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Integrated approach to subsalt depth imaging

Synthetic case study

GeoTech System, Moscow, Russia

Evgeny Landa

The Geophysical Institute of Israel, Lod, Israel

Josef Paffenholz

BHP Billiton Petroleum, Houston, Texas, U.S.

Corresponding author: jpaffenholz@fairfield.com

The first 20% of the full text of this article appears below.

Prestack depth migration has become almost mandatory in areas of complex geology for reasons that are abundantly clear. In structurally complex media, CMP traveltime curves differ significantly from hyperbolae. Thus, when using standard time-processing techniques, we cannot produce a time section containing all information related to the medium that is in the prestack gathers.

Depth migration has, in fact, quite a limited purpose—to convert seismic data from one form to another for a given velocity model. Possible variants of migration include conversion of a time section to a depth section (poststack depth migration) or of prestack gathers to a depth section (prestack depth migration). Most procedures correctly migrate only primary compressional reflection events (even when the correct velocity model is given) while all other modes (including multiples, out of plane events in the case of 2D migration, converted events, etc.) produce coherent noise in the final section.

The migration procedure is closely interlinked with the velocity model building. This model is usually determined either during the course of the migration or is assumed known—i.e., initially derived using one of the velocity inversion procedures. In the first case, model building is based on the concept that, after migrating prestack data with the correct velocity, a panel collected at a given depth point should consist of a flat event. When a trial velocity is incorrect, the correction needed to flatten a common-depth point (CDP) panel is not trivial, requires a vast amount of calculation, and greatly reduces the chances of deriving the correct velocity model in complex situations.

Another problem arises when a velocity model is constructed before migration using a traveltime inversion approach (Dix transformation, coherency inversion, traveltime tomography, etc.). In such cases, arrival times of reflected waves serve as input data. They should be picked or estimated (in one way . . . [Full Text of this Article]