Base metal applications of the radio imaging method. Current status and case studies

Abstract

The Radio Imaging Method (RIM-MET) is a geophysical technique which uses medium frequency (50?520 kHz) radio waves to evaluate subsurface geology. The method is well established in the Australian coal mining industry and has recently been applied to base metal deposits in the Mount Isa area. RIM relies on propagating radio waves in a transmission mode through rock, either from borehole to borehole, borehole to mine, or drive to drive. The resulting EM wave signal will attenuate as a direct response to variation in the conductivity of the intervening rock between transmitter and receiver. Multiple ray paths are measured through the area of interest by moving the transmitter relative to the receiver. This is repeated for a number of receiver locations until sufficient data are collected for tomographic image processing. Tomographic imaging procedures, developed by the CSIRO Division of Radiophysics, are employed to produce an image of the distribution of ore within the survey section. The production of a tomographic image is an iterative process, incorporating inverse modelling and hypothesis testing. This paper describes the results of in-mine and borehole surveys through Pb/Zn orebodies, carried out for M.I.M. Exploration Pty Ltd in and around Hilton Mine. The surveys at Mount Isa have demonstrated the role the method can play in late-stage ore definition and mine exploration. Determination of ore 'shape' in section and correlation of ore zones between boreholes is possible using the Radio Imaging Method. At this stage the technique does not appear to be applicable to green-field exploration but has the most potential as a mine-planning tool. The advantage of RIM over other EM geophysical techniques is its ability to look between boreholes. The method has responded to low-grade Zn-dominated ore intersections in addition to high-grade Pb ore. Radio-wave penetration varies according to the conductivity of the host, but useful tomographic images using 50-kHz antennas were possible at 64-m borehole separation in the country rocks encountered at Mount Isa. The Radio Imaging Method has demonstrated that it can be used to improve geological interpretation between boreholes and may reduce the amount of drilling required for mine planning.