Rayleigh-wave focal spot imaging of the polymetallic Kylylahti mine area in the Outokumpu mineral district, Finland

Abstract. The green energy transition requires the development of new seismic imaging approaches to locate and characterize deposits of critical raw materials efficiently and cost-effectively. Here, we apply the Rayleigh-wave focal spot imaging approach to dense array data collected in the Kylylahti polymetallic mine area in the Outokumpu mineral district, Finland, and demonstrate the efficiency of the method in a hardrock environment. We cross-correlate one month of ambient-noise records from the 994 vertical-component stations deployed over an area of ∼10.5 km² following standard signal processing operations. The anatomy of the correlation wavefield includes spurious arrivals associated with the local mining operations and with a mine located to the SW of the array. From the narrow-band zero lag amplitude distribution of the cross-correlations we get the Rayleigh-wave focal spots which we parameterize with a Bessel-function model to estimate the local phase velocity at each station. The employed model accounts for the anisotropic background illumination and is constrained using focal spot data in the short 1λ distance range. In the 3 − 6 Hz frequency range we obtain phase velocity maps that correlate well with the Kylylahti formation, which is resolved with lower phase velocity values compared to the host rock. The lower phase velocities are associated with abundant soapstone, while higher phase velocities are indicative of the surrounding mica schist. The similarity of the phase-velocity features to the structure resolved by an independently obtained P-wave velocity model, together with the agreement between the inferred Kylylahti formation boundaries and the contours of the geological model, suggest that the focal spot method is a valuable, low-cost passive imaging tool for modern mineral exploration that can complement established approaches.