Seismic velocity over a depth interval z. If the rock type is uniform through that depth interval, then V_int 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 z_i is the thickness of the ith interval and t_i is the one-way travel time through it, then V_int = z_i/t_i. A specific form of V_int 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.