“…However, in this case in the equation (1) there is a complex dependence of ray path ξ on the nonlinear velocity function v(x, y, z), where z is a depth. Therefore the task to determine the slowness field f (x, y, z) can be solved with linearized formulation that has been proposed and investigated by [9,10]. Considering travel times t and t 0 along ray paths ξ and ξ 0 for the two media with velocities v(x, y, z) and v 0 (z) (Figure 1) the problem of a search of f (x, y, z) can be written in the following form:…”