For reflections from a sequence of flat, parallel isotropic layers and small offsets, the interval velocity in the nth layer can be recursively extracted from the stacking velocities, using
where and are the stacking velocities from the datum to reflectors above and below the layer and and are reflection arrival times. This formula is often misused to calculate interval velocities in situations that do not satisfy Dix's assumptions. Named for C. Hewitt Dix (1905 - 1984), American geophysicist. See Dix (1955) . Some call this the Postma equation.
- Dix, C. H., 1955, Seismic velocities from surface measurements: Geophysics, 20, 68–86.