# Dictionary:Fig D-20

FIG. D-20. DMO. (a) Depth section showing the updip movement of the reflecting point for an offset geophone for constant velocity; ${\displaystyle \Delta =(h^{2}/D)cos\xi sin\xi }$, where ${\displaystyle \xi }$; is the dip (Levin, 1971). To avoid reflection point smearing, an offset trace should be gathered with the updip zero-offset trace at a distance ${\displaystyle G=(-h^{2}/D)sin\xi }$, but such a gather is not hyperbolic; the DMO correction makes this gather hyperbolic. (b) A diffraction in location-offset space, a Cheops pyramid, is not a hyperboloid. (c) Applying NMO changes the Cheops pyramid into a saddle-shaped surface. (d) Applying DMO along with NMO yields data that can be stacked without reflection-point smear. (e) NMO corrects for the time delay on an offset trace assuming horizontality, DMO moves the data to the correct zero-offset trace for a dipping reflection, and migration further moves it to its subsurface location. (After Deregowski, 1986, 13.)