Near-surface characterization and delineation of water preferential flow-pathways at South Deep Gold Mine, South Africa

Abstract. Velocity models of the shallow subsurface (a few hundred meters) are important in near-surface characterization, improving seismic mapping resolution at depth, and constraining deeper geological models. It is therefore interesting to retrieve them from deep seismic exploration data. We compute the near-surface shear wave velocity model in the vicinity of South Deep Gold Mine, using surface waves present in the small-offset 2D and 3D seismic reflection data acquired between 2022 and 2023 at the mine for research, mine planning, and development purposes. The obtained near-surface model is then used to (1) characterize the near-surface, and (2) better constrain the interpretation of possible water preferential flow-pathways (faults, fracture zones, and dykes) mapped at mining levels, that enable the migration of water from overlying aquifer systems (< 0.5 km depth) to the mining levels (∼ 3 km depth). The analysis is carried-out on reflection seismic data acquired for deep mineral exploration, where the acquisition parameters were not optimized for surface wave techniques and the reciprocity principle is used to improve the data density, coverage, and near-surface mapping resolution. The lithostructural information retrieved from the produced pseudo-2D and 3D shear wave velocity models is consistent with information obtained from available surface borehole data and published records in the study area. To investigate the structural linkage between the deep mining levels and shallow groundwater aquifers, we integrated the near-surface shear wave velocity model produced from the small-offset 2D and 3D reflection seismic data with the large-offset 2003 3D reflection seismic data, and geological structures derived from underground mapping, and exploration drilling. The shear wave velocity models help define the faults, fractures, and dykes that compartmentalize the near-surface groundwater aquifer systems. The large-offset legacy 2003 3D seismic data, underground mapping, and exploration drilling provide a better definition of the orebody and its offsets (e.g., faults) at the mining level. The integrated data show that several geological structures (e.g., faults and dykes), defined by legacy seismic data, underground drilling, and mapping, cross-cut the mining levels at ∼ 3 km depth and intersect the near-surface aquifers, thus making these structures possible preferential flow-pathways for water migration to the deep mining levels. The results of the interpretation illustrate the advantages of integrating shallow and deep subsurface information to constrain the timing of geological events and mitigate the risks associated with water ingress to the mining levels. The final model produced can be used for future mine development, improving safety and production, and for the extension of the Life of Mine (LoM).