These days "simple" oil fields and reservoirs are well researched and oil companies face a complex task of improving the accuracy of the study of small capacity collectors, or fractured reservoirs. These new challenges require new methods for processing and interpretation of seismic data.
Significant velocity anomalies are usually observed in areas with complex geological structure, e.g. in the presence of faults, salt-dome tectonics, or steep dips of the reflecting horizons. Time migration in such environments can not get a reliable image of the complex geological structures. Its resultis distorted due to underestimated lateral velocity changes. Successful method of restoring the correct structure of the environment in complex geological conditions is a pre-stack depth migration. During depth migration seismic data are transformed to depth image of the geological structure, with correctly positioned reflecting boundaries and correct dips due to rightful consideration of seismic-wave refraction/. Comparison of the results of time and depth prestack migration is presented in Figures 1 and 2.
These days, we are using a large number of algorithms to perform pre-stack depth migration, whether based on ray tracing methods or the wavefield restoration: classical Kirchhoff migration, migration of full-wave solutions on the basis of the wave field (WEM), reverse time migration (RTM), different variations of Gaussian Beam, etc. Which algorithm is used usually depends on the geological structure of the area and processing goals. One of the determinants for pre-stack depth migration performance for each algorithm is the quality of deep-velocity model.