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This study was carried out within the framework of the Gambia River Basin Development Organization Energy Project (OMVG), which brings together Gambia, Guinea, Guinea-Bissau and Senegal. The construction of a high voltage interconnection network fed by a hydro-electricity power station to supply power to OMVG member states must be done on stable ground. During the construction of the installation pits for the pylons in Labé Prefecture, cavities in the geological formations were discovered on the initial planned route. Regarding to geotechnical context of subsurface cavities risk it is mandatory to know their exact location in study areas to reduce the potential degree of risk. A field study was conducted to investigate the lithology, as well as in detecting the presence of apparent faults. Geophysical data was collected using Two Dimensional ERT with different electrode spacing to delineate associated subsurface cavities on each site and each cavity respectively. The target depth was 20 m. The result shows that the observed cavities were because of the dissolution of certain minerals constituting the weathering crust due to the infiltration and the seasonal variation in the level of the watercourses. The cavities were almost entirely empty such as in the cases of pylons 213 and 210 indicated that the weathering crust is due to the groundwater infiltration and the seasonal variations in the river flow level. The finding of this study showed that the ERT method have a good applicability in the detection of underground cavities in the study area.

The increasing demand for Aluminium pushes the miners to multiply the research in order to answer this demand. The objective of this work is to characterize the bauxitic deposit of Limbiko. The methodology consists in collecting samples, and establishing stratigraphic logs, sections and geological maps. The analyses were carried out in the laboratory of the Company des Bauxites de Guinea. Statistical methods were used to process the geochemical data. The study area is characterized by sedimentary formations of the Devonian, in which the dolerites of the Mesozoic were injected. It is on these Devonian and Mesozoic formations that the Limbiko bauxite deposit was developed. The stratigraphic logs show the succession of formations. The parent rock is surmounted by an alteration crust. The petrographic study is based on the bauxitic and transitional zones characterized by ferruginous laterites. Both zones contain some ferriplantite. Mineralogical analysis shows the presence of gibbsite, goethite, alumogoethite, rutile and clay minerals. The geochemical study of the major elements shows that the more the contents of SiO 2 , Fe 2 O 3 decrease, the more the content of Al 2 O 3 increases and those of TiO 2 , Al 2 O 3 increase in the same direction. The PCA confirms the mineralogical results by classifying the samples into clay, bauxite and ferrite.

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