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Presentation Chapter 4: Interpretation of Raw Seismic Records
 

Presentation Chapter 4: Interpretation of Raw Seismic Records

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

    • 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)
    • Signal and Noise Signal: desired Noise: not desired So for reflection seismology: - Primary reflections are signal - Everything else is noise!
    • Signal and Noise (2) Direct wave: noise Refraction: noise Reflection: (desired) signal
    • Signal and Noise (3) Direct wave: noise Refraction: noise Reflection: signal Multiply reflected : noise
    • 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
    • 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
    • Processing of Signal (Primary-reflected energy) Goal of processing: Focus energy to where it comes from
    • 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
    • The seismic record
    • Body waves: Surface waves:
    • A seismic shot record on land Linear event: Direct/refraction Linear event: Slower, Direct Slightly hyperbolic event: Reflection Linear event: Very slow, Direct
    • A seismic shot record on land Linear event: Direct/refraction Linear event: Slower, Direct Slightly hyperbolic event: Reflection Linear event: Very slow, Direct
    • 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
    • (Old: Interpretation of seismic land record)
    • 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
    • Modelling of seismic land record Observed data Modelled data (ray theory approx.)
    • Modelling of seismic land record Observed data Modelled data (ray theory approx.) refraction direct P-wave surface wave reflection reflection direct air wave
    • Wassenaar-beach data: Wednesday
    • Wassenaar-beach data: Wednesday
    • Wassenaar-beach data: array data (Thursday)
    • A seismic shot record at Wassenaar beach Linear event: Direct/refraction Linear event: Slower, Direct Many slightly hyperbolic events: Reflections ! Linear event: Slow, Direct
    • Interpretation of Wassenaar record
    • Modelling of Wassenaar record Observed data Modelled data (ray theory approx.)
    • 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
    • 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
    • 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
    • Old: Modelling of marine record Observed data Modelled data (ray theory approx.)
    • Modelling of marine record Observed data Modelled data (ray theory approx.) refraction direct P-wave (water) reflections multiple reflection multiple reflection