Crosswell Imaging with LCB Beamlet
Yueming Ye; Wu Ru-shan; Zhuang Xijin
Lots of oil and gas reservoirs are distributed within thin layers and minor faults which can not be clearly described by surface seismic data imaging due to the low resolution. In cross-well seismic survey geometry, shots and receivers are located in separate wells, so it has little effect from the surface. Crosswell seismic survey has proven to be an effective, high-resolution method of reservoir characterization because of its high, wide frequency band. In this paper, we introduce local cosine basis beamlet migration into crosswell prestack depth migration which greatly improved the imaging quality of thin layers and minor faults between two wells.
For the specific crosswell acquisition system, there exist many kinds of different reflected wave in crosswell acquisition system. VSP-CDP which based on constant velocity diffraction stack is conventional crosswell pre-stack depth migration method which doesn't work well in area with strong velocity variation. Wave equation based one way prestack depth migration can adapt to area with strong velocity variation, however, conventional one way migration method can not directly used for corsswell data migration. In this paper, we perform the crosswell prestack depth migration based on the principle of wavefield superposion, wavefield downward continuation from the surface, seismic data or source wavefield can be added where the depth intersect receivers or shots located in the well, which eliminate the influence of different depth distribution of shot and receivers. In order to improve the image quality of these thin layers, Local cosine basis beamlet migration operator was introduced which has localized reference velocity. The localized velocity reduces the perturbation velocity and improves the calculation accuracy.
A complex crosswell model was build, where exist two steep faults and many thin layers. Crosswell wavefield was separated into upgoing and downgoing wavefield. With similar method and different wavefield continuation direction, we respectively obtain the crosswell images by upgoing and downgoing wavefield prestack depth migration. Test of the crosswell model migration has proved the validity and effectiveness of our method. Compare with the conventional method, thin layers and minor faults got clearer image and migration noise was better suppressed.
Key Words: Crosswell Prestack depth migration Beamlets High resolution
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013