Seismic Records </li></ul><ul><li>Seismic Instrumentation </li></ul><ul><li>Processing of Seismic Reflection Data </li></ul><ul><li>Vertical Seismic Profiles </li></ul><ul><li>Practical: </li></ul><ul><li>Processing practical (with MATLAB) </li></ul>
2. 2. Signal and Noise Signal: desired Noise: not desired So for reflection seismology: - Primary reflections are signal - Everything else is noise!
3. 3. Signal and Noise (2) Direct wave: noise Refraction: noise Reflection: (desired) signal
4. 4. Signal and Noise (3) Direct wave: noise Refraction: noise Reflection: signal Multiply reflected : noise
5. 5. Signal and Noise for P-wave survey <ul><li>Noise: </li></ul><ul><li>direct wave through first layer </li></ul><ul><li>direct air wave </li></ul><ul><li>direct surface wave </li></ul><ul><li>S-wave </li></ul><ul><li>Multiply reflected wave </li></ul><ul><li>Refraction / Head wave </li></ul><ul><li>Desired signal: </li></ul><ul><li>primary reflected P-waves </li></ul>
6. 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. 7. Processing of Signal (Primary-reflected energy) Goal of processing: Focus energy to where it comes from
8. 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. 9. The seismic record
10. 10. Body waves: Surface waves:
11. 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. 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. 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. 14. (Old: Interpretation of seismic land record)
15. 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. 16. Modelling of seismic land record Observed data Modelled data (ray theory approx.)
17. 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. 18. Wassenaar-beach data: Wednesday
19. 19. Wassenaar-beach data: Wednesday
20. 20. Wassenaar-beach data: array data (Thursday)
21. 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. 22. Interpretation of Wassenaar record
23. 23. Modelling of Wassenaar record Observed data Modelled data (ray theory approx.)
24. 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. 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. 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. 27. Old: Modelling of marine record Observed data Modelled data (ray theory approx.)
28. 28. Modelling of marine record Observed data Modelled data (ray theory approx.) refraction direct P-wave (water) reflections multiple reflection multiple reflection
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