INTRODUCTION
1.1 General
The 1996 constitution of South Africa considers the availability of drinking water a basic
human right. Considerable emphasis is therefore currently placed on the identification of
sustainable water supplies. Groundwater is the sole source of potable water for many
urban and rural communities in South Africa and efficient methods of exploration,
development and management of groundwater resources are required.
The vast majority of South African aquifers are of the secondary type, that is, their waterbearing
properties were developed through secondary processes such as faulting or
fracturing. It is estimated that approximately 90% of South African groundwater occurs in
secondary aquifers. More than 50% of South Africa is underlain by rocks of the Karoo
Supergroup. These rocks are typically dense and not extensively fractured so that their
permeabilities are low when compared with primary porous formations.
The existing geophysical techniques, particularly magnetic, electromagnetic and
resistivity methods, have been used for many years to locate groundwater in South Africa,
but with varying degrees of success. The magnetic method is used almost exclusively to
locate and delineate intrusive magmatic bodies and the contact zones between the
intrusive bodies and the host rock are then considered to be targets in groundwater
exploration. The electromagnetic and resistivity methods are used to map the subsurface
conductivity distribution of the area under investigation. From the conductivity
information inferences regarding the subsurface geology are made and geological
structures are mapped.
Although highly weathered zones containing groundwater will in some instances produce
large enough conductivity contrasts to allow direct detection by means of the
electromagnetic or resistivity method, groundwater exploration by geophysical means is
in general an indirect process whereby subsurface geological structures that may act as, or
be associated with aquifers are identified.
In the past groundwater exploration in Karoo rocks mostly targeted intrusive dolerite
bodies. When such bodies do not occur in the area being explored, the magnetic method
is of little use for groundwater exploration. The electromagnetic and resistivity methods
on the other hand are also frequently unsuccessful in locating Karoo aquifers due to a lack
of conductivity contrast between the fractured zones and the surrounding host rock.
5
One method that promises to circumvent many of the difficulties associated with
conventional geophysical techniques in siting water in Karoo aquifers, is the
electroseismic (ES) technique. This method, which is based on the conversion of seismic
energy to electromagnetic energy in deformable fluid-filled porous rocks, does not only
depend on the rock properties but also the fluid. Indeed there are indications that the
method may hold particular advantages for Karoo formations, where bedding-parallel
fractures often form the main conduits of water to boreholes Botha et al. (1998).
The rock properties that play a particular important role in ES techniques include the bulk
and shear moduli of deformation. The same parameters also determine the extent to
which rocks will deform under the influence of stresses, with a corresponding reduction
in the apertures of fractures present in the rock (Makurat, 2001) and the ability of the
fracture to transmit water (Botha and Cloot, 2002). Indeed, Botha et al. (1998) show that
a 20% decrease in the aperture of the fracture will cause a drop of almost 50% in the yield
of a fracture and that the over-pumping of a borehole could cause the fracture to collapse.
ES techniques may therefore not only be useful in siting boreholes, but also provide
information on the elastic properties of the rock matrix that is vital for the management of
boreholes in fractured rock formations.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-08252004-121644 |
| Date | 29 September 2005 |
| Creators | Fourie, Daniel Francois |
| Contributors | PROF J F BOTHA |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-08252004-121644/restricted/ |
| Rights | mixed, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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