Geophysical Geometry of Fracture Zones in the Basement Rocks of the Donga of Benin

Abstract

The Donga Department is located in the northwest of Benin in an area made up of crystalline and crystallophyllic basement rocks where most of the groundwater resources are found in the area of weathered and conductive fractures. The carrying out of drilling campaigns in this department are often crowned with a significant number of negative boreholes (<0.7 m3/h) due to the poor choice of sites for drilling. The objective of this study is to use geophysical methods to identify and characterize the fractured basement areas, with a view to improving the implantations and the sustainable management of the aquifers they contain. The Electric Resistivity Tomography method was used to characterize the geometry of the fracture zones and the thickness of the weathered zone. The results showed that in the department of Donga, the thickness of the weathered zone is between 5 and 35 m and the width of the fracture zones varies from 15 to 40 m with a vertical dip and a contrast >100. The determination of the fracture zones granulometry with T2 * values in 5 different localities of the study area are between 150 and 212, 5 ms. It emerges from this study that in the department of Donga, few fracture zones are identified by the ERT below thick weathering layers (>20 m) and that the particle size. T2 * of the fractured zone is also a function of geology with medium-grained gneiss and coarse-grained quartzites.