After having previously successfully compared the factorial kriging filter to their most common geophysical counterparts, a full processing of VSP data using probability modelling (geostatistics) has been developed and named sigma processing of VSP data. A case study is presented where the wavefield separation of upgoing and downgoing waves is operated using a specifically designed kriging filter called gamma filter. The very promising results obtained are related to the robustness and the practical flexibility of the theoretical background of the sigma processing of VSP data.