Interpretation of RIM surveys in underground potash mines

Abstract

RIM surveys in underground potash mines in Saskatchewan, Canada, have been undertaken for several years with generally positive results. The methodology is similar to surveys carried out in underground coal mines in the U.S. and Australia, where TEM-mode magnetic-field amplitudes are measured and subsequently processed to produce absorption-coefficient images of the seam being exploited. The primary differences between the coal and potash situations are the great transmission range possible in potash, which exceeds 3000 m, and the nature of the target anomaly, which for potash is always the anomalous presence of brine. The RIM method responds to zones of anomalous conductivity in a manner that does not well match the commonly-applied ray assumption. There is a difference between how two rays respond to the same anomaly where one of the rays has both end points outside the anomaly and the other has either the beginning or end of the ray within the anomaly. In each case grossly different measurements are made of essentially similar volumes of earth which, if unrecognised, can lead to serious interpretational pitfalls. These effects have been recognised in field data and turned to advantage: if both ray "modes" traverse the same anomaly the thickness and conductivity of the anomalous material can be derived by a curve-matching method.