# Translations:Seismic tomography - book/8/en

What is the third step in tomography? The third step consists of a nonlinear *iterative-improvement method*. In each iteration, the set of picked traveltimes obtained from the seismic data is matched to the computed traveltimes obtained from the traveltime equations. The traveltime equations require a velocity or slowness model. This velocity model starts with an initial best guess. For each iteration, the error (for example, the mean squared error) between the two sets of traveltimes is used to update the previous velocity model. That is, in each iteration, the slowness vector is adjusted so that the traveltimes given by the model agree more closely with the picked traveltimes. The iteration terminates when the agreement is deemed satisfactory. The result is the desired final velocity model.