Seismic Data Processing Flashcards
Where is initial processing done?
In the field
What is demultiplexing?
Data sorted so that a seismic trace is obtained for each channel for every shot
How does manual editing occur on land?
Every shot displayed and examined for noisy geophone stations, bad electrical connections, static noise burst, electrical interference, animal and wind noise
Occasionally wrong polarity can also be corrected
How does manual editing occur at sea? How often is it done?
Rare
Noise problems generated when cable steering birds break the surface or make mechanical sounds
What is the aim of F-K filtering?
Remove ground roll that has not been corrected in field from record
Why do amplitude corrections need to take place?
As the source generated wavefront expands, its energy is reduced meaning deeper reflections will be weaker. Energy reduction is predcitable
Why is the NMO correction required? What is the formula?
Correction needed to bring pulses to horizontal alignment as if they all came vertically from source
NMO - Tx - To
What is a static correction?
A constant time shift applied to a seismic trace to allow it to be compared and combined with others within the survey
What is the elevation correction?
Adjusts all traces for elevation to a common surface reference datum
How is the elevation correction done in areas of low relief and high relief?
Low relief = corrected against mean sea level
High relief = higher reference datum used to minimise corrections needed and reduce errors from the calculation
What is the gun and cable correction?
Adjusts all traces to mean sea level to compensate for depth of operation of the gun arrays and the average depth of the hydrophone stations in the cable
What is the weathering correction? Why?
Removes the effect of a weathering layer
On most land surveys unconsolidated layers directly below surface give anomalous readings
Why might small delays still be present after static corrections?
- Using incorrect parameters for field static corrections
- Lateral velocity variations
- Non-ideal reflection ray paths
Residual statics tries to correct what was missed in static corrections by applying small shifts. How is this done?
- Automatically by cross-correlating a pilot trace
- Using a specific window length to allow for ‘stretch’
Under what conditions do residual statics work?
Where signal/noise ration is good and for variations that occur accross the width of a CMP gather
Variations with longer wavelength will not be detected
What may cause long wavelength statics and how can they be identified on seismic?
Occur where large scale but subtle variations in near surface seismic velocities are present (e.g, mud-filled channels, peat)
May cause pull-ups on seismic
Give four examples of undesirable effects that data processing lessens the effects of?
Multiples: where sound is reflected repeatedly within a rock formation and registers as a deeper reflector
Reverberations: reflections between water surface and sea-bed are similar to ‘ringing’
Diffractions: hyperbolic reflections generated from sharp discontinuities in reflectors
Random noise: mainly unwanted reflections from within rock layers, horizontally proagated and refracted sound
What does stacking achieve?
Reduces multiples and random noise
What is the stacking velocity? Why do reflectors form a hyerbolic curve?
Stacking velocity is the appropriate shift to correct the reflection hyperbola and allow the reflectors to stack correctly for any time and offset
Reflections from any one reflector should form hyperbolic curve on the CMP gather as the sound takes longer to travel to more distant detectors
What is the uncorrected CMP gather?
After sorting when the traces are displayed in order of increasing offset from the zero offset or min trace time
What is the benefit of adding the NMO correction to the corrected gather (stacked trace)?
Process greatly improves signal to noise ration as stacking causes true reflection pulses to enhance one another and random noise is cancelled out
What is NMO stretch and how is it formed?
NMO stretch creates artificially low frequency reflections on the outer traces
Forms in shallow sections with long offset as NMO correction shifts are greatest
