ÖNCEL AKADEMİ: INTRODUCTION TO GEOPHYSICS
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Seismic Reflection 2: Acquisiton, Processing, and Waveform Analysis
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ÖNCEL AKADEMİ: INTRODUCTION TO GEOPHYSICS
- 1. Introduction to Geophysics Ali Oncel [email_address] Department of Earth Sciences KFUPM Seismic Reflection 2: Acquisiton, Processing, and Waveform Analysis Introduction to Geophysics-KFUPM
- 3. Reflection Seismology Introduction to Geophysics-KFUPM
- 4. Seismic Methods Introduction to Geophysics-KFUPM
- 5. Snell’s Law Introduction to Geophysics-KFUPM
- 7. Reflection “Presentation” Introduction to Geophysics-KFUPM Refraction (first arrival) Reflection 1 Reflection 2 Reflection 3 (dipping) Reflected refractions
- 8. Introduction to Geophysics-KFUPM Angle Incidence = Angle Reflection Multifold Reflections Four fold coverage of the CMP
- 9. Receiver Arrays Introduction to Geophysics-KFUPM
- 10. Common Depth Point Common Midpoint Many Source Receiver Offsets Sample Same Depth Point Introduction to Geophysics-KFUPM CMP Method Figure 6.4 of Reynolds Sixfold coverage of the CDP
- 11. Introduction to Geophysics-KFUPM CMP Method Sample Same Subsurface Point – Common Depth Point 4 Times
- 12. Introduction to Geophysics-KFUPM Processing Fold of Stack Coverage Redundancy Improve Signal to Noise Ratio
- 13. =Travel time from source, to interface, to receiver X= horizontal distance from source to receiver V 1 = seismic velocity of layer above distance T 0 = T-axis intercept (time directly down to interface, straight back up to shot location h = thickness of layer above interface V 1 = seismic layer of above interface Introduction to Geophysics-KFUPM T NMO = T f - t 0 Normal moveout is the difference between the travel time and hyperbola (T f ) and the intercept time (t 0 ). Normal Moveout See EQ 4-10 of Robinson and Coruh, 1988
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- Each receiver station commonly is not a single instrument, but rather a array of several receivers, connected electronically and a centered around a point. ARRAY or GROUP: Receiver stations of connected geophones. Geophone responses for a group add together as a single seismic trace. Response of one geophone group. If the group length is equal to the wavelength of a Rayleigh wave, positive signals from geophones moving up cancel negative signals from those moving down, the Rayleigh wave is thus attenuated. A reflected P-wave at close distance to the source moves the group of geophones up or down in unison, enhancing the arrival. The geometry of an array ( or a geophone group) is designed to cancel certain unwanted signals, while enhancing reflected waves. GROUNDROLL: Rayleigh waves which are example of unwanted noise.
- CMP method provides redundancy of information that enhances signal-to-noise ratio and facilitates the determination of seismic velocities used in processing and interpretation. EXAMPLE: During the first shot, the midpoint between the source (S1) and first receiver (R1) is the same point as S2/S3 and S3/S5, for shots 2 and 3 respectively.
- SHOT GATHER: Each record of six seismic traces is known as a shot gather. CMP Gather: A display of traces corresponding to reflections around a each source, is known as CMP or CDP. NMO: The differences in travel time are called normal moveout. PRIMARY REFLECTION= Resulted from energy that undergoes only one reflection before returning to the surface.