An equation used to calculate the interval velocity within a series of flat, parallel layers, named for American geophysicist C. Hewitt Dix (1905 to 1984). Sheriff (1991) cautions that the equation is misused in situations that do not match Dix's assumptions. The equation is as follows:
Vint = [(t2 VRMS22 − t1 VRMS12) / (t2 − t1)]1/2,
Vint = interval velocity
t1 = traveltime to the first reflector
t2 = traveltime to the second reflector
VRMS1 = root-mean-square velocity to the first reflector
VRMS2 = root-mean-square velocity to the second reflector.
Reference: Dix CH: "Seismic Velocities from Surface Measurements," Geophysics 20, no. 1 (January 1955): 68–86.