High-resolution Integrated Geophysical Investigation at the Lancaster Gold Mine, Krugersdorp, South Africa

An integrated geophysical approach using seismics and geoelectrical techniques was employed to investigate the architecture of historic narrow-reef workings and a proposed open-pit mine at Lancaster Gold Mine, near Krugersdorp, South Africa. The mining activities in the area were mainly carried out within the Kimberley Reef Package in the upper Central Rand Group of the Witwatersrand Supergroup, which hosts several gold-bearing conglomerates (locally known as reefs). The reefs are generally thin (≤ 2 m thick) and dip between 28° and 32° south. The low-velocity weathered layer introduces significant static shifts in the reflection seismic data. Moreover, environmental noise from drilling and trucking, and the prominent bedrock-overburden contact that produces various wave conversions (P-S conversion), caused undesirable noise that contaminates the shot gathers as high-amplitude, source-generated and monochromatic noise. The noise was removed from the shot gathers using frequency and velocity filtering techniques. The final depth-migrated sections are characterised by high-resolution images of the subsurface from ~10 to ~150 m depth, which are constrained by the borehole information. The reflection seismic data delineate the interfaces between different rock layers and the stopes. The refraction and resistivity tomograms, on the other hand, provide more detailed images of the top 20–50 m of the subsurface and depict the approximate shallow geometry of fluid migration paths, mined-out areas, and bedrock-overburden boundaries. The integrated results indicate that the study area is characterised by a weathered surface layer with variable low P-wave velocity (400–1200 m/s) and resistivity (150–800 Ωm). The deeper layer reveals an increase in resistivity and velocity, and it’s characterized by discontinuities, weak zones, cavities or water-bearing zones due to the mining activities. The combined borehole and geophysical data provide valuable information regarding the physical characteristics of the subsurface and can be helpful for future risk management decisions, environmental and engineering studies in the area.