seismic migration

1. Migration performs two functions-
Event positioning
 Focusing to achieve image sharpness.
Why migration?
The CMP gather performs poor because of-
 Dipping reflector
 Focusing due to syncline structure
 Diffraction through sharp edges.

2. MIGRATION
• Migration is a process, which repositions the reflected energy from
its CMP to true subsurface locations.
• After migration the seismic section appears to be the geologic
section.
• Migration moves dipping events in up dip direction to their true
subsurface position and collapses diffraction to a point.

3. • To understand why seismic expression does not show the true
position of events, consider the following figures. At the surface the
common mid point (CMP) lies at the middle of shot and receiver
positions. For a horizontal bed the common depth point (CDP) lies
exactly below the CMP.

4. Migration process
Cour
©An introduction to Geophysical Exploration Philp
Kerry at all.

5. Dipping Reflector
 Usually for horizontal reflectors
the reflection point is vertically
below the source/receiver.
For dipping layers the reflection
comes from a point up dip.
Therefore, a travel-time section
will always show a reduced dip.
 tan αt = Sin αs
©An introduction to Geophysical Exploration , Philp Kerry at
all.

6. Depth model Time representation
Diffracting point Diffraction hyperbola.
Dipping reflector Shifted down dip and dip
decreased.
Tight syncline “Bow tie” shape
Geophysicists know well the simple examples of images in time deformed or displaced in relation to the depth
model

7. TYPES OF MIGRATION
PRE STACK MIGRATION
TIME
DEPTH
POST STACK MIGRATION
TIME
DEPTH

8. TYPES OF MIGRATION(CONTD…)
• POST STACK MIGRATION:
Post-stack migration operates on the stacked section. Post stack- is much faster than Pre
stack, because stacking reduces by an order of magnitude the number of traces that must
be processed.
• For post stack migration to be successful the assumptions made in stacking must be
satisfied: the amplitude of the stacked trace and reflected arrivals must be approximately
hyperbolic. These suppositions are valid only when variations in lithology and fluid
content over the span of the gathered traces can be ignored and when the structure is
simple.
• PRE STACK MIGRATION:
When the subsurface structure is complex the various traces in a CMP are not actually
coming from same point in subsurface. These situations can be solved by pre-stack
migration. It avoids the stacking process by migration the unstacked data directly.

9. TIME AND DEPTH MIGRATION
DEPTH MIGRATION-
 Needed when -stacked section contains
structural dip.
Stacked section contains large velocity
gradients.
Successfully used in areas with lateral velocity
variations, which tend to be the areas that are
most interesting to petroleum geologists.
Some of the popularly used depth migration
algorithms are: Kirchhoff depth migration,
Reverse Time Migration(RTM), Gaussian Beam
Migration and Wave- equation Migration.
Time Migration-
 Needed when stacked section
contains structural dip or diffractions.
 Valid for vertically varying velocity.
 Acceptable to mild lateral velocity
variations
 Some popularly used time migration
algorithms are: Stolt migration,
Gazdag and Finite-difference
migration.

10. Diffraction Events

11. Structural events