Radio-frequency tomography (RFT) normally is assumed to sense the geological structure in a plane between the transmitters and receivers, loosely termed the image plane. In practice, out-of-plane objects also affect the tomograms. We illustrate these effects on synthetic crosshole tomograms generated for a conductive sphere in a more resistive host. The sources and receivers are vertical magnetic dipoles. The tomographic image of a highly conductive sphere located wholly or partially between the sources and the receivers is a conductive feature the shape of which is governed by the conductivity of the host. As the sphere moves around one of the boreholes at a fixed distance, its expression on the tomogram is transformed from a conductive feature to a resistive feature. This reversal also occurs as the conductive sphere moves radially away from the transmitter. The period of these reversals can be related to the change in path length from source to sphere to receiver. Thus, if only amplitudes are recorded, an out-of-plane conductor could be misinterpreted as a resistive object in the plane; the two cases could probably be differentiated if phase data were also recorded. The influence of the sphere on the tomogram is also negligible at certain azimuths and radial separations determined by the host conductivity and source frequency. A good conductor close to a borehole could be missed by an RFT survey if it were located in such a null. The risk of this occurrence is greatly reduced if multifrequency data are collected. Also, the shape of the tomographic expression of the sphere is insensitive to azimuth and radial separation.

The radio imaging method is utilised in mines and oil fields to obtain detailed geological information between drill holes or mine roadways. When radio waves are transmitted through the ground at a fixed frequency, variations in absorption as the transmitter-receiver geometry changes are indicative of variations in conductivity of the geological section. With operating frequencies typically between 103 and 106 Hz, radio frequency electromagnetics (RFEM) is intermediate in range and resolution between low frequency electromagnetics used in exploration and high frequency ground penetrating radar (GPR).At many metalliferous mines, the ore is characterised by high conductivity contrasts and well-defined boundaries. Radio tomography between holes or mine roadways has a role to play in orebody delineation, but standard SIRT reconstruction produces unrealistic smooth images. In order to generate tomographic images with sharp boundaries, a weighted SIRT algorithm has been developed. A ‘clamping weight’ has been designed to fix the absorption coefficient at the low (host) value in regions that do not attenuate the radio signals, thereby localising high absorption into discrete zones. A ‘central weighting’ has also been introduced to concentrate high absorption towards the centre of the image in an attempt to compensate for the sensitivity of the acquisition system to variations in conductivity close to the receiver or transmitter.The weighted tomography has been tested on simulated cross-hole radio frequency data, as well as data collected from the Levack mine, Canada. Generally, the inclusion of weights in tomographic reconstructions has produced more realistic images of the geology. For the synthetic data sets, the resulting tomograms resembled the true model more closely than the standard SIRT images. For the Levack data set, the weighted tomography improved the definition of a mineralised lens.