Theory
Tomography is solved as an inverse problem. First measurements are made of seismic waves passing through a material. The character of these measurements is then analyzed to make inferences on the material such waves have passed through (velocity, density, etc.). The velocity of p and s waves depends on the rheology of the material that they travel through (density and elasticity). In short, variations in chemical compositions and thermal structure result in a change of velocity. Such waves can travel faster through relatively colder material. It is observed that wave velocity increases with increasing depth from 2–8 km/s in the crust up to 13 km/s in the mantle. Current research is based on this premise.
Read more about this topic: Seismic Tomography
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