Weighted tomographic imaging of radio frequency data

Abstract

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 l03and 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.