Gravity Model for an Anomalous Body Located in the NW Portion of the Douala Sedimentary Sub-Basin, Cameroon (Central Africa)

Abstract

Gravity studies have been carried out in the Douala sub-basin which is a sedimentary basin located both onshore and offshore on the South coast of Cameroon between latitudes 3 ° 03'N and 4 ° 06'N and longitudes 9 ° 00' and 10 ° 00'E, covering a total surface area of 12,805 km 2 . On its onshore portion, the Douala sub-basin has a trapezoic shape and covers a total surface area of about 6955 km 2 while the offshore part covers an area of about 5850 km 2 . Gravity data used in this study are constituted of 912 gravity data points located between longitudes 8 ° 10.2' to 10 ° 59.4'E and latitudes 2 ° 30.6' to 4 ° 59.4'N and the study area is located to the NW section of the onshore portion of the Douala sub-basin. This study area is characterised by considerably high positive anomalies attaining peak values of about 104.1 mGals at longitude 9 ° 9.9' and latitude 4 ° 1.1' with contour lines which are mostly oriented in the NNE direction. Residual anomalies were extracted by upward continuation of the Bouguer anomaly field at an optimum height of 30 km. This residual field and those obtained by the separation of polynomial of order 4 had a very high correlation coefficient factor of 0.979. The multi-scale horizontal derivative of the vertical derivative (MSHDVD) method was applied on the extracted residual anomalies for the delimitation of possible contacts in the source while the amplitude spectrum was used to estimate the depth to the top of the potential field source. The MSHDVD method did not delimite any clear cut contacts in the source but the amplitude spectrum estimated the potential field source at a depth of about 4.8 km. The ideal body theory was used to determine the density contrast along a 65 km NW-SE profile yielding a value of 0.266 g/cm 3 . 2.5D modelling aimed at bringing out the underlying structural layout of this study area presents a source body which is very probably an intrusive igneous block surrounded by sedimentary formations and having a density of 2.77 g/cm 3 at a depth of about 5.88 km below the surface and an average thickness of about 26.95 km.