High-resolution geophysical imaging and characterization of severe landslides vicinities at South-Eastern Nigeria for uni-vario-seasonal degradation monitoring and civil engineering remediation.

Abstract

This research work used high-resolution geophysical methods to explore the near surface of severe gully-erosion sites. The erosion that mesmerized the environment as landslides is the major landscaping challenge facing Anambra Basin in the South Eastern Nigeria. The geophysical techniques used include electrical resistivity tomography (ERT), induced polarization (IP), vertical electrical sounding (VES), and geotechnical analysis to study the subsurface conditions of severe gullies. Principally Wenner, Schlumberger, and dipole-dipole are the geophysical tomography technique used during the survey, depending on the peculiarity of each selected site. Geotechnical data analysis was used to confirm the results of vertical electrical sounding at a specific location, with the aid of resistivity formations. The geodynamics of the sites as related to rocks’ susceptibility to failure, and the mechanisms of slope failure was investigated, and foundation depths of the immediate surroundings of the Nanka gully were studied using geotechnical data. The surrounding and the subsurface of the eroded portions were monitored through imaging and analysis across the basin to measure hydrological contribution to the gully risks and other prevailing factors. The geoelectrical data was acquired with the ABEM Terrameter SAS 4000 and the ABEM LUND ES464 electrode selection system (using resistivity method) and processed with the RES2DINV software to produce 2D subsurface images. The VES resistivity curve matching was developed by a partial curve matching technique; and interpreted by Minitab 18 software to produce subsurface contour maps. In addition, the subsurface contour maps were qualitatively analysed with mapped surface geology and information on current geological typical rocks. In the geotechnical analysis, Spangler and Handy sampling techniques were used to collect eighteen (18) samples from dug gully walls, and the laboratory tests were to ascertain the soil properties index. The results of the models indicate that the study area is mainly clayey and sandstone formations that exhibited low resistivity values corresponding to the shale layers and groundwater zones. Many features that may lead to slope failure are present in the study areas, such as fractures, boulders, weak zone, and saturated zone. The results also showed that the soils in the study areas are friable hence easily washed off whenever there is storm water runoff from the surface, making the landslide active over the years despite every protective precaution put in place. In conclusion, this research work has identified lithologies, structural deformations, and distinguishing clayey zones from water-saturated zones, proving that the geophysical technique is the most successful tool in the landslide investigation.