Generalized stacking velocity analysis tools like the Common-Reflection-Surface stack method provide stacking parameters beyond the conventional stacking velocity. These parameters can be expressed in terms of useful geometrical wavefront properties. However, this requires a good estimation of the tuned velocities which are valid for imaging in the vicinity of source and/or receiver of the recorded data. Using the downgoing first arrivals, we introduce a simple and efficient method to determine the velocities at every receiver level in a walkover VSP experiment. The calibrated wavefront properties are subsequently used for the decomposition into upgoing waves arriving either as P- or S-waves at the receivers.