Application of microgravity and electrical resistivity imaging techniques to identify ground subsidence prone zone

Owing to the disastrous consequences of ground subsidence in the urban area, great attention had always been accorded to areas with suspected signs of subsidence occurrence in order to mitigate its effect. Microgravity and electrical resistivity imaging (ERI) survey techniques were conducted at a parking lot of which possible occurrence of subsidence is anticipated due to the increased exposure of surface cracks and very slight depression. This study aimed to map and delineate the subsurface condition of the area with the view of revealing probable subsidence prone zones within the study area. To this end, the microgravity survey data was acquired at a station interval of 2 m. Pole-dipole configuration with 2 m electrode separation was adopted for the electrical resistivity imaging survey. The two methods showed good correlation with each other and proved their effectiveness in imaging the subsurface. A large proportion of the subsurface area was found to be residual soil (mainly silt and or clayey sand) with saturated zones identified by low gravity (<-40.94 mGal) and resistivity (<150 Ω.m) values, which have the tendency to undergo expansion and contraction processes due to rise and fall in moisture content. The subsurface condition was found to be relatively stable, devoid of any subsidence triggering features such as voids or cavities. However, it is inferred to be unsuitable for engineering structures due to the expansive and contractive properties of the subsurface geomaterials (residual soil). Therefore, it is concluded that the area is not prone to subsidence and the surface cracks presents are mere effects of the expansion and contraction process, which could be avoided by the excavation of the expansive soil or good engineering design before the establishment of any structure.