Seismic Data Processing

Question 1

Where is initial processing done?

Answer 1

In the field

Question 2

What is demultiplexing?

Answer 2

Data sorted so that a seismic trace is obtained for each channel for every shot

Question 3

How does manual editing occur on land?

Answer 3

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

Question 4

How does manual editing occur at sea? How often is it done?

Answer 4

Rare

Noise problems generated when cable steering birds break the surface or make mechanical sounds

Question 5

What is the aim of F-K filtering?

Answer 5

Remove ground roll that has not been corrected in field from record

Question 6

Why do amplitude corrections need to take place?

Answer 6

As the source generated wavefront expands, its energy is reduced meaning deeper reflections will be weaker. Energy reduction is predcitable

Question 7

Why is the NMO correction required? What is the formula?

Answer 7

Correction needed to bring pulses to horizontal alignment as if they all came vertically from source

NMO - Tx - To

Question 8

What is a static correction?

Answer 8

A constant time shift applied to a seismic trace to allow it to be compared and combined with others within the survey

Question 9

What is the elevation correction?

Answer 9

Adjusts all traces for elevation to a common surface reference datum

Question 10

How is the elevation correction done in areas of low relief and high relief?

Answer 10

Low relief = corrected against mean sea level

High relief = higher reference datum used to minimise corrections needed and reduce errors from the calculation

Question 11

What is the gun and cable correction?

Answer 11

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

Question 12

What is the weathering correction? Why?

Answer 12

Removes the effect of a weathering layer

On most land surveys unconsolidated layers directly below surface give anomalous readings

Question 13

Why might small delays still be present after static corrections?

Answer 13

• Using incorrect parameters for field static corrections
• Lateral velocity variations
• Non-ideal reflection ray paths

Question 14

Residual statics tries to correct what was missed in static corrections by applying small shifts. How is this done?

Answer 14

• Automatically by cross-correlating a pilot trace

- Using a specific window length to allow for ‘stretch’

Question 15

Under what conditions do residual statics work?

Answer 15

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

Question 16

What may cause long wavelength statics and how can they be identified on seismic?

Answer 16

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

Question 17

Give four examples of undesirable effects that data processing lessens the effects of?

Answer 17

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

Question 18

What does stacking achieve?

Answer 18

Reduces multiples and random noise

Question 19

What is the stacking velocity? Why do reflectors form a hyerbolic curve?

Answer 19

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

Question 20

What is the uncorrected CMP gather?

Answer 20

After sorting when the traces are displayed in order of increasing offset from the zero offset or min trace time

Question 21

What is the benefit of adding the NMO correction to the corrected gather (stacked trace)?

Answer 21

Process greatly improves signal to noise ration as stacking causes true reflection pulses to enhance one another and random noise is cancelled out

Question 22

What is NMO stretch and how is it formed?

Answer 22

NMO stretch creates artificially low frequency reflections on the outer traces

Forms in shallow sections with long offset as NMO correction shifts are greatest