Voxel solution and 3D sub-surface imaging in the Douala sedimentary sub-basin (Cameroon)

Abstract

Gravity data consisting of a total of 128 data points located between latitudes 3° 21′N and 4° 9′ N and longitudes 8° 27′ E and 9° 16′ E have been computed for Bouguer and residual anomaly maps in this study. From the plots of these maps which are characterized by very high positive ring shaped contours towards the centre, which are suggestive of a subsurface intrusive body, three anomaly zones as well as two minor peaks were identified. The maps were also characterized by W-E alignment of contours to the N and S as well as approximately NNW-SSE alignments to the E and to some extent, the W of the study area. A layered earth model which constitutes a depth model built using the geosoft montaj extension, GM-SYS 3D, was used to provide a surface-oriented model for this intrusive body. Using constant layers added to a 3D model at depths of 5 km, 10 km, 15 km, 20 km, 25 km, 30 km, 33 km, 35 km and 39 km; each in the absence of the others, forward calculations were employed to calculate the density contrast of each layer while their densities were recalculated using constant density inversion. Structural inversion of each of these layers then generated their surface-oriented models. The optimum inverse model for the intrusive body was obtained at a depth of about 33 km. The model has a surface and surroundings with a density of about 2.5 g/cm3 which corresponds to that of sedimentary basins. It also presents an intrusive body with density of about 2.77 g/cm3 located at a depth of about 6.1 km with its bottom located at about 33 km giving an average vertical height of about 26.9 km. This 3D relief sub-surface model was used to define the geometry of the starting model for 3D data voxels to generate the block structure of the intrusive body. Applying the basic statistical kriging algorithm, the 3D voxel solution of the intrusive body underneath this study area as well as its corresponding calculated maps were generated. This solution is similar in features to that of the surface-oriented solution and presents a block structure of the intrusive body with a major peak at a depth of about 6.2 km and having a relatively sharp sloping boundary towards its NE edge and gradual sloping edges towards its NW, W and E limits. Its density of 2.77 g/cm3 is further confirmation to the fact that this body is of igneous origin. This model however shows evidence that this block extends further south beyond this study area and consequently requires more data in this direction to image it fully. These results from the 3D surface-oriented solution and the 3D voxel solution are in good agreement with previous results from 2.5D studies in this very area of study.