The groundwater prospection requires the use of tools which allow the different aquifers characteristics related to be highlighted. Among these tools, geophysics is the most solicited because it allows to highlight them through the analysis of the physical parameters of the recorded geological formations. However, a prior knowledge of certain geological and structural indices is essential not only for the choice and implementation of the geophysical method, but also for the validation of the ranges of the measured properties, in order to realise an accurate interpretation. To this end, a geoelectrical investigation combined with satellite remote sensing was conducted in the Messondo area, which has a ferralitic gneissic basement (Claude, 2014). The approach of this study consists to measure the apparent resistivity and chargeability in areas with high fracture density observed from spatial images, by a geoelectric survey. The investigation methods used include remote sensing, electrical resistivity (ER) and induced polarisation (IP).
Although most IP works are applied to mineral exploration (Allis, 1990 ; Izawa et al., 1990 ; Locke et al., 1999 ; Moreira et al., 2012), this method can be used in parallel with DC for some hydrogeological investigations (Slater et al., 2007), as the contrasts highlighted by one method in some cases are not highlighted by the other. In general, in mineral exploration, IP effects can be very strong at the surface of conductive metallic mineral grains (Keller and Frishknecht, 1970) whereas the non-polarizability of waterwill induce low IP values.
This paper presents the results of the combined application of remote sensing, DC and IP methods to the study of aquifers in the Messondo locality. The inverted and interpolated data using Res2Dinv and Oasis Montaj software, provided 2D and 3D models allowing a very broad understanding of the investigated structures. Location, geology and tectonics of the study area
The area covered by this study is located in southern Cameroon, between 402250 m and 403350 m North latitude, 653600 m and 655120 m East longitude in UTM zone 32N projection. It is located in the Edéa - Eséka region, which lithologically (Fig. 1) straddles the Paleoproterozoic (1800 to 2400 Ma) Nyong complex (NyC), Oubanguides chain (OC) and sedimentary (SC) age (Feybesse et al., 1998 ; Maurizot et al., 1986 ; Nedelec et al., 1993 ; Pénaye et al., 2004). The NyC consists of orthogneisses, charnockites, mylonites, amphibolites, metasyenites and metagranodiorites. The OC consists of paragneiss, micaschists, chloritoschists and quartzites. The SC consists of conglomerates, coarse sandstones, siltstones, clays, limestones, iron sands and enriched marls. Tectonically, the NyC was affected by a polyphase deformation marked by NE-SW oriented shear zones (Feybesse et al., 1998 ; Maurizot et al., 1986 ; Nedelec et al., 1993 ; Pénaye et al., 2004 ; Lerouge et al., 2006 ; Owona et al., 2011).
The local geological reconnaissance in Messondo reveals the lithological dominance of:
-
Gneisses, forming the main substratum of the area, they are rich in leucocratic minerals (quartz and feldspar), and contains some ochre to rust coloured iron oxide flakes (Fig. 2.a and c); their alteration yields rusty hematite (Fig. 2.d).
-
Amphibolites : found on the hillsides, they are greenish to be light green (Fig. 2.e). They are rusty brown and they are mainly composed of ferromagnesian minerals. They weather to isalerites (Fig. 2.f) found on the lower hills. The constituent minerals are mainly kaolinite, gibbsite, and iron oxide.
Structurally, the gneiss outcrops encountered show significant fractures and foliation (Fig. 2.a and b) all oriented NNE-SSW, SW-NE E-W and N-S0. Figures (3) shows structural features such as :
-
Dextrous moving folds, showing symmetrical flanks ;
-
A subduction zone between two veins driven by opposing forces ;
-
Fractures and foliation ;
Hydrographically, the Edéa - Eséka region straddles the catchment areas of the Sanaga, Nyong, Dibamba rivers, Lake Ossa and Lake Mévia. This hydrographic network is dendritic and flows in the N-S, NE-SW and NNE-SSW directions.