Figure 3. A great deal of mathematics involved in the extrapolation of surface source (S) to a deep virtual source (VS) (Figure 2) is summarized by this diagram. First, time reverse the surface-source data trace X1 acquired at the intended virtual-source position. Second, “convolve” this reversed-time trace with all other data traces (X2, X3, etc.) produced by the surface source to transform these traces into data that are equivalent to data that would be produced by a source positioned at station X1.You now have a deep source to go with the deep sensors, and you are starting to look into hidden places that cannot be seen with surface-positioned sources and sensors.