Doctoral Degrees (Institute for Groundwater Studies (IGS))

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Browsing Doctoral Degrees (Institute for Groundwater Studies (IGS)) by Subject "Aquifers -- South Africa"

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    ItemOpen Access
    Application of electroseismic techniques to geohydrological investigations in Karoo rocks
    (University of the Free State, 2003-11) Fourie, Francois Daniel; Botha, J. F.
    English: The possibility of using surface electroseismic (ES) methods for groundwater exploration in fractured Karoo rocks is studied by investigating the criteria of vertical and lateral resolution of surface ES data and the ES thin bed response. The ES Fresnel zones for monochromatic excitation are found to be larger than their seismic equivalents and the lateral resolution of surface ES data is consequently poorer. Seismic velocity increases with depth result in larger ES Fresnel zones and poorer lateral resolution. As in seismics, only a single Fresnel zone can be identified for broadband excitation. Higher dominant frequencies and broader bandwidth result in higher lateral resolution. Rayleigh's criterion for vertical resolution applied to ES data requires that the imbedded layer has a thickness of at least λ/2 to be deemed resolvable, where λ is the wavelength of the seismic wave under consideration. There are, however, two wavelengths to consider for ES phenomena − those of the Biot fast pressure and slow pressure waves. Since the wavelength of the slow pressure wave in saturated Karoo rocks may be a couple of orders of magnitude smaller than the wavelength of the fast pressure wave, the theoretical limit of resolution is determined by the slow pressure wave. This wave is, however, strongly dissipative and the practical limit of resolution seems to depend more strongly on the wavelength of the fast pressure wave. A simplified approach to examine the ES thin bed response suggests that imbedded layers with thicknesses smaller than λ/2, where λ is the wavelength of the fast pressure wave, may be classified as electroseismically thin. Investigations by means of a full waveform approach that simultaneously takes the influence of the different wave types into consideration, supports the above observation. The results of an ES field survey on a site where a localised fracture is known to occur, supports the idea, but does not conclusively proof, that the lateral and vertical resolution of surface ES data is insufficient to detect fractures with apertures in the millimetre to centimetre range. The influence of porosity contrasts on ES reflection is studied by examining the change in the magnitude of the reflection coefficients when varying the porosity (and all porositydependent parameters, such as permeability) of a Karoo sandstone overlain by a Karoo mudstone. Increases in the sandstone porosity results in larger reflection coefficients, and consequently, stronger reflection. The possibility therefore exists to map porosity contrasts in a certain geological environment and thus identify zones more likely to be suitable as aquifers. The possibility of using ES techniques as a non-invasive means of obtaining information on the elastic properties of the rock matrix is investigated by examining the electroseismic and magnetoseismic transfer functions at positions in boreholes. The transfer functions are dependent on various physical and chemical parameters, including the elastic parameters and none of these parameters can be determined unambiguously from the evaluation of the different transfer functions. The shear modulus of the porous frame may be estimated from measurements of the shear wave velocity in a fluidsaturated porous system. Since an approximate linear relation exists between the shear and bulk moduli of the porous frame, the bulk modulus may be estimated once the shear modulus is known. Porosity changes due to aquifer deformation may lead to detectable changes in the transfer functions. The electroseismic transfer function of the fast pressure wave is insensitive to porosity changes in consolidated material. Although the electroseismic transfer function of the slow pressure wave is very sensitive to porosity changes, this wave is strongly dissipative and is notoriously difficult to measure. The magnetoseismic transfer function is sensitive to porosity changes and is likely to yield the most useful information on aquifer deformation.
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    ItemOpen Access
    Risk based decision tool for managing and protecting groundwater resources
    (University of the Free State, 2001-11) Van der Voort, Ingrid; Van Tonder, G. J.
    English: The limited number of water resources in South Africa has resulted in increased emphasis being placed on groundwater. Groundwater supply of acceptable quality and quantity is a very important factor in the development of communities. The availability of water for various uses is directly related to the management of water quantity, quality and/or elimination of diseases. A risk can be defined as the probability that an adverse event will occur under specified circumstances. Effective decision-making involves the management of risks: the identification, evaluation, selection and implementation of actions to reduce risk. The aim of the research discussed in this thesis is to develop a decision tool to aid groundwater resource managers in the task of optimising the utilisation of groundwater. The decision tool will include: • Information concerning aquifer parameters: Pumping test analysis methods have been developed primarily to investigate and characterise flow within idealised confined radial flow systems. Unfortunately these assumptions are usually invalid with regard to the shallow fractured rock aquifers in South Africa. Notable attempts have been made to expand pumping test methodologies. A worthwhile method to consider when analysing a pumping test was developed by Barker (1988), where he generalised the Theis equation by including a term called the non-integer flow dimension, thereby making it applicable to arbitrary fractured confined aquifers. • Information concerning contaminant parameters: Dispersivity is a scaledependent property of an aquifer that determines the degree to which a dissolved constituent will spread in flowing groundwater. No in-depth investigation was conducted concerning this parameter, but as it plays an important role in the movement of contaminated groundwater, it is briefly discussed. Although matrix diffusion can influence groundwater contamination, very little research has been conducted in South Africa on this topic. The project therefore includes laboratory matrix diffusion experiments. The results of these experiments are included in the decision tool. • A framework for risk assessments: the project introduces tools based on fuzzy logic to assist in decision-making by systematically considering all possibilities. This tool takes into account the sustainability of a groundwater resource, the potential contamination of groundwater, human health risks and impacts of changes in groundwater (quantity and/or quality) on aquatic ecosystems. • Methods to make cost-effective decisions: Negative impacts can place heavy burdens on society and economics. Cost-benefit-risk assessments are therefore considered to define, compare and measure benefits and costs with regards to an impact. • Possibilities of remediation: Remediation forms an important component of many groundwater investigations and a few experiments were therefore conducted, the results, of which were included in the decision tool. The results provide the groundwater manager with an indication of the possible success of a remediation project.