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| R Number: R6232 |
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| Contractor: British Geological Survey | |
| Dates: 1 April 1995 to 31 August 1997 | |
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To improve well and borehole siting and hence alleviate water scarcity. To assess and develop a new, non-invasive surface geophysical technique (EKS) claimed to map the permeability of sub surface rocks and predict the presence and depth of groundwater. |
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| Executive Summary |
| Objectives |
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To increase the success rate of well and borehole siting in diverse hydrogeological environments (all major aquifer types), by developing and assessing EKS methodology. In particular, to assess to what extent EKS is able to map the permeability of the subsurface and predict the presence and depth of groundwater.
1) An assessment and refinement of existing EKS equipment and field practice. 2) An EKS user-manual. 3) Case-studies demonstrating the suitability of EKS in all main overseas aquifer types. Quite early in the project numerous problems were discovered with the existing equipment and methodology and hence, following the Project Review of May 1996, it was agreed that Output 2 (the user manual) would be replaced by a scientific overview report of EKS, to include an indication of the way forward. |
| Methodology |
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The project was started using the first commercially available EKS instrument and early improvements were made to data processing, by writing software to facilitate the filtering of high frequency noise, the graphical overlay of individual EK traces and data stacking. It was felt that the existing interpretation routine (supposedly yielding absolute permeabilities and borehole cumulative flow) was too ambitious in view of the large range of variables involved and the complex nature of the inducing seismic pulse. The interpretation was constrained to a qualitative indication of likely permeable horizons based solely on the rise-time of the EK signal. Experimentation in the UK was followed by an extensive field programme in Zimbabwe and a lesser programme in Egypt. Further field trials in the UK (involving array rotation, electrode watering, a range of electrode materials etc) led to the routine collection of improved data. The project also experimented with a range of dipole lengths and dipole separations and incorporated seismic observations in 'moveout' experiments. Latterly such experiments have been facilitated through the use of a sophisticated high-gain 8-channel instrument (TEKA), developed in-house largely independently of this project. Limited progress was made in forward numerical modelling (assuming simple subsurface geometries) to predict the form and amplitude of EK response given different aquifers and their associated parameters (fluid conductivity, porosity, permeability, tortuosity etc). |
| Results |
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A large amplitude EK signal was frequently observed from very early times even where the water table is deep (+/-40m) and in such cases the signal must be generated in the vadose zone. This is in accord with recent theory that predicts that EK coupling will occur across any streaming potential coefficient boundary that is disturbed by a seismic pulse. It follows that the technique cannot unequivocally detect the water table. More disturbing are the results of the moveout experiments. All the EK events demonstrate spatial moveout which indicates that these events are generated by horizontally propagating seismic waves. This finding is endorsed by the joint acoustic/electric moveout observations, in which both sets of results demonstrate moveouts with similar apparent velocity and similar frequency. Measurement of the permeability/depth profile (ie EK sounding) clearly requires EK signal generation at successively deeper boundaries by a vertically propagating seismic pulse and this would give rise to simultaneous arrivals at surface dipoles. Numerical modelling indicates that the amplitude of EK signals generated by a vertically propagating seismic pulse will attenuate rapidly with depth and will, even in the best case (shallow source) be an order of magnitude lower than those signals induced by horizontally propagating disturbances. It is also clear that such signals will be largely overwhelmed by the much stronger responses resulting from horizontal seismic propagation. |
| Conclusions |
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It was concluded that some important assumptions made by the suppliers of the EKS instrument are flawed. A vertical sounding capability has not been established. The routine use of the 2-channel equipment and the modus operandi established in the manufacturer's manual is likely to:
In the context of increasing the success rate of well siting it is necessary to be pragmatic. The results obtained in the study suggest that the current state of EKS is sufficiently research based to rule out a simple application. |
| Further Information |
| List of Publications |
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EK measurements in various hydrogeological environments of Zimbabwe. BGS Technical Report WC/97/31 (Beamish, Peart, Davies). Well and borehole siting by EKS and associated experimental observations in Bikita District, southern Zimbabwe. BGS Technical Report WC/97/02 (Peart, Beamish, Matthew) EK measuremnts in various hydrogeological environments of Egypt. BGS Technical Report WC/97/32 (Peart, Beamish, Davies). Electrokinetic geophysics: a review. Terra Nova 10, 48-55. 1998. (Beamish, Peart). |
| Follow-up Activities |
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Dr Beamish presented a summary of our findings entitled "EKS: more waves than water" at Geoscience 98 (Keele University). The BGS Regional Geophysics Group continues to investigate the EKS phenomenon. Numerous copies of the earlier reports have been distributed, both in the UK and overseas. |
| Contact Details for Further Information |
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R.J. Peart Regional Geophysics Group British Geological Survey Keyworth Notts NG12 5GG
Tel: +44 (0) 115 9363256 |