occurs when wave encounters sharp discontinuities in the medium important in defining faults

1. Diffraction • occurs when wave encounters sharp discontinuities in the medium • important in defining faults • generally considered as noise in seismic sections • seismic migration usually corrects for this effect

2. Seismic eventsNon-primary events • Diffractions • generated by abrupt lateral changes in lithology • T-X curve is a hyperbola • Amplitude falls off rapidly away from the apex. • attenuated by migration

3. X Earth model Z X Seismic Section T Diffraction

4. Seismic eventsPrimary reflections • generated by waves that have been reflected once from an interface • formed by the convolution of the source wavelet with the RC of the interface • considered the most important part of the seismic section • comprise the “signal” while everything else on the section is considered “noise”

5. Seismic eventsNon-primary events • Direct waves • source-generated wave that travels directly from source to receiver • usually first arrivals at near offsets • T-X curve is a straight line with intercept = 0 • attenuated by NMO muting and stacking • Head waves (refractions) • generated by critically refracted waves from near-surface layers • Usually first arrivals at far offsets • T-X curve is a straight line with intercept  0 • attenuated by NMO muting and stacking

6. Seismic eventsNon-primary events • Ground roll • generated by source • They are surface waves traveling along the ground surface. • T-X curve is a straight line with intercept = 0. • frequency content < 10 Hz • low velocities (100-1000 m/s) • attenuated by arrays, frequency filtering, or F-K filtering

7. Seismic eventsNon-primary events • Multiples • generated by waves reflected more than once • T-X curve is a hyperbola. • NMO correction does NOT flatten them. • Very high reflection coefficients are needed to produce distinctive multiples on the seismic section. • Most common type is water reverberation. • attenuated by various advanced techniques

8. Non-primary Linear Events XC XO Direct Head wave Ground roll

9. Direct P-wave Ground roll Earth’s surface R1 S R2 Subsurface reflector First multiple Head wave (refraction) Primary Seismic eventsNon-primary events

10. Seismic eventsNon-primary events R1 R2 Direct Head wave (refraction) Primary First multiple Ground roll

11. Seismic eventsNon-primary events • Incoherent noise • generated by near-surface effects (e.g., humans, animals, machines, equipment, wind, plants, scatterings, … etc) • random in nature • Signal-to-noise ratio (S/N) is the ratio between signal and noise energies in a specific portion of the section. • Poor sections have S/N < 1.0. • attenuated by stacking • Stacking M traces enhances the S/N by M1/2.

12. The seismic wavelet • Ideally, we want an impulse of zero width. • Practically, the best we can attain is a narrow wavelet with minimum sidelobes. • Examples of practical wavelets are: • Ricker wavelet, which approximates that of a dynamite. • Klauder wavelet, which approximates that of a Vibroseis.

13. The seismic wavelet Ideal wavelet Klauder wavelet (f = 20 – 80 Hz) Ricker wavelet (f = 30 Hz)

16. Seismic resolution • ability to distinguish two closely-spaced events on the seismic section • We can distinguish two events: • vertically, depending on: • the signs and magnitudes of the events • the separation between the events • dominant frequency • horizontally, depending on: • the receiver spacing • dominant frequency • velocity • dip angle

17. Seismic resolutionVertical