In the problems of three-dimensional (3D) travel time seismic tomography where the data are
travel times of diving waves and the starting model is a system of plane layers where the velocity is a function
of depth alone, the solution turns out to strongly depend on the selection of the starting model. This is due to
the fact that in the different starting models, the rays between the same points can intersect different layers,
which makes the tomography problem fundamentally nonlinear. This effect is demonstrated by the model
example. Based on the same example, it is shown how the starting model should be selected to ensure a solution
close to the true velocity distribution. The starting model (the average dependence of the seismic velocity
on depth) should be determined by the method of successive iterations at each step of which the horizontal
velocity variations in the layers are determined by solving the two-dimensional tomography problem. An
example illustrating the application of this technique to the P-wave travel time data in the region of the Black
Sea basin is presented.