A PRELIMINARY INVESTIGATION FOR PROPOSED ROAD CONSTRUCTION PROJECT USING GRAVITY SURVEY METHOD OF GEOPHYSICAL INVESTIGATION IN IGBEYIN-ADUN AREA 1 SUBURB OF IBEJU-LEKKI SOUTH-WEST NIGERIA

Abstract

Gravity method of geophysical investigation was utilized in acquiring gravity anomaly data aimed at investigating gravity variations in relation to subsurface geological conditions for proposed road construction project in Igbeyin-Adun Area 1 suburb of Ibeju-Lekki, South-West Nigeria. CG-5 Autograv meter was employed for the acquisition of field data. Nine gravity stations were occupied, the first reading was taken at the base station and at one-hour interval, then readings were taken at the base station and also consecutive readings were taken from different points at every 5m distance in a 40m profile along a traverse line against time. In the course of the field operation, four cycles each were adopted for four minutes to achieve four different readings which are indicated as N1, N2, N3 and N4 with the end of N4 assigned as the last reading taken at the base station. 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 five (5) 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 readings of the study area scanning from N1 to N4 produced the following outputs. The geometry of the subsurface structural variations was determined from the interpretation of the anomaly data. The transverse section (T1 to T6) revealed a gravity profile of a total length 30 m, 40m, 35m, 40m, 35m and 20m respectively with station spacing of 5m. the residual anomaly profile shows major anomalous regions which occur at lateral distance between 5m to 17 m and 17m to 25m (T1), 22m to 32m and 32m to 40m (T2), 3.5m to 15 m, 13m to 19m and 22.5m to 35m (T3), 0m to 40 m (T4), 1m to 9 m (T5), and 1m to 8.5m (T6) with gravity value of 42000 and -30000 mGal in T1, 1400 and -1800 mGal in T2, -2100, -600 and 1400 mGal in T3, 700 to 1500 mGal in T4, 12200 mGal in T5 and 1.3 mGal in T6 respectively for all the transverse lines and these anomalous zones are indicative of low-density and high-density body in T1, low-density and high-density body in T2, low-density and high-density body in T3, high-density body in T4, high- density body in T5 and a low density body in T6 which could represent possible buried objects within the study area. The results conclusively delineated the study area has a high-density bodies and low-density body. 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 affected area for compaction and stabilization.