GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.1 557 1993, Genik, 1993 gave an overview on petroleum geology of the rift basins located in Niger, Chad, and Central African Republic, based on seismic reflection and well-log data of Exxon (1969-1991). The main objectives of this work are: 1) estimation of the depth of the basement under the Chad basin through a joint analysis and interpretation of satellite and terrestrial gravity data (BGI) with borehole data and density values of Cameroon-Chadian rock samples taken from the collection of Prof. A. De Min of the University of Trieste. 2) estimation and interpretation of the Bouguer and residual gravity anomalies. The results obtained will give crucial information ( i.e. depth of the geologic basement, thickness of sediment infill of the basin) for drilling a scientific core onshore of Lake Chad, in the framework of the International Continental Drilling Project (ICDP). This borehole will bring to scientists an archive of the paleo-climates and paleo-environments of continental Africa over some 10 million years. This area has experienced the variability of the Monsoon since several million years, that in turn has strongly impacted environmental changes (droughts and desert versus hydrographic network reactivation) through time and space, at a period that is crucial for the emergence and evolution of early hominids. Methods. For this project, we used the following methods: 1- to determine the density values of each rock samples we took several measurements of their weight with a hydrostatic balance and �� ���������� ��� ������� ������� we calculated the average values; 2- to calculate the complete Bouguer anomaly in forward modelling, topography of the study area is divided in more than ten thousand spherical prisms (“ tesseroids ”) and the gravity effect produced by each one was calculated; 3- the regression analysis of Bouguer anomaly field map and topography data is a method particularly suitable for geophysical surveys in areas almost uncovered by data like the Chadbasin (Braitenberg et al. , 2013;Braitenberg, 2015). Indeed,withvery fewparameters, it is possible to obtain information about the isostatic conditions of the study area. The processing with the regression analysis, observing the relation between topography and the gravity field, consists in a Spectral Analysis on topography and gravity data, previously discussed in other works. The processing with the regression analysis is needed for several scopes: 1) to determine the anomalies due to the crustal mass inhomogeneity, 2) to reduce the Bouguer anomaly for the effect of crustal thickness variations; 3) to allow the regression analysis to identify various tectonic features, which produce a different correlation between topography and gravity; 4) to use the residuals of the regression line to invert for the depth of the basement. The results of the regression analysis are displayed in Fig. 1. The black line and the blue points displayed in Fig.1 represent the average regression between topography and gravity, and local deviations (anomalies) from the regression line, respectively. We can further observe that the intercept of the regression line is ~ -25 mGal, indicating a prevalence of negative anomalies inside the Chad basin. The value of the slope is -0.08 mGal/m and is strictly Fig. 1 - The results of regression analysis (regression line) and the correlation between topography and Bouguer gravity anomaly (Eigen-6c4) for Chad basin and surrounding areas.