# Translations:Migration - book/11/en

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Let us now outline how 3D imaging is done. The 3D volume (*x*,*y*,*z*) represents the earth, where (*x*,*y*) represents the surface coordinates and *z* represents the depth coordinate. On the surface plane (*x*,*y*), the sources and receivers are arranged in a 2D grid. The (two-way) traveltime *t* is the duration of the passage of a primary reflection down from the source to a depth point (*x*,*y*,*z*) and then up to the receiver . A digital trace is a discrete sequence of amplitudes as a function of discrete traveltime. Thus, the amplitude represents the sum of all primary reflections with traveltime *t* originating from source and recorded at receiver .