Background With the advancement in smart city construction in China, the development and utilization of urban underground space have become a focal point. Geophysical methods, as a crucial technical means for underground geological structure exploration, face new challenges when applied to complex exploration environments in urban areas.

Objectives and Methods  To address issues in underground space exploration in the coastal area of Qingdao, this study investigated the reclamation area along the eastern coast of the Jiaozhou Bay. Using a detection technique that combined the transient electromagnetic method (TEM) using a conical source and high-density resistivity method, this study constructed a 3D resistivity model through multi-source data fusion. This model revealed the stratigraphic structures and the extent of fracture zones within the study area, demonstrating the technical advantages of the novel TEM method with a conical source when applied to complex urban environments.

Results and Conclusions  A three-layer electrical structure was revealed in the study area: an artificial fill layer, the Quaternary sedimentary layer, and bedrock. A clear bottom interface was observed in the artificial fill layer. Data processing and geological interpretation contributed to the successful identification of unfavorable geological bodies such as water-rich anomalous zones and structurally fractured zones, with the former exhibiting laterally continuous resistivity variation bands and the latter characterized by dislocated and concave contours. The varying degrees of bedrock weathering influenced the developmental degrees of faults and fracture zones, especially near the Cangkou fault. The established integrated geophysical exploration system can effectively identify the spatial distribution of strata and the characteristics of geological structures, providing a new technical pathway for achieving “transparent” urban underground space. The results of this study offer a critical geological basis for engineering applications such as metro tunnel siting and underground pipeline network planning.