The seismo-deformation of Karoo aquifers induced by the pumping of a borehole
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Date
2003-09
Authors
Dzanga, Panganai
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Publisher
University of the Free State
Abstract
The research unequivocally showed that pump-induced seismo-deformations exist, especially in boreholes equipped with electric and diesel pumps. Central to seismo-deformations are the mechano- and the geo-acoustic effects on both the borehole water and the aquifer. These aspects of the pump instigate the proposed diaphragmatic deformation hypothesis. In an aquifer the pump vibrations manifest as Rayleigh waves. By virtue of the driving source, the waves are harmonic (White, 1965). To be effective in engineering the proposed diaphragmatic deformation on a hydrostratigraphic unit situated several metres below the ground surface, the waves propagate along the borehole. The component of the Rayleigh wave orthogonal to the unit instigates the deformation and thus oscillations are generated in drawdown curves. Oscillations are a common feature in pumping test data and the masking effect of abstraction makes them inconspicuous. However, the derivative technique when applied to the signal enhances the phenomenon. The technique suppresses the abstraction effect in the data leaving the time-dependent oscillations. The spikes obtained in some of the derivatives of the pumping test data may suggest a one-sided deformation, probably the upper lip to a hydrostratigraphic unit. Coupled with the effects of Rayleigh waves is the pressure variation on the water level caused by the sound. The humming sound of a pump in operation is capable of subjecting borehole water to constant pressure variations thus leading to inherent oscillations in the pumping test data. The conspicuousness of oscillations in the raw pumping test data (Figure 1.2) can be attributed to the high resolution of the equipment used and interferences of different wave forms existing in the system. For a borehole that is continuously in operation, the impact of seismo-deformations on the physical integrity of an aquifer can be detrimental. In hard-rock fractured Karoo aquifers, the induced seismo-deformations can be of astronomical severity. The fact that successful boreholes are drilled in the Karoo within a short distance of a failed or "dried up" borehole is a clear evidence of structural failure. Bedding fractures in particular are very susceptible (Rasmussen, 1998). The radial extent of failure depends on the strength of the vibrations which are habitually intense close to the borehole and decay exponentially-like with distance. The ultimate failure of hydrostratigraphic units after prolonged pumping, which is common in the Karoo, may be due to the evolution history of deformation that accumulates as residual strains and stresses (Botha and Cloot, 2002). The research also revealed that ground displacements are smaller when discharging than when not. According to the hypothesis, this is due to the decreasing hydrostatic pressure essential in opposing the normal stress from a pump. It is in this view that groundwater should be considered as a hydroskelatal component of an aquifer and the rationale of lowering the discharge rate should not only be seen as a process to prevent dewatering but as a mechanism for preserving the dynamic equilibrium between the normal stress emanating from a pump and the hydrostatic force offered by water. Offsetting the equilibrium can lead to irreversible plastic deformation and aquifer subsidence (Botha and Cloot, 2002). The failure of numerical models to simulate the oscillations completely indicates that the wave that triggers the proposed diaphragmatic deformation of a fracture is more complicated than the simple harmonic waveform caused by pump vibrations. The wave should be a convolution of the vibrations, sound and the natural ground roll. Besides, the pump vibrations on reaching a geological feature of interest in a multi-stratigraphic geologic terrain would have undergone multiple reflections and transmissions with the ultimate waveform being a distortion of the primary signal. The research conducted showed that pump-induced seismo-deformations should have the same status as the discharge rate in aquifer management. It is clear that the introduction of measures aimed at reducing the transmission of seismic waves to an aquifer should be a priority in order to deter the pump-induced seismo-deformations. This includes preventing the pump and the borehole casing from coming into direct contact; incorporating gadgets to tone down the frequency of the mains; and installing shock absorbers between the pump and the ground. The selection of pumps should be based on the seismological properties of the aquifer that is to be used and its hydrogeological properties.
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Keywords
Thesis (Ph.D. (Geohydrology))--University of the Free State, 2003, Groundwater flow -- South Africa -- Karoo, Oscillations, Sound-waves, Aquifers, Geohydrology, Pumping stations