ÖNCEL AKADEMİ: INTRODUCTION TO GEOPHYSICS

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Seismic Reflection 4: Acquisiton, Processing, and Waveform Analysis

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  • Whether the reflected wave has the same or opposite polarity as the input seismic signal depends on whether the reflection coefficient is positive or negative.
  • Convolution means replacing each reflection coefficient with the input seismic signal, scaled to the amplitude and polarity of the reflection coefficient.
  • ÖNCEL AKADEMİ: INTRODUCTION TO GEOPHYSICS

    1. 1. Introduction to Geophysics Ali Oncel [email_address] Department of Earth Sciences KFUPM Seismic Reflection 4: Acquisiton, Processing, and Waveform Analysis Introduction to Geophysics-KFUPM
    2. 2. Types of Velocities Average Velocity Root mean Square Velocity (RMS) Interval velocity Previous Lecture Introduction to Geophysics-KFUPM
    3. 3. Example: Disk Formula v 1 =1500 m s -1 v 2 =2000 m s -1 v 3 =2345 m s -1 t 1 =2.14 s t 2 =1.21 s t 3 =1.13 s What is v rms at the base of layer 3? Introduction to Geophysics-KFUPM
    4. 4. Introduction to Geophysics-KFUPM
    5. 5. Introduction to Geophysics-KFUPM Fig 5.21 of Lillie
    6. 6. Introduction to Geophysics-KFUPM
    7. 7. “ 12 elephants dancing in unison” (LITHOPROBE, CANADA) A vibrator truck Vibroseis images from the Lithoprobe Project, Canada www.lithoprobe.ca Introduction to Geophysics-KFUPM
    8. 8. Introduction to Geophysics-KFUPM Fig. 5.23 of Lillie
    9. 9. Reflection and Transmission <ul><li>The relative proportions are determined by the acoustic impedance – the product of density ( ρ ) and velocity ( v ): </li></ul><ul><li>Generally speaking, the greater its acoustic impedance is resulted from the “ harder ” the rock. </li></ul><ul><li>Maximum transmission of seismic energy requires a matching of acoustic impedances. </li></ul><ul><li>The total energy of a transmitted and reflected ray must equal the energy of the incident ray, due to partitioning energy as: </li></ul><ul><li>Incident Amplitude = Reflected Amplitude + Transmitted Amplitude </li></ul>Introduction to Geophysics-KFUPM From Kearey, Brooks, and Hill, 2002
    10. 10. Reflection and Transmission It becomes for a normally incident ray: <ul><li>A negative value of R indicates a 180 o phase change in the reflected ray. </li></ul>Introduction to Geophysics-KFUPM <ul><li>Reflection coefficient R is a numerical measure of the amplitude and polarity of the wave reflected from an interface, relative to the incident wave. It is the ratio between the amplitude ( A 1 ) of reflected ray and the amplitude ( A 0 ) of the incident ray: </li></ul>T 12 = 2I 1 / (I 1 +I 2 ) T 21 = 2I 2 / (I 2 +I 1 ) Transmission Coefficients
    11. 11. Example: Air-water reflection ρ air =0; Vp=330 m/s ρ water =1; Vp=1500m/s R AirWater = (I Water -I Air ) / (I Air +I Water ) R AirWater = (I Water -0) / (0+I Water ) R AirWater = 1 Introduction to Geophysics-KFUPM Air Layer 1 Layer 2
    12. 12. ρ air =0; Vp=330 m/s ρ water =1; Vp=1500m/s Example: Water-air reflection R WaterAir = (I Air -I Water ) / (I Air +I Water ) R WaterAir = -1 ( A negative reflection coefficient) Introduction to Geophysics-KFUPM Air Water Layer 1 Layer 2
    13. 13. Introduction to Geophysics-KFUPM
    14. 14. Introduction to Geophysics-KFUPM <ul><li>The input seismic signal is a specific waveform with a certain amplitude ( A i ). </li></ul><ul><li>b) The seismic response to an interface has the same waveform as the input signal, scaled to the amplitude of the reflection coefficient ( A r ); it appears at two way travel time ( T ) dependent on the thickness ( h ) and velocity ( V 1 ) of the material above the interface. </li></ul>Fig. 5.25 of Lillie
    15. 15. Introduction to Geophysics-KFUPM b) Seismic response for reflection at interface with positive reflection coefficient c) Seismic response for negative reflection coefficient Fig. 5.26 of Lillie a) Input minimum phase and zero phase seismic signals
    16. 16. Introduction to Geophysics-KFUPM High Frequency Input
    17. 17. Introduction to Geophysics-KFUPM Low Frequency Input
    18. 18. Introduction to Geophysics-KFUPM
    19. 19. Introduction to Geophysics-KFUPM
    20. 20. Convolution <ul><li>Earth as a Filter of Seismic Energy </li></ul><ul><li>G(t) * F(t) = H(t) </li></ul><ul><li>Source Earth = Seismogram </li></ul><ul><li>Wavelet Ref. Coeff. </li></ul>Introduction to Geophysics-KFUPM
    21. 21. Source and Earth Response <ul><li>Mathematical Description of Filter </li></ul><ul><li>Convolution </li></ul>Introduction to Geophysics-KFUPM See pp. 353 of Reynolds, 2002

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