Factors of effect on accuracy of velocity field in south edge of Tianshan and its effect on depth imaging

摘要

We design the seismic geological model in the area of the south edge of Tianshan which includes a shallow conglomerate layer with high velocity of group Xiyue of Quaternary at the north of Yaken, and a low velocity gypsum-rock layer whose structure varies widely in a deep layer of group Jidieke of Tertiary. The shot gathers are synthesized based on the field observation system. Then stacking velocities (Vs) and a migration section are obtained and Vs is compared with the actual root-mean-square velocity (Vrms) of the model. The results show that Vs is smaller than the actual Vrms when the velocity varies laterally over the target layer. Vs can also be larger than the actual Vrms due to a shallow conglomerate. As a result of the synthetic effect, Vs is 8% larger than the actual Vrms. Moreover, the shallow high velocity conglomerate slightly affects the time image of deep layers, but the Vs of the deep layers at two ends of the conglomerate is increased, which results in the false synclines in deep layers and enlarges the height of the structure near the synclines.