Electronic Theses and Dissertations (PhDs)

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Browsing Electronic Theses and Dissertations (PhDs) by Author "Manzi, Musa"

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    3D seismic constraints on the strato-structural evolution of the deep-water Orange Basin, South Africa
    (University of the Witwatersrand, Johannesburg, 2023) Maduna, Nombuso Gladys; Jinna, Zubair; Manzi, Musa
    This research utilizes seismic attributes and advanced machine learning methodologies to analyse high-resolution 3D reflection seismic data from the deep-water Orange Basin, located offshore western South Africa. The primary goal is to gain valuable insights into the basin's tectonic setting, depositional environment, and hydrocarbon potential. Significant features are delineated within the basin including (1) a gravitational collapse system in the Mesozoic Late Cretaceous, (2) mass flow features in the Cenozoic, (3) natural gas and fluid escape structures, (4) a large slope-perpendicular submarine canyon cutting Oligocene strata, and (5) multiple slope-parallel, sinusoidal channel features in the Miocene. The Late Cretaceous succession exhibits a gravitational collapse system with a translational and compressional domain detaching on seaward-dipping Turonian shales. Gravitational collapse during margin uplift formed fold-and-thrust belts along the slope characterizing the compressional domain. As they are commonly linked to hydrocarbons, the compressional domain of these systems has been extensively studied, while the translational domain has been poorly constrained due to its structural complexity. In this research, the translational domain is shown to contain a mixture of extensional tectonics (normal faults) up-dip and compressional tectonics (thrusts) down-dip, with extensive oblique-slip faults cutting thrusts perpendicularly during the translation of sediment. Variance and chaos, conventional seismic attributes, were used to manually pick and interpret the >500 regional-scale faults arising from the gravitational collapse system. Fault-net, a convolutional neural network (CNN), was compared with these edge-enhancing seismic attributes for extracting faults from the seismic volume. The CNN offers several notable advantages over conventional seismic attributes, such as automation, accelerated analysis, and improved time-efficiency on large datasets. Analysing the distribution, type, and geometry of faults within the basin gave valuable insights into the potential hydrocarbon system at work. Numerous natural gas and fluid escape features are identified in the seismic volume including an elongated mud volcano, pockmarked surfaces, and polygonal faults. The stability of the evolving margin is influenced by the underlying structure of a Late Cretaceous gravitational collapse system, also referred to as a deep-water fold and thrust belt (DWFTB) system. The fault framework within provides primary migration pathways for hydrocarbons. Major seafloor slumping occurs directly above a syncline of the Late Cretaceous DWFTB system. This slumping surrounds a structurally controlled, 4.2 km long elongated mud volcano situated between the translational and compressional domains of the underlying DWFTB system. The late Campanian has the largest accumulation of hydrocarbons evidenced by (1) an anticline with a positive high amplitude anomaly situated at the intersection of the two domains, and (2) >950 pockmarks preserved on the palaeo-surface compared to the 85 pockmarks observed on the seafloor. In addition to tectonics, the onset of stratified oceanographic circulation patterns and climate played a large role in changing depositional trends since the mid-Cenozoic. The Oligocene is characterized by a ~2.3 km wide, >13 km long, slope-perpendicular canyon formed at ~30 Ma during a major sea-level fall by a turbidity current. The Miocene is characterized by a ~14 km wide zone of slope-parallel, sinusoidal channels between water depths of 1 200–1 500 m. The formation and preservation of these features during the Miocene are attributed to the erosive interaction between two distinct water currents: (1) the Antarctic Intermediate Water flowing northwards, and (2) the deep North Atlantic Deep Water bottom currents flowing southwards; and the effects of the Benguela Upwelling System and a dry climate prevailing in southwest Africa all intensifying around 11 Ma. While pre-Miocene hydrocarbons originate from Turonian and Aptian source rocks, the origin of hydrocarbons on the seafloor is likely biogenic, arising from organic-rich sediment in the Miocene
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    Utilizing legacy seismics and non-seismic geophysical methods for deep mineral targeting and near-surface characterization: implications for mine development planning
    (University of the Witwatersrand, Johannesburg, 2023) Mutshafa, Ndamulelo; Manzi, Musa
    This thesis demonstrates through several case studies how the reprocessing of legacy reflection seismic data using advanced algorithms can be of value to mineral exploration and mine development, especially in hard-rock environments. The thesis also showcases how the integration of seismic and non-seismic geophysical datasets can assist in delineating near- surface geological structures (e.g., boulders and fractures) for mine planning and designs. Papers I and II demonstrate how the reprocessed legacy reflection seismic data have been used to delineate and image the world-class gold deposits such as the Black Reef (BLR) and Ventersdorp Contact Reef (VCR) of the Witwatersrand goldfields in South Africa. The two legacy reflection seismic profiles (Paper I) were acquired in 1988 for deep mineral exploration and mine planning over an area that is dominated by dolomitic outcrops that cause scattering of seismic energy at the near-surface, preventing energy propagation into the subsurface. Various migration approaches, namely, pre-stack time, pre-stack depth, and post-stack time migration were applied to test their capabilities in improving structural imaging. Reprocessing results from the pre-stack depth migration using the Kirchhoff algorithm provided the most improved subsurface images, especially the deeper targets due to its ability to honour complex lateral variations in the velocity field. In addition, Kirchhoff's pre-stack and post-stack time migration techniques improved the continuity of the near-surface reflections below the dolomitic rocks. Paper II presents the results from the recovered and processed 25.3 km long legacy seismic survey that was acquired in 1983 by the Gold Division of Anglo-American as part of the Witwatersrand goldfields exploration program. The reprocessing of the data improved the imaging of the gold-bearing horizon termed Ventersdorp Contact Reef (VCR), which is situated at depths between ~2400 and ~4100 m below the ground surface near the South Deep mine in Fochville, South Africa. The pre-stack time and phase-shift migration approaches were tested during processing, and both revealed a dipping reflection associated with the gold-bearing horizon and major steeply dipping faults that crosscut and displace the deposit. The interpretation of the results was constrained using borehole logs and surface geology. This is encouraging and motivates the use of legacy seismic data in the exploration of deep-seated targets. Papers III, IV and V present the results from the use of multi-geophysical methods (resistivity, magnetics, seismic, ground penetrating radar and multichannel analysis of surface waves) at Tharisa platinum mine to provide a comprehensive understanding of the subsurface geology by accurately delineating and locating boulders, mapping fractures and groundwater aquifers to improve Platinum Group Elements (PGEs) mining efficiency and reduce risks. The results from these integrated geophysical methods were successful in complementing each other in terms of providing a clear picture of the near-surface geological structures to help the mine plan better for future operations.