Botha, J. F.Fourie, Francois Daniel2015-08-142015-08-142003-11http://hdl.handle.net/11660/869English: 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.Afrikaans: Die moontlikheid om oppervlak elektroseismiese (ES) metodes te gebruik vir grondwatereksplorasie in gefraktureerde Karoo-gesteentes word bestudeer deur die kriteria van vertikale en laterale resolusie van oppervlak ES data en die dunlaagresponsie te ondersoek. Die ES Fresnel-zones vir monochromatiese opwekking is groter as hul seismiese ekwivalente en die laterale resolusie van oppervlak ES data is gevolglik swakker. Seismiese snelheidstoenames met diepte lei tot groter Fresnel-zones en swakker laterale resolusie. Soos in seismies kan slegs 'n enkele Fresnel-zone vir breëbandopwekking geïdentifiseer word. Bronne met hoër dominante frekwensies en breër bandwydte het hoër laterale resolusie tot gevolg. Rayleigh se kriterion vir vertikale resolusie toegepas op ES data vereis dat die ingebedde laag a dikte van ten minste λ/2 moet hê, waar λ die golflengte van die seismiese golf in ondersoek is, om as oplosbaar beskou te word. Daar is egter twee golflengtes om in ag te neem - die van die Biot vinnige en stadige drukgolwe. Aangesien die golflengte van die stadige drukgolf in versadigde Karoo-gesteentes 'n paar ordegroottes kleiner as die golflengte van die vinnige drukgolf mag wees, word die teoretiese perke van resolusie bepaal deur die stadige drukgolf. Hierdie golf is egter sterk kwistend en die praktiese perke van resolusie blyk sterker afhanklik te wees van die golflengte van die vinnige drukgolf. 'n Vereenvoudigde benadering om die ES dunlaagresponsie te ondersoek dui aan dat ingebedde lae met diktes van kleiner as λ/2, waar λ die golflengte van die vinnige drukgolf is, as elektroseismies dun geklassifiseer mag word. Ondersoeke deur middel van 'n volle golfvorm-benadering wat gelyktydig die invloede van die verskillende golftipes in ag neem, ondersteun bostaande observasie. Die resultate van 'n ES veldopname op 'n terrein waar 'n bekende gelokaliseerde fraktuur voorkom, ondersteun die idee, maar bewys nie onweerlegbaar nie, dat die vertikale en lateral resolusie van oppervlak ES data onvoldoende is om frakture met openinge in die millimeter- tot sentimeterbestek waar te neem. Die invloed van porositeitskontraste op ES refleksie word ondersoek deur die verandering in die grootte van die refleksiekoeffisiënte te bestudeer wanneer die porositeit (en alle porositeitsafhanklike parameters, soos permeabiliteit) van 'n Karoo-sandsteen, oorlê deur 'n Karoo-moddersteen, verander word. Toenames in die sandsteenporositeit lei tot groter refleksiekoeffisiënte, en gevolglik sterker refleksie. Die moontlikheid bestaan dus dat porositeitskontraste in 'n spesifieke geologiese omgewing gekarteer kan word en dat zones wat meer waarskynlik geskik is as akwifere geïdentifiseer kan word. Die moontlikheid om ES-tegnieke te gebruik as 'n nie-indringende metode om informasie oor die elastisiteitseienskappe van die gesteente-matriks te bekom word ondersoek deur die elektroseismiese en magnetoseismiese oordragsfunksies by posisies in boorgate te bestudeer. Die oordragsfunksies is afhanklik van verskeie fisiese en chemiese parameters, insluitend die elastisiteitsparameters, en geeneen van hierdie parameters kan eenduidig uit die evaluasie van die oordragsfunksies bepaal word nie. Die skeurmodulus van die poreuse raamwerk kan uit metings van die skeurgolfsnelheid in 'n vloeistofversadigde poreuse sisteem beraam word. Aangesien daar 'n bykans linieêre verband tussen die skeur- en die bulkmoduli van die poreuse raamwerk bestaan, kan die bulkmodulus geskat word sodra die skeurmodulus bekend is. Porositeitsveranderinge as gevolg van akwifeerdeformasie mag lei tot waarneembare veranderinge in die oordragsfunksies. Die elektroseismiese oordragsfunksie van die vinnige drukgolf is onsensitief vir porositeitsveranderinge in gekonsolideerde materiaal. Alhoewel die elektroseismiese oordragsfunksie van die stadige drukgolf baie sensitief is vir porositeitsveranderinge, is hierdie golf baie kwistend en berug om moeilikheid te wees om te meet. Die magnetoseismiese oordragsfunksie is sensitief vir porositeitsveranderinge en sal waarskynlik die mees bruikbare inligting oor akwifeerdeformasie lewer.enThesis (Ph.D. (Geohydrology))--University of the Free State, 2003Groundwater -- South AfricaAquifers -- South AfricaSeismology -- InstrumentationHydrogeologyThesisUniversity of the Free State