During the processing of pre-stack depth migration (PSDM) of seismic data in complex structures in western China, an anomaly of interval velocity inconsistent with geological features appears, even when the CRP gather is flattened and the residual velocity (or the residual moveout) returns to zero(Reshef, 2001). The model data analysis in this paper suggests that velocity errors in the shallow layer lead to these anomalies. In order to improve PSDM results of near-surface complicated zones, the velocity variation trend of the deep layer is restrained by geological constraints and residual spectra and afterwards an updated velocity modeling method using the corrected shallow layer velocities is applied. The crucial point in this paper is to understand the reasoning for velocity anomalies and to build a relatively correct velocity model using the PSDM velocity analysis.


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