Application of the cross-hole radio imaging method in detecting geological anomalies, MacLennan township, Sudbury Ontario

Abstract

The occurrence of conductive sulphide in an otherwise highly-resistive host rock is the ideal situation for exploring using high-frequency electromagnetic methods. The FARA radio imaging (RIM) system was deployed to explore the rock properties between two boreholes MAC104 and MAC100G, which are about 182 m apart, on the Nickel Rim South property (MacLennan Township) 22 km northeast of Sudbury. Tomographic data were collected and processed at 625 kHz and 1250 kHz. One data set has the transmitter in MAC100G and the receiver in MAC104; the other “reciprocal” data set has the transmitter in MAC104 and the receiver in MAC100G. The amplitude data were reduced, edited, and processed to generate tomograms employing the SIRT algorithm. Separate tomograms were created for the reciprocal data sets in the ImageWin software. A sensitivity analysis was conducted to assess the influence that perturbations in the ImageWin processing parameters have on the resulting tomograms. The sensitivity study of the tomograms along with the information obtained from the value of fit analysis can be used to select appropriate processing parameters. Finally, the two reciprocal sets of conductivity values were averaged and imported into Geosoft to create a final tomogram for the panel. The resistivity values of the studied zone obtained from the FARA modeling package agree fairly well with the conductivity data set generated by the ImageWin modeling package when compared using the Geosoft and GOCAD visualization software. Differences between the two tomograms are attributed to the different solver methods employed by FARA and ImageWin and the statistical analysis used for averaging the attenuation value over ray paths. Furthermore, it is shown that the tomographic results are consistent with the location of conductive zones that were identified using down-hole geophysical logging. The main focus of the project is to understand how the radio imaging (RIM) data is processed with the ImageWin software to construct an attenuation tomogram. This research showed that both tomograms created by ImageWin and FARA illustrate the same pattern with two conductive zones at the same depth; however, the values of conductivity are slightly different. The FARA resistivity values obtained for the upper zone is a factor of two lower than the resistivity calculated by ImageWin. The resistivity values obtained for the lower zone using the FARA processing is a factor of eight lower than the resistivity calculated by ImageWin. Also, there is a slight discrepancy in the orientation of the upper and lower zones on the two tomographic images generated using the two processing packages. In the tomograms generated by FARA software both upper and lower zones are continuous linear zones from one hole to the other with dips from MAC104 towards MAC100G, whereas in the tomograms created by ImageWin the upper and lower zones are less linear and do not have obvious dips.