Res3Dinv - Resistivity Software
In areas with complex structures, there is no substitute for a fully 3D survey. The RES3DINV pro- gram is designed to invert data collected with a rectangular grid of electrodes (Loke and Barker 1996). The arrays supported include the pole-pole, pole-dipole, inline dipole-dipole, equatorial dipole-dipole and Wenner- Schlumberger and non-conventional arrays.
The RES3DINV program uses the smoothness constrained least-squares inversion technique to produce a 3D model of the subsurface from the apparent resistivity data. A Pentium 3 or Pentium 4 based microcomputer with at least 512 megabytes RAM and a 40 gigabytes hard-disk is recom- mended. It supports parallel calculations on Pentium 3 and 4 based computers 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.
The program will automatically choose the optimum inversion parameters for a data set. Howev- er, the parameters which affects the inversion process can be modified by the user. Three differ- ent 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 for areas where the subsurface resistivity varies in a smooth manner, and another optimised for are- as with sharp boundaries. Topographic effects can be incorporated into the model by using a distorted finite-element grid such that the surface of the grid matches the topography (Sasaki 1994).
The results from a 3D resistivity and IP survey over a copper and gold porphyry prospect in Southeast Asia is in Figure 2 displayed by a 3D Virtual Reality Modelling program to show se- lected regions of the model. The survey area covers a 5000 by 5000 meters grid. The main targets are regions with high IP values over 20 and 30 mV/V . All images courtesy of Geotomo Software sdn. Bhd.
An example of the results obtained from an electrical imaging survey in an area with com- plex geology is shown in Figure 1. This survey was carried out at Lernacken in Southern Swe- den over a closed sludge deposit using the pole -pole array (Dahlin et al. 2002). A fairly large survey grid of 21 by 17 electrodes was used. The former sludge ponds containing highly con- taminated ground water show up as low resis- tivity zones in the top two layers. This was con- firmed by chemical analysis of samples. The low resistivity areas in the bottom layer are due to saline water from a nearby sea.