A MULTISCALE ELECTRICAL SURVEY OF A LATERITIC SOIL SYSTEM IN THE RAIN-FOREST OF CAMEROON

Resistivity investigations were carried out on an elementary watershed in SW Cameroon, firstly to assess the applicability of direct-current (DC) resistivity methods to solve various pedological problems in int ertropical regions, and subsequently to determine the relationships be tween electrical resistivities and pedological properties of lateritic soil systems. The survey included measurements in pits with a small W enner fixed-spacing array (SWA), vertical electrical soundings (VES) a nd vertical electrical ''quick soundings'' (VEQS) both using the Schlu mberger configuration. The VES data were interpreted using a conventio nal multilayer inversion program to obtain best-fit models. Constraint s to the interpretation of these data were provided by SWA and pedolog ical information from existing observation pits. The results of the in terpretation reveal five distinct geoelectrical layers overlying a res istive bedrock. The first is a thin organo-mineral upper layer with lo w resistivities in the range 250-450 Omega m. The second layer corresp onds to micro-aggregated clayey materials and is more resistive (1300- 1800 Omega m). The third represents the main part of ferruginous mater ials and is even more resistive (2000-4500 nm). The fourth corresponds to unsaturated saprolite and the last to saturated saprolite (ground water) with resistivities ranging from 800 to 1500 Omega m and from 15 0 to 250 Omega m, respectively. Estimates of soil volumes for the enti re study area were obtained from VEQS interpretations. Most of the soi l cover corresponds to saprolite (74%, 1/4 being saturated by ground w ater), while topsoil and ferruginous materials represent 14 and 12%, r espectively. Finally, geophysical results based upon 1-D inversion pro vide a satisfactory approximation of the various lateritic components' 3-D geometry over the watershed. The study provides original quantita tive results concerning the behaviour of intertropical soil systems as well as some geomorphological keys for soil mapping at a regional sca le.