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1
Migration Artefacts
2
Overview
• There are a number of problems associated with migration:
– Migration can generate noise, especially if the traces are too far apart
(spatial aliasing).
– Steep dips are often missing.
– Any pre-existing noise-spikes, on stacked sections, migrate to form a
“migration smile”.
– If the velocity used for migration is too high, or too low, an incorrect
structure is produced in the image.
– Missing data (eg at the edge of a section or below salt structures) produces
“edge diffractions”.
Migration Noise
& Spatial Aliasing
• Noise here is generated
by inadequate spatial
sampling.
• Example courtesy of
Western Geophysical.
Missing
steep dips
Migration
noise
Migration Noise
& Spatial Aliasing
• Noise here is generated
by inadequate spatial
sampling.
• Example courtesy of
Western Geophysical.
Real
Data
Migration
Noise
Migration Smiles
a
Earth Model Normal Incidence Section
Migration
• A real semi-circular reflector in the subsurface would produce a spike on a
stacked section (because of focussing).
• Hence any spikes, due to noise on the stacked section, will migrate to give a
semi-circular reflection event.
Earth Model Stacked Section
Migration
Smiles
• Semi-circular
artefacts occurring
particularly at depth.
Migration with wrong V
a
Stack Correct Migration
Under MigrationV too low
Over MigrationV too high
Migration with wrong V
Correct Velocity Velocity Too High Velocity Too Low
Overmigration Undermigration
Migration with wrong V
• Over Migration
Example
• Under Migration
Example
“Edge Diffractions”
a
MissingData
Migration
Espirito Santo Courtesy PetroBras
• Edge diffractions can
appear internally in a
section if data is
missing for any reason.
Here, there is data
missing because of the
salt (stacking has
failed).
• As a consequence,
the space below the
salt is filled with
migration noise which
is entirely fictitious.
Espirito Santo Courtesy PetroBras
• Edge diffractions can
appear internally in a
section if data is
missing for any reason.
Here, there is data
missing because of the
salt (stacking has
failed).
• As a consequence,
the space below the
salt is filled with
migration noise which
is entirely fictitious.
Depth Migration
Example Courtesy Western-
Geco
• When lateral velocity
changes are large (so
that stacking and
migration fail), the
solution is Depth
Migration (details not
covered in this course).
Standard Processing Depth Migration