# Dictionary:Normal moveout (NMO)

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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.[1]

The difference in reflection arrival time because the geophone is not located at the source point, i.e., because of source-to-geophone distance (offset). Usually applied to common-midpoint gathers, it is the additional traveltime required because of offset, assuming that the reflecting bed is not dipping and that raypaths are straight lines. This leads to a hyperbolic shape for a reflection. Because the raypath actually curves as the velocity changes, fitting a hyperbola assumes that the actual velocity distribution is equivalent to a constant NMO velocity, but the NMO velocity changes with the offset. However, the assumption often provides an adequate solution for offsets less than the reflector depth.

The NMO correction applied to long-offset data generally creates a ‘‘hockey-stick’’ effect giving long-offset traveltimes that are too small and causing waveshape broadening and loss of resolution. For long-offsets the reflection curvature becomes nonhyperbolic because of vertical changes in velocity and anisotropy and a nonhyperbolic normal-moveout analysis (q.v.) has to be used. Reflector dip also has effects that often require a DMO correction. The functions of the NMO and DMO operations are illustrated graphically in Figure D-20e.