Normal Move Out is a term used in seismics and geophysics to describe the effect of the separation between receiver and source on the arrival time of a reflection that does not dip, abbreviated NMO. Filters and correcting algorithms must be used to properly image the subsurface.
In a land-based seismic survey blasts of dynamite or vibroseis units are used to create waves that will reflect off different layers of rock and are collected by equally spaced geophones or receivers placed along the surface. If a horizontal interface is present in the subsurface, seismic waves will reflect toward the surface recorded by geophones. Reflections from this surface will be received by the closest geophone first, arriving later at farther geophones. Because the wave must travel along the hypotenuse created between the depth of the event and source-receiver offset, the time delay increases hyperbolically along equally spaced geophones. The hyperbolic distortion must be corrected in ordered to accurately image the subsurface.