Seismic tomography uses digital seismographic records to image the interior of the Earth.
The basic scheme is to first localize and characterize a set of significant earthquakes. These earthquakes are then considered to "illuminate" the interior of the earth with seismic waves.
The time that the waves arrive at seismic stations can then be used to calculate the waves' speed through the Earth. By combining analyses from many earthquakes, in different places around the Earth, a three dimensional map of wave speed through the Earth can be constructed.
Seismic tomography can be roughly categorized into traveltime tomography and full-waveform tomography. Traveltime tomography uses the high-frequency contents of the seismic data while the full-waveform tomography uses the whole seismic data including amplitudes and phases. Seismic tomography is an inverse problem, in which the seismic data are given and the goal is to determine the attributes of the medium such as wave speed and medium density. Seismic tomography is a nonlinear inverse problem where the traveltime tomography is much less nonlinear than the full-waveform tomography.
