Seismic imaging in complex structures by hybrid gridded tomography velocity model

Integrating advantages of layer based and tomography velocity model building methods could partially resolve some of the problems in seismic imaging. This integration resulted to introduction of the hybrid tomography methods which try to overcome the problem of handling large velocity contrast by tomography methods. In this study, the velocity model building strategy in the hybrid gridded tomography method was modified to reduce the effect of velocity contrast in depth seismic imaging. The proposed strategy then was applied on a seismic data from a complex geological structures and results depicted that the hybrid gridded tomography, with the presented strategy could be used as an alternative to conventional velocity model building.SummaryThe hybrid gridded velocity model uses the concept of soft and hard regions and boundaries in the initial velocity model. In this approach, regions with anomaly velocity values are defined as hard regions and the rest of the media is considered as soft media. The contacts between regions with low uncertainty in position are defined as hard boundary and experience minimum changes in updating steps. Whilst region contacts with high uncertainty in position are considered as soft boundaries. IntroductionThese boundaries vary in position during velocity model updating. These regions and boundaries are simultaneously and separately updated in each model updating sequence. Updating of hard regions doesn’t disturb soft regions updating results and vice versa. In the proposed strategy, the conventional velocity model is obtained to be used for defining large velocity contrasts. Afterwards soft and hard regions are simultaneously updated by gridded tomography method. Hard boundaries face small variation in place while soft boundaries might largely vary in location. After sufficient iteration, sharp boundaries are smoothed and finalized in location and together with velocity values. Methodology and ApproachesIn the proposed approach, the seismic imaging with velocity model obtained in each iteration was changed from pre-stack depth migration to post stack depth migration. The selected seismic data contains a large salt dome with surrounding dipping layers, faults and an unconformity beside the salt. Through the hybrid velocity model building, top boundary and the body of salt dome were considered as hard regions. Bottom of the salt was considered as soft boundary.Results and ConclusionsResult of applying the proposed strategy showed that this method could handle large lateral velocity contract in depth imaging. Therefore, depth imaging by the final hybrid velocity model obtain seismic image with more accuracy in positioning of seismic reflectors and body of geological structures in comparison to images obtained by conventional velocity modelling.