# Translations:Migration - book/16/en

Over the years, various methods have been devised to obtain a sampling of the velocity distribution within the earth. The velocity functions so determined vary from method to method. For example, the velocity measured vertically from a check shot or vertical seismic profile (VSP) differs from the stacking velocity derived from normal-moveout measurements of common-depth-point (CDP) gathers. Ideally, we would want to know the velocity at each and every point in the volume of earth in which we are interested. The velocity is needed especially in regions where significant and intensive lateral or vertical differences in velocity occur. Migration requires an accurate knowledge of vertical and horizontal seismic velocity variations. Because velocity depends on the types of rocks through which a wave travels, a complete knowledge of the velocity *v*(*x,y,z*) is essentially equivalent to a description of the geologic structure *g*(*x*,*y*,*z*), as far as that can be obtained by conventional seismic methods. However, as we have stated above, the velocity function is required to get the geologic structure. In other words, to get the answer *g*(*x*,*y*,*z*), we must know the answer *v*(*x,y,z*).