1. The locations of midpoints assuming plane horizontal reflectors. Reflection points do not lie under these positions if the reflectors dip. For a single-ended spread, the subsurface coverage extends from the midpoint between the source and the nearest geophone group to the midpoint between the source and the most distant geophone group, therefore of length equal to half the spread length. 2. Sometimes the migrated positions of the actual reflection points, thus allowing for dip (and possibly for velocity variations). See Figure S-27. 3. The multiplicity associated with common-midpoint data.