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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. The arrays supported include the pole-pole, pole-dipole, inline dipole-dipole, equatorial dipole-dipole and Wenner-Schlumberger and non-conventional arrays.
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. 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. A Intel multi-core (or compatible CPU) based microcomputer with at least 2 GB RAM and an 500 gigabyte hard-disk is recommended. It supports parallel calculations that significantly reduces the inversion time. On a modern microcomputer, the data inversion takes 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. 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, a data compression technique is used. It enables the inversion of very large data sets with over 50000 data points and model cells. As an example, RES3DINVx64 took about 3.6 hours to carry out 5 iterations for the inversion of a data set from an area with significant topography with 147,355 data points, 9396 electrode positions, 64,400 model cells and over 670,000 nodes on a PC with a 3.5GHz Hex-Core Intel 3930 CPU with 64GB RAM. An example of the results obtained from an electrical imaging survey in an area with a very complex subsurface geology is shown in Figure 1. This survey was carried out at Lernacken in Southern Sweden over a closed sludge deposit using the pole-pole array (Dahlin, T. and Bernstone, C., 1997. A roll-along technique for 3D resistivity data acquisition with multi-electrode arrays, Procs. SAGEEP’97 , vol 2, 927-935.). A survey grid of 21 by 17 electrodes with a 5 metres spacing between adjacent electrodes 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. Figure 2 shows a 3D plot of the inversion model using the Slicer/Dicer plotting program.
Figure 3 shows an interesting example of a 3D resistivity and IP survey provided by Arctan Services Pty. and Golden Cross Resources, Australia. Copper Hill is the oldest copper mine in NSW, Australia. Copper porphyry with minor gold and palladium mineralization were found to occur in structurally controlled fractures and quartz veins. However, due to the very complex geology, large differences in ore grades were found in drill-holes that were less than 200 meters apart.
To map the ore deposit more accurately, a new 3D resistivity and IP survey using the pole-dipole array was used. The survey covered a large (1.6 x 1.1km) area using a series of 1.6 km lines with a spacing of 25m between adjacent electrodes. The entire survey took 10 days giving a total of over 7000 measurements (Denne, R., Collin, S., Brown, P., Hee, R. and White, R.M.S. 2001. A new survey design for 3D IP inversion modelling at Copper hill. ASEG 15th Geophysical Conference and Exhibition, August 2001, Brisbane). A copy of the paper describing the survey and results can be downloaded from the www.arctan.com.au web site. Other interesting information about the mineralization at the Copper Hill area is available at the Golden Cross Resources Ltd. www.reflections.com.au/GoldenCross/ web site.
The data was inverted with the RES3DINV program that produced a 3D resistivity as well as a 3D IP model for the area. The 3D IP model below shows the location of the mineralized zones more clearly. The inversion model output from the RES3DINV program was rearranged into a VRML format that could be read by a 3D visualization program (please contact Arctan Services for the details) that enables the user to display the model from any direction.
The above model shows two en-echelon north-south trends and two approximately east-west trends forming an annular zone of high chargeability. The results from existing drill-holes that had targeted the shallower part of the western zone agree well with the resistivity and IP model. A drill-hole, CHRC58, intersected a 217m zone with 1.7 g/t gold and 0.72% copper coincided well an IP zone of greater than 35mV/V. The lower boundary of the western zone with high chargeability coincides well with low assay results from existing drill-holes. The eastern zone with high chargeability and resistivity values do not outcrop on the surface and very little drilling has penetrated it. Further surveys, including drilling, is presently being carried out.