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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 s 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 from E-SCAN (Li and Oldenburg 1992) type of surveys where the electrodes are arranged in a rectangular grid. The arrays which are most commonly used for 3D surveys are the pole-pole, pole-dipole and dipole-dipole. Other arrays probably do not provide sufficient horizontal coverage to fully map the area of interest.

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. It is completely automatic and the user does not even have to supply a starting model. This program has been optimised for the inversion of large data sets. The use of available memory is optimised so as to reduce the computer time by minimising disk swapping. A Pentium Pro or Pentium II based microcomputer with at least 64 megabytes RAM and a 3.2 gigabyte hard-disk is recommended. 

Depending on the size of the survey grid and the number of field measurements, as well as the speed of the computer system used, the inversion of a single 3D data set can take anywhere from a few minutes to more than 12 hours! Topographic corrections can also be carried out with this program by using a distorted finite-element grid such that the surface of the gird 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. Three different variations of the least-squares method are provided; a very fast quasi-Newton method, a slower but more accurate Gauss-Newton method, and a moderately fast hybrid technique which incorporates the advantages of the quasi-Newton and Gauss-Newton methods. Two different variations of the smoothness constrained least-squares method are provided; one optimised to reduce the difference between the calculated and measured apparent resistivity values, the other which guaranties models with smooth resistivity variations even with noisy data sets.

An example of the results obtained from an electrical imaging survey in an area with a complex subsurface geology is shown in Figure 1. This survey was carried out at Lernacken in Southern Sweden over a closed sludge deposit (Dahlin and Bernstone 1997). A resistivity survey using a grid of 21 by 17 electrodes with a 5 metres spacing between adjacent electrodes was carried out. The pole-pole array was used. The former sludge ponds containing highly contaminated ground water show up as low resistivity zones in the top two layers. This was confirmed by chemical analysis of samples. The low resistivity areas in the bottom two layers are due to saline water from a nearby sea.

As RES3DINV is a Windows based program, all graphics cards and printers with a Windows driver are automatically supported. It has been tested for graphics modes with a maximum resolution of 1600 by 1200 pixels and 256 colours.