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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 3 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.

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Résumé Ce travail a pour objectif de proposer une méthodologie de cartographie des accidents régionaux, basée sur une analyse combinée d'images radar et optique, afin de déterminer leur rôle dans les écoulements souterrains. Cette méthodologie est mise en oeuvre en zone de socle, dans la région de Dimbokro-Bongouanou (Côte d'Ivoire). Dans un premier temps, cette démarche méthodologique consiste à spatialiser les linéaments régionaux susceptibles d'être en relation avec la fracturation du socle à partir de l'analyse combinée d'images radar (Radarsat) et optique (Landsat), puis à valider ces linéaments aux moyens de données géologiques. Dans un second temps, une analyse permettant d'identifier le rôle de ces linéaments dans les circulations souterraines est proposée. Celle-ci consiste à étudier les corrélations entre la position des forages et le tracé des linéaments régionaux. Plus précisément, le débit des forages est analysé en fonction de leur distance au linéament le plus proche, ce qui permet de constater une augmentation du débit inversement proportionnelle à la distance entre forages ayant un débit supérieur à 5 m 3 /h à l'air lift (forages à fort débit) et linéaments majeurs. Aucune corrélation avec le débit des forages ayant un débit inférieur à 5 m 3 /h (forages à faible débit) n'est constatée. Cette corrélation négative, entre la productivité des forages à fort débit et la distance au linéament le plus proche, permet de confirmer que ces linéaments sont les expressions surfaciques d'accidents majeurs de la région et jouent un rôle primordial dans les écoulements souterrains.Mots clefs fracturation; images radar; Landsat; linéaments régionaux; productivité de forage; Dimbokro-Bongouanou, Côte d'IvoireMapping regional discontinuities and identification of their role in underground hydrodynamics in a base zone: case study of the Dimbokro-Bongouanou region, Ivory Coast Abstract This study aims to investigate a methodology to map regional discontinuities, based on a combined analysis of radar and optical images, to assess if they play a noticeable role as groundwater flow paths. This method is implemented in the region of Dimbokro-Bongouanou (Ivory Coast). In a first step, the methodological approach consists of spatial analysis of the regional lineaments likely to be related to the fracturing of the basement by means of combined analysis of radar (Radarsat) and optical (Landsat) images, and then validating these lineaments with the mean geological data. In the second step, analysis is proposed that enables the role of these lineaments in groundwater circulation to be identified. This consists of determining the correlation between the major lineament traces and the location of boreholes. The yield of boreholes is analysed as a function of their distance from the nearest lineament, which shows that borehole yield decreases proportionally with the distance between the lineament and high-yield boreholes (discharge greater than 5 m 3 /h). No correlation between distance to the lineament and lowyield boreholes ...