Technical Reference Documentation

2.5 Migration Aperture

The migration aperture is an area to be added to ensure that dipping events and diffraction are properly migrated and collapsed respectively during seismic processing.

3D Acquisition Plan Zones

There are three defined areas (from inside to outside) within a model for a 3D acquisition plan:

  • The inner area (full-fold migrated image) to be used by the interpreters reliably.
  • The next area (full fold stack but partial fold migrated) to be used in processing.
  • The third area, which is the taper area needed to have the full fold of coverage area.

The second and third areas are necessary to create the first area and then interpretable 3D seismic sections. In practice, the second and third areas could overlap with each other partially.

Velocity Medium Dependencies

For a constant velocity medium, the migration aperture could be defined as Z tan 30° for dipping events with small angles (less than 30°), where Z is the depth of the reflector. This aperture value will collapse 95% of the energy from a diffraction (with the 30° point of the diffraction curve separating it from the 70% of migrated energy captured closer to the apex in the Fresnel zone).

For a V(z) medium and the resulting ray bending, the required migration aperture will be smaller. Due to the migration aperture being velocity and dip dependent, this value could change for the different edges of the survey area. That means the dip/inline direction could require a larger value than the strike/crossline direction.

Ray Tracing in Complex Structures

Ray-tracing modelling is a very useful tool to calculate migration aperture, especially in complex geologic subsurface conditions such as thrust belts. Profiles extracted from a 3D model of a thrust belt area usually show high dip layers and strong lateral velocity variations. In these environments, ray tracing modelling is a must to calculate a proper migration aperture.

The maximum offset, recording patch size, and migration aperture calculations are all interrelated. Evaluating a migration aperture map generated from ray tracing shows how the parameter changes across different edges of a target horizon layer. The migration aperture itself is the lateral component of a 30.0 degree diffraction traced from the layer to the surface, where the diffraction angle is relative to the vertical.