With an increasing worldwide demand for critical raw materials, among which Rare Earth Elements (REE), mine-waste deposits in Sweden are considered as secondary sources of critical minerals. These so-called mine tailings are abundant, and located close to the surface, facilitating their excavation. To assess the economic potential of these tailings as a source of critical minerals, their volume must be estimated. The horizontal-to-vertical spectral ratio (HVSR) method, which estimates the fundamental frequency of sedimentary sites, is particularly suited for the purpose of mapping the depth-to-bedrock. We apply this method to three-component ambient-noise recordings that were recently collected over the Bäckegruvan mine tailings in the Bergslagen province, central Sweden, to delineate their depth.
Additionally, we employ 3C beamforming to characterize the ambient-noise wavefield, with the aim of supporting HVSR curve interpretation. However, the results of the 3C beamforming prove unreliable, which we attribute primarily to the array's suboptimal geometry - specifically, its sparse station spacing and highly anisotropic wavenumber resolution.
We estimate the thickness of tailings at each station by combining the fundamental site frequency with a constant S-wave velocity. We then interpolate these individual measurements to construct a three-dimensional model of the tailings-bedrock interface, from which we derive the total volume of the tailings deposit. To assess the reliability of our model, we compare the interpolated depths with findings from a recent investigation in the same region, which utilized geoelectrics and electromagnetic geophysical methods. Comparing the results, we find strong concordance between the two approaches.
Our case study highlights the HVSR method’s potential as a cost-effective, fast, and robust approach for obtaining a preliminary estimate of mine-tailings depth.