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.
Diffraction tomography can locate seismically reflective structures in the crust.
Linearized, least-squares approaches to this have been very successful in the earth's deep interior. However, the crust is much more heterogeneous. Monte-Carlo optimization Linearized inversions often return false results in crustal tomography for velocity modeling from time data. As a matrix inverse problem, seismic tomography exhibits singularity and ill-conditioning. However, the forward-projection problem for times from velocity models is now deterministic and very fast on common computers.