As in x-ray tomograms done in a hospital, tomograms of the earth can be constructed by passing rays of seismic energy between a source and a receiver.
Rays can be direct or bounce off reflectors.
Unlike x-rays, in the earth seismic rays bend and diffract significantly. Each ray produces a seismogram. We arrange seismograms from adjacent rays as strips in an image, with time going down instead of around the drum, and the amplitude of the stylus swing coded as a color. Basic seismic tomography uses the picked travel times of coherent phases to interpret the rock property of seismic velocity. Velocity distributions can be related to geologic environments.