A method of forming a geological model of a region of the earth includes obtaining seismic data relating to the region, the seismic data including seismic traveltime uncertainty. A seismic velocity model of the region may also be provided and includes velocity uncertainty. Ray tracing may be performed on the seismic data using the velocity model to determine the three dimensional positions of a plurality of points of the region. The three dimensional positional uncertainties (7-12) of at least some of the points can be calculated from the traveltime uncertainty, the velocity uncertainty, and uncertainty in the ray propagation direction. This can be combined with the positions determined by the ray tracing to form a geological model.