Doctoral Degrees (Institute for Groundwater Studies (IGS))
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Browsing Doctoral Degrees (Institute for Groundwater Studies (IGS)) by Author "Fourie, Francois Daniel"
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Item Open 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.