Apport de la géophysique et de la télédétection a l’étude de la structure interne de la localité de logbadjeck (littoral – Cameroun)

Abstract

Geophysics and remote sensing are frequently used for a large number of applications. They explore the Earth using indirect and non-invasive methods to determine the physical properties of the soil. In the framework of this study, the internal structure of the Logbajeck locality is to be characterised through satellite images and vertical electrical soundings. To this end, satellite images (Landsat7 ETM+ and SRTM) were processed and geophysical data were acquired by combining the trails and electrical soundings with the Schlumberger device. Eighty vertical electrical soundings spaced 50 m from each other were carried out along fourteen profiles approximately 250-450 m long and oriented NE-SW direction. The rocks identified in this area consist of migmatites, gneisses, quartzites, and breaches. Processing of satellite images reveals the presence of 285 lineaments in the Dibamba region with lengths ranging from 0.6 km to 9.5 km. Major linear structures with directions N20°-40°, N50°-80° and N100°-140° and minor ones with directions N0°-20°, N80°-100° and N140°-180° have been highlighted. The mapping of lateral and vertical variations of resistivities permitted the identification of the internal structure and to determine the presence of aquifers in the study area. The vertical electrical sounding curves obtained are of types KH, K, H, AK and Q and the soil consists of 3 and 4 layers. Electrical resistivity generally varies between 11 Ω.m and 8131 Ω.m and decreases with depth except in the northern and eastern sectors of the study area. Hydraulic conductivity ranges from 0.1 to 16 while transmissivity ranges from 5 to 600 . The transmissivity results obtained show that the study area has a low to moderate hydrogeological potential. The drilling results show that the shallow aquifers are around 6 m deep and the deep aquifers around 56 m deep. The longitudinal conductance varies from 0.2 to 1.24 and the central and north-western sectors are considered to be areas of medium to good aquifer protection capacity. The central sector appears as the most conductive and fractured sector of the study area with Overburden thicknesses ranging from 10 m to 19.91 m. The electrical tomography confirms the presence of a major lineament varying between 60 and 140 m in width and more than 50 m in depth in the locality of Logbadjeck. This lineament long for 9.5 km has an electrical resistivity range of 27 to 300 Ω. m.