Electrical resistivity imaging and multichannel analysis of surface waves for mapping the subsurface of a Wetland Area of Lagos, Nigeria

Abstract

Electrical resistivity imaging (ERI) and Multichannel Analysis of Surface Waves (MASW) surveys were carried out over a reclaimed wetland area of University of Lagos, Nigeria. The main purpose of the geophysical surveys was to characterise the site for better understanding of the subsurface conditions before building construction is carried out. For this purpose, seven traverses were established for electrical resistivity and surface waves measurements. The PASI resistivity metre was used for the ERI surveys with minimum electrode spacing of 5 m using the Wenner array. The MASW measurements were carried out using Terraloc seismograph with 24 channel 4.5 Hz vertical component geophones. The results of the ERI surveys show that the subsurface strata are composed of peat/organic materials with resistivity values of (0.7– 3) Ω-m, silty clay having resistivity values of (5– 50) Ω-m and sandy clay sediments of resistivity between (51– 105) Ω-m, all were mapped at different depths. Also, the shear wave velocity models from MASW measurements show that three zones were clearly mapped. These zones are the low shear strength strata of peat/organic materials with Vs between (25– 70) m/s, silty clay with Vs ranges between (70– 120) m/s and sandy clay of Vs ranging from (120– 150) m/s. The combined approach has helped to better define the interface between layers, their thicknesses and consistency of each stratum. Thus, moderate to very strong correlations between the measured resistivity and velocity and the boreholes drilled were achieved. The regression models obtained compared reasonably well for all the traverses. The engineering implication of the geological units mapped is that the site is characterised with weak/incompetent materials not suitable for hosting the foundation of especially massive engineering structures. Hence, deep foundation through pilling to the competent layer is to be giving consideration or soils improvement techniques can be employed for the stabilisation of the soils.