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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