Normal moveout may be used to distinguish primary from multiple reflections when the average velocity changes with depth. Approximate values of normal moveout may be found using an average velocity formula, but such values are always too large. The error appears to be controlled by the ratio of the standard deviation of the velocities within the section to the average velocity. Ray tracing provides normal moveout values which agree much better with observations.As a consequence of large dip, the reflections added by the CDP method do not represent identical areas of the dipping bed. However, if the various reflection zones lie on an inclined plane, they are added in-phase and assume the time of a reflection from a point on the plane.