# Dictionary:Rms velocity

Root-mean-square velocity. For a series of parallel layers of velocity *V*_{i}, where the traveltime for seismic energy perpendicularly through each is *t*_{i}, the rms velocity for travel perpendicular to the layers is

The average velocity for this path, , on the other hand, is

The **stacking velocity**, the velocity determined from velocity analysis based on normal-moveout measurements, is often used to approximate rms velocity; this is valid only in the limit as the offset approaches zero where interfaces and velocities are horizontal and isotropic.