Predicting Depth of Mineral Deposit using Gravity-Density Downward Correlation by Fourier Transform
A Thesis Submitted to the School of Postgraduate Studies, University of Lagos
Changes in rock densities produce variations in the distribution of the earth’s gravity field observed on the earth’s surface. These variations are partly responsible for the differences between observed gravity value and the theoretical value in a given place. These differences are usually referred to as gravity anomaly. The anomalies are known to be caused by both the local variations of earth densities and the isostatic or rift effects of the earth’s crust which are long wave in nature. In geophysical prospecting for minerals, the anomaly due only to shallow layers are desirable. The major challenge therefore is to isolate them from the observed anomalies using a combination of methods in a sequential but complimentary way. In this research, the Pratt-Hayford and Airy Heiskanen isostatic models (in planar and spherical approximations) were used in removing the rift effects from the anomalies leaving us with only residual gravity anomalies which relate to the densities of the minerals of the subsurface in the study area. The shallow layers were also isolated from the deep masses using the concept of second vertical derivative (SVD). Residual gravity anomalies are the superposition of effects originating from several interfaces in the subsurface. The determination of the depth of the particular substructure which accounts for the bulk of the residual gravity anomalies can only be resolved with the aid of additional subsurface data such as density log. In this research, density log was used to determine the weighting density function, delineate the multiple layers and correlate with the gravity-density downward variation for predicting the mineral depth using the Fourier transform method which was implemented using the Fast Fourier Transform (FFT). The output from the SVD processing was used to determine the basin’s sedimentary thickness. In addition, geoid undulations are known to be related to the anomalous mass density distributions within the Earth’s subsurface. In this study, the lateral density distributions of the shallow layers of interest were confirmed using geoid undulations obtained within the study area. The geoid undulations computed from the isostatic models were subtracted from the Earth Gravitational Model (EGM 2008) geoid to obtain the residual geoid. .The graphical interpretation of the patterns of residual gravity contours at the surface, leads to the initial conclusion that the South-East (SE) and North-East (NE) zones contain negative residual gravity values which shows the dominant presence of low density sedimentary or volcanic sections while the North-West (NW) and South-West (SW) zones contain positive values which correlates with the intrusions of high density crystalline materials. The SE zone is favourable for hydrocarbon accumulation at depth between 2015m and 2170m.The NE at depth between 500m and 7000m contains anomalous mass with a density range of between which is associated with the mineral is identified as granodiorite using standard density calibrations. The basement depth obtained showed a maximum basement depth of 5.27km in the South Eastern part and 7.0Km in the North Eastern part of the study area. The downward continuation and basement depths obtained from the gravity models were compared with that obtained from reflection seismic observation from the study area and the relative error percent were 1.37% and 0.46% respectively. The geoid undulation of the basin ranges from 19m to 21m. The residual geoid undulation maps and the geoid/ gravity admittance maps showed the SE as a region with low density mass deposits, while the NE has high density mass deposits. These results corroborate the findings obtained from the gravity model. The basin is a good target for solid minerals.