Abstract：During the seismic exploration, the stack velocity based on hyperbolic NMO formula is equivalent velocity, not mean-square-root (MSR) velocity in common meaning when the velocities in overburdens of target laterally change rapidly.In seismic data processing, the velocities used for stack imaging of targets are badly affected by this kind of lateral variation of velocities.In order to in-phase stack of aligned reflection events on CMP gathers, the stack velocities may have some higher or lower anomalies.The reason is that the moveout,the rays crossing overburdens at near and far offsets on CMP gathers,deviates from hyperbolic tendency because of the lateral change of velocities, making the NMO change.So,the relationship between the stack velocity taking aligning the reflection events on the CMP gathers as a law and MSR velocity is very complicated, the paper tries to analyze this phenomenon by theoretical models and real data, deepening the knowledge of the issue.
