# Translations:Migration in the case of constant velocity/3/en

Here we consider the case of a homogeneous isotropic medium. Within a homogeneous isotropic material, the velocity *v* has the same value at all points and in all directions. The rays are straight lines because, by symmetry, there is no preferred direction along which they can deviate from the straight path. The two-way traveltime *t* is the elapsed time for a seismic wave to travel from its source to a given depth point and return to a receiver at the earth’s surface. Thus, the two-way traveltime *t* is equal to the one-way traveltime from the source point *S* to the depth point *D* plus the one-way traveltime from the depth point *D* to the receiver point *R*. Note that the traveltime from *D* to *R* is the same as the traveltime from *R* to *D*. We can write , which in terms of distance is