Abstract: In view of the insufficiency of former velocity analysis, this paper proposes the method of velocity analysis under the shaft: to carry on P-wave velocity analysis and NMO correction under the shaft based on the time-distance equation of offset distance and the VSP reflection time, as first break time of down-going P-wave is known. This article uses digital NMO correction method similar to surface seismic. Because the VSP ray path is asymmetrical, this paper studies and summarizes the method of calculating the position of spot in pressure wave after calculating the position of reflection spot, obtains common-reflection point-gather on binning, then carries on NMO correction, stacks with changed fold number. The authors process the vertical well Walk-away VSP P-wave record and the practical data, verifying the validity of this method and the ability to solve practical problems

Key words: VSP, velocity analysis, NMO correction, position of reflection spot