Figure 2. Zero or near offset VSP (left panel) and a corridor stack of VSP traces (right panel). The VSP has been corrected to two-way time so that reflections from horizontal reflectors appear at the same time on traces recorded at different levels. The corridor stack (right) is a partial summation or stacking of the VSP traces (left). Stacking, a summing of data to produce a single output trace, enhances the signal to noise ratio of seismic data.