DETERMINATION OF THE GRAVITY DISTRIBUTION IN THE WEST AFRICAN LITHOSPHERE USING MODIFIED GEOPOTENTIAL GRAVITY MODEL

Abstract

Exploitation of minerals and other factors including density variations reduce the stability of the lithosphere from changes in Earth’s compactibility. The Geopotential Gravity Models (GGMs) are presently used to determine the stability of the continental and oceanic regions separately, but could not handle regions of the lithosphere which combine continental and oceanic regions. This study was therefore designed to develop a modified GGM that will cater for combined continental and oceanic regions. A Two-Staged modelling method was employed. The density of the continental region was varied with that of the oceanic region to obtain a compensation factor 𝜂. This was used to normalise the density contrast, which resulted from gravity field variation. The normalised density component was then used as an input factor into the existing GGM to produce the Modified Geopotential Gravity Model (MGGM). Space coordinates within longitude 30°W-15°E and latitude 3°-20°N were used in spherical harmonic model to simulate Gravity Data (GD) for different pre-chosen coordinates. Also, measured GD of Itakpe area in Kogi State, Nigeria were obtained from the Nigerian Geological Survey Agency (NGSA), Abuja. Results were validated by comparing the values of gravity of the simulated GD using MGGM and GGM with measured gravity values. The simulated GD from MGGM, GGMs and measured GD of Itakpe served as input data into Markov Chain Monte-Carlo (MCMC) inversion process to obtain density anomalies. Density data were analysed using descriptive error estimation approach. The modified model was: where V is the gravity potential, G is gravitational constant, γ is the number of Earth layers, g is the theoretical gravity, ρ_o is the density of the crust, k is bulk modulus and Z is the depth coordinate. V values of continental and oceanic lithospheres were obtained for latitudes 〖4.9358〗^° N to 〖12.4273〗^° Nand longitudes 〖13.4213〗^° W to 〖12.3502〗^° E which ranges from 85.6 to 249.7m2/s2 and 73.8 to 215.6 m2/s2, while the measured V for latitudes 〖5.8156〗^(° ) N to 〖14.3912〗^° N and longitudes 〖15.0498〗^°W to 〖10.4657〗^° E were 79.4 to 261.5 m2/s2 and 62.7 to 208.3 m2/s2, respectively. The density of the Earth’s lithospheric segment obtained from MCMC with depth ranged from 0 to 860 Km for the continent, varied between 9.7 and 26.5 x 103Kg/m3, while oceanic region varied between 14.6 and 33.2 x 103Kg/m3 with a depth range of 0 to 255 Km. The obtained values of density vary slightly from the observed density of the Earth’s lithosphere having values of 8.5 to 24.9 x 103Kg/m3 and 12.7 to 30.5 x 103Kg/m3 for continent and ocean, respectively. Average errors ranging from ±0.3% to ±11.4% and ±0.5% to ±28.5% were obtained when densities from MGGM and GGM using MCMC were compared with the density of Itakpe. These indicated that MGGM performed better than GGM.

The MGGM determined the lithospheric stability of combined continental and oceanic regions of West Africa simultaneously. This model could be extended to other regions of Africa.