Teleseismic tomography uses relative arrival time residuals from distant earthquakes to image wave speed variations in the upper mantle beneath a seismic array.
The inverse problem is solved using a subspace inversion method.
The forward and inverse steps can be used iteratively in order to address the non-linear nature of the tomographic inverse problem.

Spherical coordinates are used to account for the curvature of the Earth. Travel times from the distant source to the edge of the 3D model are computed using a global reference model.