Integrated geophysical methods for near-surface site characterization in South Africa

Onyebueke, Emmanuel Onyebuchi
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The aim of this research is to integrate reflection seismics with other geophysical methods (such as first-arrival traveltime refraction tomography (RT) and electrical resistivity tomography (ERT), multichannel analysis of surface waves (MASW), horizontal-to-vertical spectral ratio (HVSR) and magnetic field methods) for site investigation. The results will assist the environmental planners, infrastructural engineers, geologists and hydrologists to plan and conduct comprehensive site characterization investigation in the study area. Where possible, the geophysical datasets were constrained by borehole and surface geological map information. This research work was carried out by way of four different case studies, which are discussed in this thesis. Chapter 2 is a brief review of seismic theory and its applications, and basic principles of electrical resistivity and magnetic methods. Chapter 3 presents the integration of high-resolution reflection seismics with RT, ERT and MASW for hydrogeological investigation in the Nylsvley Nature Reserve, South Africa. The integrated data were used to image near-surface geology. The interface between the unsaturated sand and saturated sand-gravel was interpreted as the water table, while the undulating bedrock erosional surface with associated weathered/fractured system was interpreted to play a role with respect to the underground water movement and storage in the survey area. In Chapter 4, a similar geophysical approach was adopted. High-resolution reflection seismic, RT and ERT were combined to investigate the historic shallow narrow-reef and bulk open-pit mining at the Lancaster gold mine, Krugersdorp, South Africa. The reflection seismic data produced images that were used to delineate stopes and the geological boundaries, while the refraction and resistivity tomograms, on the other hand, generated detailed images of the top 20-30 m of the subsurface. Together they were used to map the shallow geometry of the fluid migration path, mined-out areas, weak zones and boundaries between bedrock and overburden. Chapter 5 presents the integration of legacy 2D reflection seismic and magnetic data acquired within the Transvaal Basin to investigate and characterize the subsurface architecture, and insights into hydrogeological prospecting and engineering applications were emphasized. The study area is dominated by agricultural activities, biosphere reserves, mining sites and residential houses. The major Rustenburg Fault (RF) was mapped, as well with other several secondary faults, heterogenous fracture systems, slumps and palaeo-karst features. The fault and fractured structures were interpreted as brittle shear-type deformation. The interpreted fault structures were shown to provide a flow path for fluid migration, while fractured zones were regarded as potential fluid storage regions. Moreover, the intrusive structures were observed to provide several paleo-compartments which could increase groundwater storage within the confined zones. Chapter 6 focuses on the stability and reliability of the microtremor signal characteristics (predominant frequency and H/V spectral ratio) for site effect assessment of the seismological stations spread across South Africa using the HVSR technique. The results show that the fundamental natural frequency of the near-surface low-velocity-layer (LVL) was essentially the same in all the analyzed time windows, irrespective of the time-of-day or season, while the amplification amplitude varied slightly with the recording duration. The estimated predominant frequency was deemed to be reliable and stable as a representation of the natural fundamental resonance frequency of the ground motion at the site location. Furthermore, the estimated values were used to approximate the thickness of the LVL and the vulnerability index for liquefaction potential at the stations that satisfied the criteria for a clear H/V peak. In conclusion, the research methodologies and results in this thesis provide a set of case studies of the use of seismic methods integrated with an appropriate geophysical method, to investigate the near-surface as a part of a comprehensive site characterization in South Africa
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, 2020