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Page 1: Seismic Reflections, Seismic Processing & Seismic Artefactsnm2.rhul.ac.uk/wp-content/uploads/2016/10/Migration... · 2016-10-10 · 2 Overview • There are a number of problems associated

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Migration Artefacts

Page 2: Seismic Reflections, Seismic Processing & Seismic Artefactsnm2.rhul.ac.uk/wp-content/uploads/2016/10/Migration... · 2016-10-10 · 2 Overview • There are a number of problems associated

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”.

Page 3: Seismic Reflections, Seismic Processing & Seismic Artefactsnm2.rhul.ac.uk/wp-content/uploads/2016/10/Migration... · 2016-10-10 · 2 Overview • There are a number of problems associated

Migration Noise

& Spatial Aliasing

• Noise here is generated

by inadequate spatial

sampling.

• Example courtesy of

Western Geophysical.

Missing

steep dips

Migration

noise

Page 4: Seismic Reflections, Seismic Processing & Seismic Artefactsnm2.rhul.ac.uk/wp-content/uploads/2016/10/Migration... · 2016-10-10 · 2 Overview • There are a number of problems associated

Migration Noise

& Spatial Aliasing

• Noise here is generated

by inadequate spatial

sampling.

• Example courtesy of

Western Geophysical.

Page 5: Seismic Reflections, Seismic Processing & Seismic Artefactsnm2.rhul.ac.uk/wp-content/uploads/2016/10/Migration... · 2016-10-10 · 2 Overview • There are a number of problems associated

Real

Data

Migration

Noise

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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

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Migration

Smiles

• Semi-circular

artefacts occurring

particularly at depth.

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Migration with wrong V

a

Stack Correct Migration

Under MigrationV too low

Over MigrationV too high

Page 9: Seismic Reflections, Seismic Processing & Seismic Artefactsnm2.rhul.ac.uk/wp-content/uploads/2016/10/Migration... · 2016-10-10 · 2 Overview • There are a number of problems associated

Migration with wrong V

Correct Velocity Velocity Too High Velocity Too Low

Overmigration Undermigration

Page 10: Seismic Reflections, Seismic Processing & Seismic Artefactsnm2.rhul.ac.uk/wp-content/uploads/2016/10/Migration... · 2016-10-10 · 2 Overview • There are a number of problems associated

Migration with wrong V

• Over Migration

Example

• Under Migration

Example

Page 11: Seismic Reflections, Seismic Processing & Seismic Artefactsnm2.rhul.ac.uk/wp-content/uploads/2016/10/Migration... · 2016-10-10 · 2 Overview • There are a number of problems associated

“Edge Diffractions”

a

MissingData

Migration

Page 12: Seismic Reflections, Seismic Processing & Seismic Artefactsnm2.rhul.ac.uk/wp-content/uploads/2016/10/Migration... · 2016-10-10 · 2 Overview • There are a number of problems associated

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.

Page 13: Seismic Reflections, Seismic Processing & Seismic Artefactsnm2.rhul.ac.uk/wp-content/uploads/2016/10/Migration... · 2016-10-10 · 2 Overview • There are a number of problems associated

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.

Page 14: Seismic Reflections, Seismic Processing & Seismic Artefactsnm2.rhul.ac.uk/wp-content/uploads/2016/10/Migration... · 2016-10-10 · 2 Overview • There are a number of problems associated

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


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