Travel-time tomography

Summary

Travel-time tomography is seismic tomography that uses only the arrival times of seismic phases. The travel time of a wave is the integral along its ray of the slowness, the reciprocal of the wave speed. So the data are line integrals of the unknown, as in the Radon transform. By Fermat's principle, small changes of the ray path do not change the travel time to first order. So the problem can be linearised along the rays of a reference model. It is the oldest and most robust form of seismic tomography, used from local to global scales.