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Ground gravity survey was recently carried out in the Lolabe-Campo area, which constitutes the southern onshore portion of the Kribi-Campo sub-basin in South Cameroon. The obtained gravity data were processed and interpreted in order to elucidate the subsurface geology of the area in which preliminary studies highlighted the presence of an intrusive igneous body. In order to determine the nature and the characteristics of this body, spectral analysis, ideal body solution and 2.5D modelling methods were applied to the gravity data. The results suggest that the emplacement, breaking up and separation of the intrusive igneous body were the outcome of two tectonic events corresponding to an upward vertical translational tectonic movement followed by an anticlockwise rotational faulting tectonic movement. From a density contrast of 0.13 g&#183cm&#453, the two isolated bodies have an estimated density of 2.80 g&#183cm&#453 which is comparatively higher than the average density of 2.67 g&#183cm&#453 of the autochthonous rocks. These two blocks are completely surrounded by both sedimentary and metamorphic formations and are confined between the depths of 0.6 km and 3.5 km. The highly dense nature of the rocks and their location around the Atlantic Ocean led to the conclusion that the two intrusive bodies are composed principally of gabbro. The disposition and shape of the open zone resulting from the separation of the blocks are suggestive of a trap for the accumulation of hydrocarbons and mineral resources.

The magnetotelluric method has been used in this work to compare the nature of the sedimentary-metamorphic contacts of the Douala and Kribi-Campo sub-basins in Cameroon. The results show that the sedimentary-metamorphic contact zones for the two sub-basins, marked by a subsidence of the low resistivity materials on the sedimentary formations and uplift of the high resistivity materials characterized by intrusive bodies of higher values of resistivity on the metamorphic formations, are structurally similar,. However, the very low values of resistivity of rocks in the Douala sub-basin is suggestive of high porosity, permeability and high level of saline ions dissociation leading to high conductivity. These rocks should be of unconsolidated sediments for the sedimentary formation and gneiss for the metamorphic formation. On the contrary, the very high values of resistivity for rocks in the Kribi-Campo sub-basin indicate the absence of free mobile electrons and ions and low porosity, permeability and non-conductivity. The sedimentary formation of this sub-basin should be composed of limestone and conglomerates rocks with some gneissic and unconsolidated granitic materials. The rocks in the metamorphic formation should be completely granitic in nature. The shallow depth of penetration of only 4 km of telluric current in the Douala sub-basin is enough evidence that the tectonic events responsible for the emplacement of this contact zone were limited to the earth crust. On the other hand, in the Kribi-Campo sub-basin the tectonic events should have originated from within the earth mantle as the depth of penetration of telluric current attains 150 km.

In order to produce a more detailed structural and geometrical information, and determine sediments thickness along the Kribi-Campo sub-basin, statistical spectral analysis and horizontal gradient analysis of residual anomalies coupled with the Euler deconvolution approach were applied on the gravity data in the area. The results obtained from the 2D spectral analysis on anomaly grids gave a depth to the basement rocks of the basin from 0.60 km to 3.93 km. This represents the thickness of the sedimentary formations overlying the basement. The interpretation of the spectral analysis results indicated that the potential hydrocarbon field areas are situated between Kribi and Lolabe and at Campo given that those areas have the highest sedimentary thicknesses values. From the analysis of the horizontal gradient, deep faults mainly striking SW-NE have been traced and a structural map of the area has been produced. By applying the Euler deconvolution method to the gravity data, information about the depth and trend of the main subsurface structures have been obtained.

A new gravity survey was carried out in the northern part of the onshore Kribi- Campo sub-basin in Cameroon. The data were incorporated to the existing ones and then analyzed and modeled in order to elucidate the subsurface structure of the area. The area is characterized in its north-western part by considerably high positive anomalies indicative of the presence of a dense intrusive body. We find, 1) from the analysis of the gravity residual anomaly map, the high positive anomalies observed are the signature of a shallow dense structure; 2) from the multi-scale analysis of the maxima of the horizontal gradient, the structure is confined between depths of 0.5 km and 5 km; 3) from the quantitative interpretation of residual anomalies by spectral analysis, the depth to the upper surface of the intrusive body is not uniform, the average depth of the bottom is h1 = 3.6 km and the depths to particular sections of the roof of the intrusion are h2 = 1.6 km and h3 = 0.5 km; 4) and the 3D modeling gives results that are suggestive of the presence of contacts between rocks of different densities at different depths and a dense intrusive igneous body in the upper crust of the Kribi zone. From the 3D model the dense intrusive igneous block is surrounded by sedimentary formations to the south-west and metamorphic formations to the north-east. Both formations have a density of about 2.74 g/cm3. The near surface portions of this igneous block lie at a depth range of 0.5 km to 1.5 km while its lower surface has a depth range of 3.6 km to 5.2 km. The shape of the edges and the bottom of the intrusive body are suggestive of the fact that it forms part of a broader structure underlying the Kribi-Campo sub-basin with a great influence on the sedimentary cover.

Atopographicalsurveyandadynamiccone penetrometertesting(DCPT)wereconductedoverthe Kekemlandslide area. The aim ofthe work was to unveil new topographic features ofthe site, and with the penetrometer soundings predict the depth of the potential rupture surface beneath the damaged area. The analysis of penetrometer data has led to the supposition that the fracturedsurface interface connectingthe basement andsediments has adepthofbetween 2.00 mand 12.20 m. The topographic data was used to determine the average slope percentage over the broken area. The slope percentage gives 33.66% from a point taken over the tearing niche and a point along the road. 2D contours map drawn with the Surfer software, over an area of 35 hectares has enabled the observation of a step gradient oriented NNE-SSW. Considering the steepness ofthe slope and the thickness ofthe sediments, the site remains dangerous for the populations especiallyduringthe rainyseason. Se hicieron un levantamiento topográfico y un ensayo con penetrómetro dinámico de cono (DCPT) sobre el área de deslizamiento Kekem. El propósito del trabajo es conocer nuevos rasgos topográficas del sitio, y con el penetrómetro predecir la profundidad a la que está la potencial superficie de ruptura debajo del área afectada. El análisis de los datos provistos por el penetrómetro ha permitido predecir que la interfaz de superficie fracturada entre el basamento y los sedimentos está a una profundidad entre 2,00 y 12,20 m. Los datos topográficos permitieron determinar la pendiente media de la zona fracturada. Medida entre un punto en la zona de desgarre y un punto a lo largo de la vía, la pendiente porcentual es de 33,66%. Líneas de contorno 2D dibujadas en el mapa con el programa Surfer, sobre una superficie de 35 hectáreas ha permitido observar un empinado gradiente orientado en dirección NNE-SSW. Tomando en cuenta la empinadapendienteyelespesorde lossedimentos,elsitiosiguesiendo peligroso paralapoblación,especialmentedurante la estación lluviosa.