1. A method for the perturbation of a background subsurface model using time preserving tomography, the method comprising: in a processor: storing values for an initial set of parameters representing an initial background subsurface model, wherein the initial model corresponds to at least a plurality of ray pairs, each ray pair comprising a traveltime; generating an altered model by altering values of the individual parameter in the set and, based on linear relationships between perturbations for the individual parameter in the set and perturbations for one or more other parameters in the set, altering values for the one or more other parameters in the set associated with an equal and opposite change in the traveltimes of each of the ray pairs in the initial model, wherein the total traveltime of each of the ray pairs in the altered model is identical to the total traveltime of each of the ray pairs in the initial model.