Velocity anisotropy in the Niger Delta Basin: A case study of prestack time imaging with isotropic and anisotropic velocity models
By Ogagarue, D.O., Ebeniro, J.O., Ehirim, C.N.
A recent study of velocity anisotropy in the Niger Delta, shows that the Niger Delta sediments are weakly anisotropic. Based on this finding, we carried out a prestack Kirchhoff 2D time migration of a pre-processed 4,767 Km 2D seismic data acquired in the Niger Delta, using both isotropic and anisotropic velocity models in the migration algorithm. The purpose of the study was to compare the imaged results and highlight the serious problems of mis-positioning and/or mis-focusing of events using the conventional isotropic imaging algorithm. We built our isotropic velocity model from a stacking (RMS) velocity analysis on CDP gathers of the pre-processed dataset while our anisotropic velocity model was based on an approach which calculates interval eta parameter from a Dix-type interval velocity field derived from the final RMS velocity field. A comparison of the imaged data shows a clear coherency improvement in structural definition and event continuity in the anisotropically imaged data than the data imaged isotropically, especially at deeper depths. Therefore, incorporating velocity anisotropy in imaging algorithms performed on long offset seismic data acquired in the Niger Delta will significantly reduce exploration risks in the Niger Delta Basin.
Key Words : Niger Delta Basin, Isotropic Velocity, Anisotropic Velocity, Kirchoffs Time Migration.
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