The equation by which the interval velocity (v_int) can be calculated for a zone between two depths on a seismic section. For two reflectors with reflected-ray travel times t₁ and t₂, and root-mean-square velocities v_rms1 and v_rms2 respectively, then: v_int = [(t₂v_rms2² – t₁v_rmsl²)/(t₂ – t₁)]^1/2. The stacking velocities are normally assumed to be true root-mean-square velocities but there may be occasions when it is necessary to correct them for the effects of dip.