Why 1D electrical resistivity techniques can result in inaccurate siting of boreholes in hard rock aquifers and why electrical resistivity tomography must be preferred: the example of Benin, West Africa
Introduction
Groundwater in hard rocks is the main water source for many human communities in Africa (Calow et al., 2010, Mukherji, 2008). Forty percent of the continent's surface area is constituted by metamorphic and plutonic hard rocks (MacDonald et al., 2012). Yields of boreholes drilled in hard rock aquifers are usually low, i.e. few hundreds to few thousands liters per hour (Gnamba et al., 2014, Louan et al., 2015, MacDonald et al., 2012, Vouillamoz et al., 2015a). Boreholes, which produce less than 700 l/h, (i.e. the minimum usually required for supplying a hand pump), are considered as negative. The resulting “failure rate” of boreholes commonly ranges in-between 10 and 50% (Wright and Burgess, 1992): in Benin and Burkina Faso for example, recent studies indicate that 30–40% of the thousands of boreholes drilled in hard rocks are negatives and then abandoned (Courtois et al., 2010, Vouillamoz et al., 2014). Such high rates of unsuccessful borehole drillings cause substantial financial losses, slowdown drilling campaigns and access to drinking water for the population.
The borehole failure in hard rock aquifers of western Africa is usually high although the boreholes siting is quite automatically systematically based on a comprehensive procedure which includes the systematic use of electrical profiling and sounding (i.e. the so-called “1D” techniques, Darboux-Afouda and Louis, 1989, Dutta et al., 2006, Keller and Frischknecht, 1966). Although some published studies use two dimensional (2D) electrical tomography in West Africa, operational geophysical surveys are still based on 1D techniques. Thus this study aims at (1) reconsidering the efficiency of 1D electrical resistivity techniques (i.e. electrical profiling and electrical sounding) currently used by geophysicists (practitioners), and (2) assessing whether the 2D electrical resistivity technique (i.e. Electrical Resistivity Tomography, ERT) can improve the borehole sitting success rate. First, the usefulness of the geophysical parameter (i.e. the electrical resistivity) is checked by assessing its efficiency to differentiate the hydrogeological compartments of hard rock aquifers (i.e. saprolite, stratiform fractured layer, subvertical fractured zone and unweathered rock). Then, numerical modeling of typical hydrogeological targets is conducted. Finally, field cases of borehole siting in Benin is presented to check the modeling results.
Section snippets
Hydrogeological targets and common geophysical practices
Various terminologies are used to describe the hydrogeological conceptual model in hard rock areas (e.g. Acworth, 1987, Comte et al., 2012, Koita et al., 2013). In this study, the terminology and conceptual model is taken from Lachassagne et al. (2014). The saprolite, i.e. the top layer of unconsolidated weathered rocks, is located above the stratiform fractured layer (Fig. 1). The stratiform fractured layer refers to the weathered layer that is fractured from the chemical action of the
Material and methods
This study combines several methods including the drilling of 20 boreholes at 7 sites (6 experimental sites and 1 test site), the implementation of comprehensive electrical resistivity surveys including 1D and 2D techniques, the analyses of a database of 2122 boreholes, 400 geophysical surveys, computations and analyses of 104 geophysical numerical models.
Detailed descriptions of the electrical methods are not in the scope of this paper, but can be found in numerous publications (e.g. Kunetz
Efficiency of electrical resistivity to differentiate hydrogeological compartments
The results of the boreholes drilled at F68 experimental site and the EL are presented in Fig. 6. Three hydrogeological compartments can be identified: (1) the saprolite extends from ground surface to about 30 m depth (where the driller switches from rotary to the down-the-hole hammer, D-T-H), (2) the stratiform fractured layer starts at about 30 m deep and ends at about 50 m depth (where there are no more weathering indices in the cuttings) and finally, (3) the unweathered rock is identified
The use of EP and ES
The common practice in Benin and West Africa is to use 1D techniques (i.e. EP and ES) to characterize 2D geological structures. However, this approach has serious limitations. EP are mainly interpreted based on the analysis of the resistivity contrast between a conductive anomaly and its surroundings. Several authors also analyzed the shape and the width of the anomalies (e.g. CIEH, 1984, Dieng et al., 2004) but modeling demonstrates that false targets as clayey zones (i.e. “C” and “D”
Conclusion
As an aid to borehole siting, the common practice in Benin, in West Africa and most countries in Africa, is to use 1D resistivity techniques (i.e. Electrical Profiling -EP- and Electrical Sounding -ES-) to characterize 2D geological structures in hard rock aquifers. Because the failure rate of borehole sited with the use of EP and ES is frequently high, (close to 40% in Benin) this study aims at estimating the interest and limitations of both 1D resistivity techniques, and 2D Electrical
Acknowledgements
This work was conducted within the framework of the GRIBA project (Groundwater Resources In Basement rocks of Africa) funded by the African Union, the European Union, and the Institut de Recherche pour le Développement (grant AURG/098/2012). The content of this paper is the sole responsibility of the authors and can under no circumstances be regarded as reflecting the position of the European Union or the African Union. This work is also funded by the support programme to capacity building of
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