Abstract: Usually，the interval velocity is computed with the stacking velocity in the structure mapping by variable velocity.When the formation velocity changes dramatically in lateral orientation，the wisdom of the use of the stacking
process is questioned.In the event that the interval velocity contains rapid lateral changes，the travel-time curve isn′t a hyperbolic curve again.The interval velocity computed with Dix interval velocity formula isn′t precise.The discrepancy increases with the degree of lateral changes.Synergetic acoustic logging velocity is precise，which is inversed by seismic data under control of acoustic logging data.At the same time，it can provide the detail of lateral velocity changes.It is interested that the relationships of different strata in the depth.Generally，roundtrip travel-time has been known.If the Interval velocity is computed，the depth of strata will be known.This paper illustrates the relationship between synergetic acoustic logging velocity and seismic velocity by the theory of wave mechanics，and the exact interval velocity for structure mapping by variable velocity is computed.

Key words: structure mapping by variable velocity, inversion, acoustic logging velocity, seismic velocity, anisotropy