Changes

Regardless of the migration approach implemented, the key parameter of the process is ''velocity''. Since migration involves pushing waves back to their reflecting points, it is essential that the waves be pushed backward through the same medium through which they have propagated. Clearly, waves will not get back to the correct position at a given time if the velocity used in the migration process differs from the actual subsurface velocity. Unfortunately, subsurface velocity is seldom well known, particularly in geologically complex areas. Today's migration algorithms are highly accurate when supplied with the correct subsurface velocity. Because subsurface velocity can only be estimated, however, migration yields only an estimate of the true subsurface.

Regardless of the migration approach implemented, the key parameter of the process is ''velocity''. Since migration involves pushing waves back to their reflecting points, it is essential that the waves be pushed backward through the same medium through which they have propagated. Clearly, waves will not get back to the correct position at a given time if the velocity used in the migration process differs from the actual subsurface velocity. Unfortunately, subsurface velocity is seldom well known, particularly in geologically complex areas. Today's migration algorithms are highly accurate when supplied with the correct subsurface velocity. Because subsurface velocity can only be estimated, however, migration yields only an estimate of the true subsurface.

Where lateral variation of velocity is modest (as in many places in the [[Gulf of Mexico]]), migration methods in the class called ''time migration'' have performed adequately. Where lateral velocity variation is severe (as in many overthrust areas), more computationally intensive ''depth migration is'' required. Note that the terms ''depth'' and ''time migration'' do not relate to whether the migrated results are presented as a function of time or depth. Results of both migration categories are most often displayed in time (as in the examples shown here) because of added uncertainties in results converted to depth. While depth migration is capable of accurate subsurface imaging where velocity is complex, the required accurate estimation of velocity is difficult and time consuming.

Where lateral variation of velocity is modest (as in many places in the [[Gulf of Mexico]]), migration methods in the class called ''time migration'' have performed adequately. Where lateral velocity variation is severe (as in many [[overthrust]] areas), more computationally intensive ''depth migration is'' required. Note that the terms ''depth'' and ''time migration'' do not relate to whether the migrated results are presented as a function of time or depth. Results of both migration categories are most often displayed in time (as in the examples shown here) because of added uncertainties in results converted to depth. While depth migration is capable of accurate subsurface imaging where velocity is complex, the required accurate estimation of velocity is difficult and time consuming.

==Poststack versus prestack migration==

==Poststack versus prestack migration==