Soro D. D., Koita M., Biaou C. A., Outoumbe E., Vouillamoz Jean-Michel, Yacouba H., Guerin R. (2017). Geophysical demonstration of the absence of correlation between lineaments and hydrogeologically usefull fractures : case study of the Sanon hard rock aquifer (central northern Burkina Faso). Journal of African Earth Sciences, 129, p. 842-852. ISSN 1464-343X.
Titre du document
Geophysical demonstration of the absence of correlation between lineaments and hydrogeologically usefull fractures : case study of the Sanon hard rock aquifer (central northern Burkina Faso)
Soro D. D., Koita M., Biaou C. A., Outoumbe E., Vouillamoz Jean-Michel, Yacouba H., Guerin R.
Source
Journal of African Earth Sciences, 2017,
129, p. 842-852 ISSN 1464-343X
The conceptualization of hard rock aquifers in terms of their geometry and structure has undergone considerable progress over the last two decades. Despite these advances, hydrogeologists are still divided by the models used to describe two central concepts: (i) the influence of weathering processes on hydraulic conductivity; (ii) the influence of tectonics on the hydraulic conductivity of hard rock aquifers. In order to provide further insight into this debate, the present study proposes a conceptual model for hard rock aquifers, based on an integrated hydrogeological and geophysical approach, using information acquired at different scales. The data and observations used for this case study were derived from the Sanon experimental site, located in Burkina Faso, which is presently exposed to a Sudano-Sahelian climate. The methodological approach consisted firstly in developing a description of the site's weathering profile at the scale of a borehole, based on lithologs and electrical resistivity logs. In a second step, the site's ridge to ridge (longitudinal) weathering profile was established from several 2D resistivity sections crossing a maximum number of lineament structures, which in some prior studies were considered to be the superficial manifestation of tectonic fractures. The results show that at that scale the weathering profile is comprised of three main layers, which from top to bottom are referred to as: the saprolite, the fissured layer and the fresh rock. This weathering profile model is consistent with other models proposed in recent years, suggesting that the hydraulic conductivity of hard rock aquifers is a consequence of weathering processes, rather than tectonic fracturing. Tectonic fractures are not visible on the 2D sections of the ridge to ridge profiles, and the lineaments originally thought to be overground representations of tectonic fractures are likely to have different origins. The lack of a substantial correlation between tectonic lineaments and fractures appears to account for the high incidence of negative boreholes in hard rock aquifers, where the siting of drillings has systematically been based on lineament studies and on geophysical studies looking for vertical fractures such as profiling and vertical electrical sounding. There is thus a need to revise current hydrogeological concepts and methodologies to site wells based on tectonic fractures represented by lineaments.
Plan de classement
Hydrologie [062]
;
Géologie et formations superficielles [064]
