SUMMARY

The potential for water infiltration is a geotechnical hazard in underground mining environments. The electrical conductivity contrast between dry and wet salt make it possible to explore for water infiltrated areas in underground salt mines using electrical resistivity imaging. We present a case history on the application of 3D ERI in an underground potash mine in Saskatchewan, Canada to delineate a water inflow to guide mitigation efforts.

INTRODUCTION

We describe the application of 3D electrical resistivity imaging (ERI) in an underground potash mine located in Saskatchewan, Canada. The active mine level is located 1 kilometer below the surface. The sources of water are aquifer layers overlaying the salt formation. The goals of the geophysical survey are to delineate regions of wet salt for exploration and monitoring purposes, and to detect void spaces. The Middle Devonian Prairie Evaporite is a sedimentary formation underlying most of southern Saskatchewan, Canada and extends as far west as Alberta. The evaporite consists mostly of halite, interbedded shales and anhydrite, and contains economic amounts of potash in its upper portion. Near the town of Esterhazy in Southeastern Saskatchewan, the Prairie Evaporite is roughly 200 meters thick with the economic ore zone situated at a depth of roughly 964 meters below the surface.

The ore is mined using a long room and pillar technique with underground access provided by two shafts near the town of Esterhazy.

The Prairie Evaporite is directly overlain by a sequence of water-bearing shale, limestone and dolomite formations. The first of these layers directly above the evaporite is a dolomitic shale referred to as the Second Red Bed. The economic potash zone is being extracted at a level 20 meters from the top of the Prairie Evaporite. This layer of salt provides an impermeable barrier separating the mine workings from the overlying aquifers. Although the geology of the evaporite is generally fairly uniform laterally, changes in the depositional environment can result in small local irregularities. Salt can also be removed at the base of the Prairie Evaporite, resulting in slumping and viscous flow which can create large structural deformations(Annan et al., 1988).

Both of these can lead to the presence of aquifers or structurally weak zones in the path of mining operations and can creates the potential for water entering the mine level. As ore is extracted, the overburden stress is redistributed and the salt undergoes a slow plastic deformation referred to as “creep”. This deformation can lead to fracturing of the evaporite which can provide pathways for water in the overlying aquifers into the mine workings. Of interest to mine personnel is the thickness and condition of the salt layer between the mining level and the Second Red Bed, the location of cracks, voids and irregularities in the evaporite formation. Seismic refraction (Gendzwill, 1969) and GPR (Annan et al., 1988) techniques have been previously used to estimate the salt thickness and condition of the evaporite. However, the ever present acoustic noise generated by mining equipment makes seismic methods difficult unless the mine is in a shutdown period.

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