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interval velocity (Vint) Seismic velocity over a depth interval z. If the rock type is uniform through that depth interval, then Vint is equal to the formation velocity. If the depth interval covers a number of rock beds, then the interval is equal to the average velocity (V) calculated over the distance z. If zi is the thickness of the ith interval and ti is the one-way travel time through it, then Vint = zi/ti. A specific form of Vint is given by the Dix formula, where the interval is defined in terms of the two-way travel time rather than by a discrete difference in depth.