Accessing Potash Mine Geohazards using Electromagnetics

Abstract:

Potash mining in Saskatchewan has made extensive use of geophysics over the years for safety and operations purposes. One serious issue potash mines have faced over the years is in-flows. In-flows have affected 12 out of the 13 conventional potash mines in Saskatchewan at one time or another. Of these mines, several have been forced to close and another has been converted into a solution mine [Prugger, CIM Distinguished Lectures, 2018]. There are three known geohazards that cause in-flows: salt-collapses, thin-backs, and connate brines [Gendzwill and Stead, Canadian Geotechnical Journal, 29, 666 – 674, 1992]. Of the three, the one that has been the most elusive to geophysical detection and forms the greatest concern is the thin-back geohazard. Thin-backs occur when the rock forming the roof of the mine is thinner or weaker than typical, and there is a anomalously porous carbonate in the vicinity. This geohazard has several features that make it challenging to detect and rectify. One is that the in-flow might not occur until months or years later. Another is that the geohazard is primarily a lithological property change (non-porous to porous) and not a 3D mound or collapse easily spotted in GPR or seismic scans. Finally, the high-volume and subsaturated nature of the brine in these geohazards is of considerable concern to mining operations when compared with the low-volume and saturated connate brine in salt.

This project sought to develop time-domain electromagnetics (TEM) in the role of detection and quantification of thin-back geohazards. There were two principal components to this project. One was computer modelling of time-domain electromagnetics responses in a full-space or mine environment performed in COMSOL Multiphysics. The second was an in-mine time-domain electromagnetics survey conducted in an area of a suspected geohazard at the Nutrien Ldt. mine at Rocanville, Saskatchewan. Potash mines in Saskatchewan are at a depth of around 1000 m. As such, geophysical surveying is conducted typically within the mine itself. This survey data was kindly collected and provided to this research project by Nutrien Ldt as part of a Mitacs Accelerate internship with the author.

The survey data showed a large and repeatable conductive response beginning around 0.1 ms for both the inline horizontal (radial) and the vertical fields when moving from “normal” conditions to the “abnormal” area. This distinctive feature was seen on both lines that were profiled. Two inversions performed with different parameter arraignments and weighting (one 11-layer and the other 2-layer) returned anomalously low resistivity values for the carbonates above the mine in the “abnormal” zone – indicating much higher porosity. These results suggest that the survey was a success at detecting a thin-back geohazard. More broadly, the project shows that TEM can be used effectively in conjunction with other geophysical tools to greatly assist with the detection, delineation, and quantification of anomalous geohazards near potash mines.