Potential Application of Nuclear Magnetic Resonance to Infer In Situ Degree of Saturation in Tailings

Reliable measurement of the degree of saturation ($S_r$) is useful for geotechnical characterization because accurately establishing a sufficiently low $S_r$ can obviate consideration of liquefaction risks. However, current technology to measure in situ $S_r$ is limited. Nuclear magnetic resonance (NMR) logging has been used for many years in the oil and gas industry and is recently being utilized more widely in the mining industry (Coates et al. 1999; Trofimczyk et al. 2018). NMR provides in situ measurement of volumetric water content (VWC) in real time during logging. This has application in the assessment of $S_r$ because through phase relationships, the following expression can be derived:where GWC = gravimetric water content; and $G_s$ = specific gravity of solids. Given that in situ NMR logging requires drilling works to provide a PVC-cased hole for subsequent instrument insertion, disturbed specimens at regular intervals are easy to obtain, enabling straightforward measurement of GWC and $G_s$. This may therefore be a promising avenue to provide another in situ measurement of $S_r$ in tailings.

A purpose-built chamber to enable specimens of reconstituted tailings to be tested with a field NMR tool was constructed and used in this study. The chamber has an inner diameter of 480 mm and height of 490 mm and was designed based on similar systems used for water calibrations of NMR tools. Test specimens consisting of copper tailings were reconstituted within the chamber at 15% and 20% GWC, such as might be produced at a filtered tailings deposit, and to a range of densities to obtain various $S_r$ values. NMR measurements were conducted with a BMR60 slimline logging tool (Orica Digital Solutions, Perth, Western Australia) with a diameter of investigation of 230 mm. NMR measurements were made allowing Eq. (1) to be used and where an independent measure of VWC and $S_r$ was available owing to the known volume and wet mass of each specimen.