# Translations:Implementation of migration/3/en

Let us give a ${\displaystyle {\rm {2D}}}$ example. We assume that the medium has a constant velocity, which we take to be one. Let the subsurface grid for depth points D be given by ${\displaystyle \left(z,x\right)}$ where depth is given by ${\displaystyle z{\rm {=}}{\rm {1,2,...,10}}}$ and horizontal distance is given by ${\displaystyle x{\rm {=}}{\rm {1,2,...,15}}}$. Let the surface locations P be ${\displaystyle \left(z{\rm {=\ 1,}}x\right)}$ where ${\displaystyle x{\rm {=}}{\rm {1,2,...,15}}}$. Suppose the source is ${\displaystyle S=(1,3).}$ We want to construct a table of one-way traveltimes, where depth z denotes the row and horizontal distance x denotes the column. The one-way traveltime from the source (1, 3) to the depth point ${\displaystyle \left(z,x\right)}$ is ${\displaystyle t\left(z,x\right){\rm {=}}{\left[{\left(z-{\rm {\ 1}}\right)}^{\rm {2}}{\rm {+}}{\left(x-{\rm {3}}\right)}^{\rm {2}}\right]}^{\rm {1/2}}}$. For example, the traveltime from source to depth point ${\displaystyle \left(z,x\right){\rm {=}}\left({\rm {4,6}}\right)}$ is