Thursday, January 15, 2015

Seismic Depth Migration

Migrated time sections may be simply stretched according to a vertical velocity function to give sections where the vertical scale is linear in depth rather than in time. However, where velocity varies apreciably in the horizontal direction, raypath bending introduces additional complications that depth migration attempts to accomodate.

Hubral (1977) observed that the apex of a diffraction curve is where the image ray, a ray that approaches the surface at right angles, emerges. Therefore, if we follow the image ray as it refracts according to Snell's law down through the earth, it will lead to the correct position of the diffracting point even if velocity surfaces are not horizontal. This concept is the heart of depth migration, migration that accomodates horizontal changes in velocity. Conventional migration collapses diffractions to the image-ray positions, so an additional step is needed to move elements to their correct subsurface locations.

The velocity model defines the major velocity surfaces where significant raypath bending occurs; key horizons on a conventionally migrated time section are mapped assuming that these are the major velocity interfaces. Cleary, defining the velocity model adequately is the key to succesful depth migration. Specifying velocities is a very difficult task because choices are not obvious. Detailed knowledge of the velocity distribution is often not available, especially in the structurally complex areas where depth migration is most needed. However, even though velocity errors create depth and location errors in the final product, the improved structural clarity often makes the procedure worthwhile and an appreciable amount of depth migration is being done today.

Subsalt imaging is important in several areas to locate hydrocarbons trapped beneath salt. Appreciable raypath bending occurs at the large contrast between the salt and sediments and the surfaces of the salt may be quite irregular. Migration is usually done in steps: conventional migration frist defines the top of the salt, then the base-of-salt reflection is defined using the salt veloctiy, and finally migration is completed with sediment velocities. Subsalt imaging provides a severe test of migration accuracy and requires very reliable data, which are usually 3-D data, and processing, often prestack migration.

(Sherrif, Exploration seismology. 1995)

Pada tahapan ekplorasi dimana konsep play menggunakan primary objektif carbonate, depth migration menjadi sangat dibutuhkan. Kontras dan distorsi velocity secara lateral menyebabkan imaging reef menjadi kurang sempurna bila processing hanya pada time migration. 

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