1. A computer-implemented method for transforming electromagnetic survey data acquired from a subsurface region to a subsurface resistivity model indicative of hydrocarbon accumulations or lack thereof, said method comprising: (a) obtaining controlled-source electromagnetic or magnetotelluric data from a survey of the subsurface region at a plurality of non-zero frequencies; (b) obtaining geologic or geophysical information about the region, and extracting structure information about the subsurface region from it; (c) assuming an initial resistivity model of the subsurface region; and (d) starting with the initial resistivity model, numerically inverting the electromagnetic survey data, said inversion involving iterative minimization of an objective function and updating of the resistivity model, said objective function including a term measuring mismatch between model synthesized data and measured survey data, and another term being a diffusive regularization term that smoothes the resistivity model based at least partly on the structure information, thereby generating an improved, anisotropic resistivity model of the subsurface region.