Urban geophysical exploration: case study in Chengdu International Bio-City

Understanding the shallow geological structure of urban areas is crucial for effective planning and development of underground spaces. Geophysical methods that are well-suited for site-specific investigation and have robustly anti-interference capabilities can provide important geological information for urban areas. In support of investigating the 3D geological structure of the shallow subsurface in Chengdu International Bio-City, a geophysical exploration study was conducted using three methods: the electrical resistivity tomography (ERT), micro-seismic exploration and opposing-coil transient electromagnetic method (OCTEM). Results from the study showed that the ERT method was greatly affected by local high-resistance bodies, construction sites and industrial currents, therefore leading to poor detection results that did not match well with the area's layered structure characteristics. The micro-seismic exploration method showed good layering effects and correlation with the drilling data in the elevation range of & SIM;350 to 436 m, but poor layering effects and low correlation with drilling data in the elevation range of & SIM;235 to 350 m, with relatively slow construction efficiency. The OCTEM showed good correlation with the drilling data for shallow depths up to 200 m and good identification capabilities for gypsum and mudstone in the area. Additionally, the instrument's anti-interference ability was suitable for complex urban conditions. Thus, OCTEM was selected for the area-based exploration with a 100 x 10 m grid, rapidly obtaining 3D resistivity information for depths up to 200 m in the study area. By integrating the 3D resistivity information with known engineering geological information, a comprehensive 3D geological model of the study area was created.