Identification of Cameroon's geological structures through a gravity separation and using seismic crustal models

Abstract

Residual gravity data provide useful information for a geophysical interpretation of geological sources. Several techniques have been developed and applied to separate a different content in gravity data. Nevertheless, few methods actually incorporate factual geophysical or geological information in gravity data processing. Instead, most of these methods relay only on mathematical techniques that do not necessarily reflect the actual geological composition and problems inherent to gravity inversion, most notably a non-uniqueness of solutions to gravimetric inverse problems (i.e. infinity many geological configurations could theoretically provide the same gravity solution) as well as the fact that gravimetric inverse solutions are generally ill-posed problems that require employment of regularization techniques. To partially address this issue, we apply a gravimetric forward modelling based on available seismic crustal structure models to separate a residual gravity signal from observed gravity data in order to interpret more realistically detailed geological and tectonic features. This procedure involves the estimation of the uppermost mantle gravity contribution that is treated as a regional gravity data. The residual gravity data is then obtained by subtracting the regional gravity data from observed gravity data. In addition, observed gravity data are typically corrected for the gravitational contributions of topography and bathymetry (and possibly other known crustal density heterogeneities, such as sediments and density variations within the underlying consolidated crust). The application of these gravity corrections to observed (free-air) gravity data yields the Bouguer gravity data. We employ this forward modelling approach to the Bouguer gravity disturbances over Cameroon. Furthermore, we apply a regional-residual separation based on a spectral analysis and a robust polynomial fitting. The performance of these three techniques is compared in terms of their interpretational quality. The numerical analysis reveals that the residual gravity disturbance map obtained by applying the forward modelling technique resembles main known geological and tectonic configuration and closely reproduces even more detailed features. The results from using a spectral analysis could to some extent reproduce realistically tectonic and geological structures, except for more localized features otherwise seen in the forward modelling result. A robust polynomial fitting, on the other hand, fails to provide residual gravity disturbances that realistically reflect geological and tectonic configuration of the study area. The reason is that this method produces either only positive or only negative gravity disturbances, but not both. Moreover, this merely mathematical technique does not take into consideration available geological and geophysical information.