Seismic migration

This process is necessary to overcome the limitations of geophysical methods imposed by areas of complex geology, such as: faults, salt bodies, folding, etc.

Migration can lead to a dramatic uplift in image quality so algorithms are the subject of intense research, both within the geophysical industry as well as academic circles.

At an interface between two rock types, with different acoustic impedances, the seismic energy is either refracted, reflected back towards the surface or attenuated by the medium.

In this case, the distance is halved because it can be assumed that it only took one-half of the total travel time to reach the reflector from the source, then the other half to return to the receiver.

Consequently, much more information is used, which results in a much better image, along with the fact that PreSM honours velocity changes more accurately than post-stack migration.

This type of migration makes the assumption of only mild lateral velocity variations and this breaks down in the presence of most interesting and complex subsurface structures, particularly salt.

[12][13] The goal of migration is to ultimately increase spatial resolution and one of the basic assumptions made about the seismic data is that it only shows primary reflections and all noise has been removed.

Noise that may be easy to distinguish pre-migration could be smeared across the entire aperture length during migration, reducing image sharpness and clarity.

This process is a central step in the creation of an image of the subsurface from active source seismic data collected at the surface, seabed, boreholes, etc., and therefore is used on industrial scales by oil and gas companies and their service providers on digital computers.

Post-stack migration begins with seismic data which has already been stacked, and thus already lost valuable velocity analysis information.

Diagram showing the raypath for a zero-offset reflection from a horizontal reflector.
Diagram showing the raypath for a zero-offset reflection from a dipping reflector and the resultant apparent dip.
A zero-offset non-migrated data set. Raw zero-offset data for a simple syncline in a constant velocity world. Notice the signature bow-tie effect in the image. This is the result of reflections occurring from both sides of the syncline, and arriving at the same receiver at different times. Migration can correct this effect.
A zero-offset migrated data set of the File:SimpleSyncline.jpg data. This data was migrated using a time-migration referred to as phase-shift which operates in the Fourier domain . The migration has replaced all events in their correct locations, successfully reconstructing a syncline. However, there are erroneous events (swinging arcs) throughout the image which are migration induced noise.
An example of simple graphical migration. Until the advent of modern computers in the 1960s and 1970s this was a method used by geophysicists to primitively 'migrate' their data. This method is obsolete with the advent of digital processors, but is useful for understanding the basic principle behind migration.