# Dictionary:Dix formula

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For reflections from a sequence of flat, parallel layers and small offsets, the interval velocity in the nth layer ${\displaystyle V_{n-layer}^{int}}$can be recursively extracted from the stacking velocities, ${\displaystyle V_{n}}$ using

${\displaystyle V_{n-layer}^{int}=\left({\frac {V_{n}^{2}t_{n}-V_{n-1}^{2}t_{n-1}}{t_{n}-t_{n-1}}}\right)^{1/2}}$,

where ${\displaystyle V_{n-1}}$ and ${\displaystyle V_{n}}$ are the stacking velocities from the datum to reflectors above and below the layer and ${\displaystyle t_{n-1}}$ and ${\displaystyle t_{n}}$ 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) [1]. Some call this the Postma equation.

## References

1. Dix, C. H., 1955, Seismic velocities from surface measurements: Geophysics, 20, 68–86.