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Figure 1. Concept of VSP depth-to-time calibration. VSP data are unique in that they are the only seismic data that are recorded simultaneously in the two domains that are critical to geologic interpretation: straigraphic depth and seismic reflection time (a). As a result, specific stratigraphic units – known as a function of depth from well log data – can be precisely positioned in their correct VSP image-time windows (b). With the exception of unit 8, each reservoir labeled here is a thin-bed penetrated by the VSP well. When the VSP image is shifted up or down to better correlate with a surface-recorded seismic image, the VSP-defined time window that spans each thin-bed unit should be considered as being welded to the VSP image, causing the stratigraphy to move up and down in concert with the VSP image as an optimal match is established between the VSP and seismic images.