A Reconnaissance Survey for Proposed Road Construction Project using Gravity Method of Geophysical Investigation in Magbon-Alade Area of Ibeju-Lekki South-West Nigeria (Published)
Abstract: Gravity method of geophysical investigation was utilized in acquiring gravity anomaly data aimed at investigating subsurface geological conditions for proposed road construction project in Magbon-Alade area of Ibeju Lekki, South-West Nigeria. CG-5 Autograv meter was used for the acquisition of field data. 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 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. At N1, it showed a maximum anomalous gravity reading of 35830 mGal at Base Station (BS) 5 and a minimum anomalous gravity reading of 27594.8 mGal at BS 1. At N2, it showed a maximum anomalous gravity value of 35848.5 mGal at BS 7 and a minimum anomalous gravity value of 27613.5 mGal at BS 1. At N3, it showed a maximum reading of 35863.6 mGal at BS 7 and a minimum value of 27632.5 mGal at BS 1 and at N4, the maximum value recorded was 37442.3 mGal at BS 7 and a minimum value of 3246.4 mGal at BS 4. The change in gravity value at different station is a reflection of subsurface structures present in the subsurface of the study area. The residual anomaly plot profile revealed the major anomalies region which occurred at lateral distance between 1m to 9m having gravity value of −2450mGal in traverse 1; lateral distance between 16-23m and 23−30m having gravity value of −5000 and 2700mGal in traverse 2; lateral distance between 4.5m−13m and 13m−20m having gravity values of 560 and 360mGal in traverse 3; lateral distance between 7.5m−16.5m, 7.5m−23.5m and 26-35m having gravity value of -1700,1450 and 1200mGal in traverse 4; and lateral distance between 11−17m and 17−25m having gravity values of 900 and −1900mGal in traverse 5. This study has therefore investigated the area using gravity method to unraveling the locations of high and low anomalous zones. 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.
Keywords: Anomalies, Bouguer, Compaction, Cycle, Litho-Structures, Reconnaissance