Geop 397 - MIGRATION

Question 1

what are the types of migration ?

Answer 1

2D and 3D

Question 2

What are the 2 kinds of 3D migration ?

Answer 2

2-Pass (IL first then XL — or vice-versa)

1-Pass (Both IL and XL simultaneously —- TIME SLICE)

Question 3

What is the position of Migration in the processing sequence?

Answer 3

Either
Pre-Stack or Post-Stack
Either way the result is a MIGRATED STACK

Question 4

What are the Migration Algorithms?

Answer 4

KIRCHHOFF algorithm

FINITE DIFFERENCE algorithm

Question 5

What are the Migration Parameters?

Answer 5

• CDP Interval
• maximum geological dip (+/- ms per trace)
• depth step (2ms or 4ms)
• Migration Aperture (greater the dip, the larger the migration aperture)
• migration velocities

Question 6

What is Migration Aperture?

Answer 6

The extents that are required for proper migration of a trace.
For 3 D, migration aperture needs to be determined in both IL and XL direction.

Question 7

What’s the difference between Pre-Stack and Post-Stack Migration ?

Answer 7

Scenario 1
In Pre-Stack Migration….
Migration (CDP gathers w traces 1-n fold) —>
Stack (stacked traces — 1 trace/cdp)

Scenario 2
In Post-Stack Migration
Stack (stacked traces – 1 trace/cdp) —>
Migration

Question 8

Talk about Post-Stack Migration

Answer 8

In Post-Stack Migration, 3D velocity model is used to NMO correct and stack the data.
NOTE:
3D velocity model for NMO correction is slowed by 5-10%.

Question 9

What are the Migration Tests?

Answer 9

range of velocity
changes 80%-120% range
you have to make a comparison to see if it is in the correct range.

Question 10

What are the 2 domains for Migration ?

Answer 10

Time and Depth

Question 11

What do you need for proper Time to Depth Conversion?

Answer 11

Accurate 3D velocity model

Question 12

How do you convert time to depth ?

Answer 12

• migration in time domain
  OR
• convert to depth domain

Question 13

What if you just do migration in the time domain?

Answer 13

convert data to depth domain.

V=D/T where T-one way seismic time

Question 14

What if you convert to depth domain ?

Answer 14

migrate in depth domain.

BUT - client needs to approve velocity model.

Question 15

Talk about Processing Sequence for Migration of CDP gathers

Answer 15

Demultiplex/Geometry
Binning/CDP gather
VA, NMO, mute
Residual statics (L) or Multiple Attenuation (M)
Fully processed, NMO corrected CDP gather
Migration
Velocity Analysis/NMO correction (optional - but then you need NMI – which removes NMO and then re-NMO)
Stack