Seismic tomography, travel-time residual কী?

The travel-time residual of an arrival is the observed travel time minus the time predicted by a reference model, $\delta t = T_{\text{obs}} - T_{\text{ref}}$, after correction for the source location and origin time, the station’s elevation and the Earth’s ellipticity. Seismic tomography is the estimation of the three-dimensional departures of the seismic speeds from a reference model, $\delta v(\mathbf x)$, from a large set of residuals (or of waveforms), by solving an inverse problem.