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In areas where the geological structures are approximately two-dimensional (2D), conventional 2D electrical imaging surveys have been successfully used. The main limitation of such surveys is probably the assumption of a 2D structure. In areas with complex structures, there is no substitute for a fully 3D survey. This program is designed to invert data collected with a rectangular grid of electrodes. The arrays supported include the pole-pole, pole-dipole, inline dipole-dipole, equatorial dipole-dipole and Wenner-Schlumberger. The RES3DINV program uses the smoothness-constrained 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. A Pentium 4 (or compatible CPU) based microcomputer with at least 512 megabytes RAM and an 80 gigabyte hard-disk is recommended. It supports parallel calculations that significantly reduces the inversion time. On a modern Pentium 4 based microcomputer, the data inversion takes less than a minute for small surveys with 100 electrodes in a flat area, to a day for extremely large surveys with 6000 electrodes in rugged terrain. Topographic effects can be modelled by using a distorted finite-element grid such that the surface of the grid matches the topography. The program will automatically choose the optimum inversion parameters for a particular data set. However, the parameters which affects the inversion process can be modified by the user.

On a modern multi-core Windows-based microcomputer, the data inversion takes from less than a minute for small surveys with 100 electrodes in a flat area to several hours for extremely large surveys with 6000 electrodes in rugged terrain. The inversion of a data set with 198 electrode positions (BLOCKS_22x9-ws.dat example data file) just takes about 17 seconds on a PC with a hex-core i7 CPU. Two different variations of the smoothness constrained least-squares method are provided; one optimised for areas where the subsurface resistivity varies in a smooth manner (as in many hydogeological problems), and another optimised for areas with sharp boundaries (such as massive ore bodies). A robust data inversion option is also available to reduce the effect of noisy data points. To handle very large data sets, the program also supports the incomplete Gauss-Newton optimisation method. When used together with a data compression technique, it enables the inversion of very large data sets with over 20000 data points and model cells. As an example, a data set with nearly 65000 data points and 32000 model cells was inverted on a 2Ghz P4 computer in slightly less than 2 days. On a more modern 2.4 GHz Core 2 Duo computer, this takes less than 1 day.