Etude hydrogéophysique de la zone de Meiganga (Adamaoua, Cameroun) par combinaison des méthodes géoelectriques (courant continu et de potentiel spontané) et imagerie satellitaire

Abstract

The aim of this study is to contribute to the identification, characterisation and map- ping of aquifers in the Méiganga area by combining different methods: hydrogeophysical, hydrogeological and remote sensing. To achieve this, a multidisciplinary methodology was adopted, and the main results can be summarised as follows: The lineament map obtained after processing satellite images (Landsat 7 ETM+, SRTM) contains lineaments of variable sizes. The validation of these different linear struc- tures was made on the basis of existing geological and photogeological maps and field knowledge. The analysis of the fracture networks revealed the following main directions: NNE-SSW, NE-SW and ENE-WSW. Overlaying the lineament map with the traditional borehole and well location map has help to identify these fractures as hydraulically active, and would constitute groundwater reservoirs or play a very important role in groundwater circulation. The results of the electrical geophysical (SEV, ERT) and hydro-geological investiga- tions were used to produce a weathering profile model. The latter shows three non-stratiform layers from top to bottom: the lateritic armour, the fissured horizon (water-bearing layers) and the healthy granite-gneissic rock. This model is in agreement with those proposed in recent years, suggesting that the hydraulic conductivity of basement aquifers is a consequence of weathering processes rather than tectonic fracturing. These results also allowed the aquifers identified to be classified into two main families: the alteration aquifers (superficial or free aquifers) with depths of 5.8-12.7m; the basement aquifers (confined or deep aquifers) with depths of 18-27m. In addition, a mathematical formulation was developed between hydraulic conductivity (Kr) and aquifer resistivity (ρ) on the one hand, and between transmissivity (Tr) and longitudinal conductance (C) on the other. A good correlation (R2=0.78) was obtained between Kr and ρ, while a medium correlation (R2=0.65) was obtained between Tr and C. The study of the spontaneous potential profiles revealed numerous rainwater infiltra- tion zones serving as groundwater recharge, which explains the intensification of fracturing in the lineament map. Finally, these results have enabled the simulation of the groundwater flow pattern in the study area, which revealed to be inward from the flanks to the central and southern parts of the studied area. The water recharge areas were located along the northwestern, northeast- ern and southwestern parts of the study area, meanwhile the discharge areas are visibly locat- ed in the central and southern parts of the study area and will serve as a groundwater collec- tion centre. Based on this information, a multi-criteria analysis based on the weighted linear combination influence factor approach was used to produce the groundwater potential map for the delineated aquifers. This map was validated using independent data and can be used as a pragmatic tool in the orientation, planning, protection, exploitation and sustainable manage- ment of the groundwater resources of the city of Méiganga.