A refraction interpretation method using reversed refraction profiles, also called Hagedoorn method. Let tAB be the surface-to-surface time between A and B and let tA and tB be arrival times at various intermediate locations from sources A and B, respectively. ‘‘Minus’’ values, tA–tB–tAB, are calculated for each location and plotted to give the velocity of the refractor. ‘‘Plus’’ values, tA+tB–tAB, are calculated for each location and plotted to give a picture of the refractor’s depth. See Hagedoorn (1959) or Sheriff and Geldart (1995, 442–443).