AN INTRODUCTION TO MIGRATION

BY MIKE LORENTZ AND ROBERT BRADLEY

INTRODUCTION

Migration is a tool used in seismic processing to get an

accurate picture of underground layers. It involves

geometric repositioning of return signals to show an

event (layer boundary or other structure) where it is

being hit by the seismic wave rather than where it is

picked up. Migration was first used in the 1920's, and

today, it is has evolved into many variations. Two of the

more important migration methods are: pre-stack and

post-stack migration.

FIG 1- (UPPER) PRE-STACK DEPTH MIGRATION, (LOWER) POST-STACK DEPTH MIGRATION

PRE-STACK MIGRATION

Pre-stack migration is essentially when seismic data is

adjusted before the stacking sequence occurs. The

popular form of pre-stack migration is depth migration

(PDM). PDM requires the user to know more about

velocites of the layers. Once the user inputs these into

the data with velocity analysis methods, there will be

some error in the image. This error is caused by dipping

reflectors or diffractions. The PDM will adjust the picture

according to the velocities given.

Pre-stack migration is often applied only when the layers

being observed have complicated velocity profiles, or

when the structures are just too complex to see with

post-stack migration. Pre-stack is an important tool in

modeling salt diapirs because of their complexity and

this has immediate benefits if the resolution can pick up

any hydrocarbons trapped by the diapir.

Overall, pre-stack migration, depth and time, is a valuable

tool in better imaging seismic data, but it is limited by

the amount of time and money required to conduct a

pre-stack migration. Most of the pre-stack migration will

be runwhen post-stacking has failed to resolve the layers

or structures. However, with advances in computers,

pre-stack migration will eventually become more

economical.

POST-STACK MIGRATION

Post stack migration is the process of migration in which

the data is stacked after it has been migrated. This

process is for many reasons, mainly because of its

reasonable cost compared to pre-stack migration. As in

pre-stack migration, post stack migration is based on the

idea that all data elements represent either primary

reflections or diffractions. This is done by using an

operation involving the rearrangement of seismic

information so that reflections and diffractions are

plotted at their true locations. The reason that migration

is needed is due to the fact that variable velocities and

dipping horizons cause the data to record surface

positions different from their sub-surface positions. The

stacking is accomplished by making a composite record

by combining traces from different records. Filtering is

involved with stacking because of timing errors or

wave-shape difference among the data being stacked.

A disadvantage of using post stack migration compared

to pre-stack migration is that it does not give as clear

results as pre-stack. Post stack usually gives good results

though, when the dip is small and where events with

different dips do not interfere on the migrated section.

LINKS


 
GEOPHYSICAL CORNER PRESTACK MIGRATION

REFERENCES

Sheriff, R.E., and Geldart, L.P., 1995, Exploration Seismology v. New York, Cambridge University Press.

Sheriff, R.E., 1981, Encyclopedia Dictionary of Exploration Geophysics v. Tulsa, OK, Society of Exploration Geophysics.

Reynolds, John M., 1997, An Introduction to Applied and Environmental Geophysics v. England, John Wiley &Sons.

Abriel, W. and Wright, R. The shapes of Gulf Coast salt intrusions related to seismic imaging. The Leading Edge. Aug 94. pp 868-878

http://www.gxtec.demon.co.uk/sirius3d.html (picture source)