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 Diffraction

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 Seismic events Non-primary events

X Z Earth model X T Seismic Section Diffraction

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” Seismic events Primary reflections

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 Seismic events Non-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 ( m/s) attenuated by arrays, frequency filtering, or F-K filtering Seismic events Non-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 Seismic events Non-primary events

Direct Ground roll Head wave XCXC XOXO Non-primary Linear Events

Primary Earth’s surface Subsurface reflector S R1R1 Ground roll Direct P-wave R2R2 Head wave (refraction) First multiple Seismic events Non-primary events

Direc t Ground roll Head wave (refraction) First multiple Primary R1R1 R2R2 Seismic events Non-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 M 1/2. Seismic events Non-primary events

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. The seismic wavelet

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

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 Seismic resolution

Seismic resolution Vertical