ERI: A Geophysical Method for Evaluating Rock Porosity Behind Concrete Mine Shafts

CIM Vancouver 2016
Dr Michael (Max) Maxwell (Chief Geophysicist - Golder Associates), Mr David Cisyk (Mosaic Potash)
Electrical Resistivity Imaging (ERI) is probably the least well known geophysical method to evaluate rock characteristics. The potash mining industry has found that ERI measurements could be very effective in identifying water flow paths through evaporite sediments in an effort to manage water ingress. The method resolves differences in the electrical conductivity of rock units which is dependent on the degree of fluid saturation and in turn is a function of porosity. More recently the method has been used to gain a better understanding of rock porosity behind concrete shafts to aid in the design of grout injection and to evaluate the result of previous grouting efforts. This paper will discuss the methods used to evaluate the source of brine seeps into a 1940’s vintage mine shaft in New Mexico and the result of the first known attempt to produce a 3D ERI image behind the liner of a mine shaft at Esterhazy, Saskatchewan. In the first case, ERI data supported a decision to adopt a shallow injection grouting process as opposed to attempting a grout ring. In the second instance, ERI data provided insight into the effects of head cover grouting during the sinking of a potash shaft in the late 1960’s, experience which could be applied to a recent sinking program of a new potash mine at Esterhazy, Saskatchewan.
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