1. A method for determining a discrete physical properties model of a subsurface region, which may be referred to as the model, by iteratively inverting measured geophysical data acquired from the subsurface region, comprising: using a Hessian matrix of an objective function, then times a vector, called “Hessian times vector,” to determine an update for the model, wherein the Hessian times vector is approximated, using a computer, with a single forward-wave simulated propagation and a single computation of gradient of the objective function, in a modified subsurface model.