Hydrogeological properties of fractured rocks (granites, metasediments and volcanites) under the humid tropical climate of West Africa

Abstract

This study aims to propose a vertical structuring of water production zones for three types of fractured rocks encountered in Ivory Coast, West Africa. In a first step, the methodology consists of the characterization of the weathering profiles based on: i) bedrocks and weathering layers observations at outcrop; ii) interpretation and synthesis of geophysical data and lithologs from different boreholes. In a second step, the evolution with depth of flow rate (air-lift discharge rates) as well as the frequency and the density of water production zones during drilling are statistically analyzed. Then, the distributions of these various properties versus the depth are fitted to probability laws. For each of the geological formations (granites, metasediments and volcanites) the related weathering profile comprises, from top to bottom, four separate layers: alloterite, isalterite, fissured layer and fractured fresh basement; these weathering profiles are systematically covered by a soil layer. In granites, the maximal values of flow, frequency and density of the water production zones (WPZ) are situated around 40 m depth systematically within the fractured fresh granite layer. In metasediments and volcanites, the maximal values of flow, as well as the maximal frequency and density of WPZ are identified at two distinct depths. The first WPZ, around 40 m depth, is associated to the fissured layer for both profiles; the second WPZ, around 80 m depth is associated to the fractured fresh sandstone layer for the weathering profile in metasediments and to the fractured fresh metabasalt layer, for the weathering profile in volcanites.