2D reflection seismic surveys to delineate manganese mineralisation beneath the thick Kalahari and Karoo cover in the Griqualand West Basin, South Africa

Abstract

The Kalahari Manganese Field (KMF) in the Northern Cape Province of South Africa hosts some of the world’s richest manganese deposits, largely concealed beneath thick Cretaceous to Cenozoic Kalahari Group and Karoo Supergroup sediments. To improve imaging of the concealed Transvaal Supergroup strata, a high-resolution 2D reflection seismic survey was conducted in November 2023 across the Severn farm area. The survey comprised five profiles totalling 18.9 km, acquired using 5 Hz 1C geophones connected to wireless nodes, enabling effective burial beneath loose aeolian sand for improved coupling. A compact 500 kg drop hammer, mounted on a Bobcat, served as the seismic source, offering excellent manoeuvrability across challenging sandy terrain. Shot spacing was 10 m, with four vertical stacks per shot to enhance signal-to-noise ratio. Refraction tomography using first-break travel times provided near-surface P-wave velocity models, revealing variable Kalahari sediment thicknesses ranging from 20 to 70 m and bedrock velocities of ∼ 5500 m s−1 associated with Karoo Supergroup strata. Despite the challenges posed by the thick sand cover, lithified calcrete horizons within the Kalahari sediments significantly aided seismic energy propagation. The data were processed using a conventional pre-stack imaging workflow. We tested both Kirchhoff pre-stack time migration (KPreSTM) and Kirchhoff pre-stack depth migration (KPreSDM) and compared the results. Both migration approaches revealed a high degree of similarity in reflector geometries and structural patterns, suggesting minimal lateral velocity variation across the study area. KPreSTM results were then used in the final seismic interpretation. Pre-stack time migrated sections exhibit nine laterally continuous high-amplitude reflectors between 0.05 and 3.42 km depth, corresponding to major stratigraphic boundaries from the Kalahari Group down to the Ghaap Group. Of particular interest is the moderate-amplitude reflection pair at 1.05–1.35 km depth, interpreted as the Hotazel Formation, the primary host for manganese mineralization. This study demonstrates that, when appropriately designed, reflection seismic imaging can be a powerful tool for delineating deep mineralized strata beneath thick sedimentary cover in arid environments.