INTEGRATED METHODS USING MULTICHANNEL ANALYSIS OF SURFACE WAVES (MASW) AND ELECTRICAL RESISTIVITY TOMOGRAPHY (ERT) FOR GEOTECHNICAL HAZARD ASSESSMENT (A Case Study of Queen Ede, Benin City, Edo State)
Gully erosion has been among the most serious environmental and geotechnical risks within the southeastern part of Nigeria, which endangers infrastructure, livelihood, and sustainable land use. This paper discusses the use of an integrated geophysical technique that consists of Electrical Resistivity Tomography (ERT) and Multichannel Analysis of Surface Waves (MASW) to study the Queen Ede gully site in Benin City, Edo State. ERT was used to describe the subsurface changes in resistivity and MASW was used to give the profile on shear-wave velocity and soil stiffness. The findings indicate that there are major lateral and vertical differences in underground materials. Areas of high resistivity are associated with sandy and erosion-prone soils and low resistivity areas are associated with clay-bearing sediments, which, while more cohesive, become vulnerable to undercutting once overlying sandy units are eroded. A type of analysis using MASW revealed that the changes in stiffness were between very soft and stiff soils with shallow weak zones directly correlated to the regions of active erosion and slope instability. This combination of MASW and ERT leads to the identification of areas of increased vulnerability, which supports the idea that the use of single methods would have given incomplete results. The given study proves that combined near-surface geophysical methods can provide an efficient and rather economical instrument of geotechnical hazard assessment in the erosion-prone areas. The results furnish essential data in the urban planning, infrastructure planning and erosion control strategies, and provide a methodological framework to be used in other sites of gully erosions in Nigeria and other regions.