Variation of Gravity Anomaly in Ayegbami Area of Ibeju-Lekki South-West Nigeria: A Reconnaissance Geophysical Survey for Proposed Road Construction Project (Published)
Gravity method of geophysical investigation was utilized in acquiring gravity anomaly data aimed at investigating subsurface geological conditions for proposed road construction project in Ayegami area of Ibeju Lekki, South-West Nigeria. CG-5 Autograv meter and accompanied accessories were used for the acquisition of data in two traverses with four (4) cycles each adopted for four minutes to achieve for different readings which were indicated as N1, N2, N3 and N4 where the end of N4 assigned as the last reading taken at the base station. The first reading was taken at the base station and at one-hour interval, then another reading was taken at the base station and also consecutive readings were taken from different points at every 5m distance in a traverse line against time. The data were inputted in Microsoft Excel format and consequently processed, plotted and interpreted. The bouguer and residual anomaly plots were used to indicate locations of low and high gravity anomaly along the two (2) traverses which were used for the ascertaining the locations of high and low densities and their corresponding subsurface litho-structures. Gravity changes were observed at different lateral extent at each base station. The gravity profile yielded results that reveal bouguer gravity anomalies with magnitude ranging from 188.1 to 23755 mGal. Station 3 in line N1 exhibited highest gravity values of 23755.7 mGal while station 5 in line N2 exhibited lowest gravity value of 188.1 in the entire investigated area. The change in gravity value at different station is a reflection of the status of subsurface structures. The residual anomaly profiles revealed the major anomalous regions which occurred at lateral distance between 92m to 100m with exhibited gravity value of -800000 mGal in traverse 1. Also, at lateral distance between 0m to 20m and 24m to 64m exhibited gravity values of 4400m and -6000 mGal were respectively observed. The detected and delineated anomalous zones were indicative of low-density body that could possibly represent the presence of metallic deposits/buried utilities; are of low strength could cause road failure in the area except appropriate geotechnical measures are integrated on the subsurface materials in the area of compaction and stabilization.
Keywords: anomalies, bouguer, compaction, cycle, litho-structures, reconnaissance