In areas where the geological structures are approximately two-dimensional (2D), conventional 2D electrical imaging surveys have been successfully used. In areas with complex structures, there is no substitute for a fully 3D survey. The arrays supported include the pole-pole, pole-dipole, inline dipole-dipole, equatorial dipole-dipole and Wenner-Schlumberger and non-conventional arrays. The RES3DINVx32/x64 programs use the smoothness-constrained Gauss-Newton least-squares inversion technique to produce a 3D model of the subsurface from the apparent resistivity data alone. Like RES2DINV, it is completely automatic and the user does not even have to supply a starting model. It supports parallel calculations that significantly reduces the inversion time. Topographic effects can be modeled by using a distorted finite-element grid such that the surface of the grid matches the topography.