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Unocal Indonesia is using 3-D prestack depth migration to better image Indonesian prospects in difficult seismic areas. The goal is to delineate the deep fault geometry beneath image-distorting carbonates. Previous 2-D prestack depth migration has resulted in significantly better structural imaging, which is motivating the 3-D work.
Lateral velocity variations of 2 to 3 km/s within distances of less than 1 km make conventional time migration methods inadequate to accurately image structures and faulting. Iterative 2-D depth migration has provided better structural imaging in depth due to more accurate interval velocities and structural geometry resolution. 3-D prestack depth migration provides better migration velocity analyses and superior structural imaging, especially in comparison to 2-D lines acquired in directions oblique to major faults.
The most important step in depth migration is construction of a 3-D interval velocity model. All available data are used in constructing the model: Dip Moveout (DMO) stacking velocities, poststack time migration, well logs, check-shot surveys, gravity and magnetic data, seismic interpretations, and the 3-D prestack depth migration velocity analyses. The model is developed iteratively from the surface downward. Modeling packages such as GOCAD assist the data integration and 3-D visualization quality control. The 3-D migration algorithm is a Unocal proprietary Kirchhoff migration with ray - trace traveltimes and migration computed on a 3 Gigaflop, 10 CPU Silicon Graphics Power Challenge.
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