Exploring karst caves in an urban area using surface and borehole geophysical methods

Karst-related geological disasters pose great threats to public safety. Geophysical methods for detecting geological hazards, such as electrical resistivity tomography (ERT), seismic tomography, ground penetrating radar (GPR), and borehole radar methods, have flourished in the past decades. However, the application of a single method among them has limitations in terms of probing depth, imaging resolution, and data interpretability. Therefore, to enhance detection accuracy and improve imaging resolution, selecting and combining multiple geophysical methods is indispensable before conducting a survey in a karst-affected area. In this paper, the location, boundary, and infill of a karst cave in an urban area are explored using surface and borehole geophysical methods. The experimental results demonstrate that ERT is appropriate for roughly detecting the location of the karst. The results indicate the presence of a U-shaped low-resistivity anomaly corresponds to a karst cave filled with water-bearing sand. Accordingly, locations of boreholes are determined. To further characterize the boundary and infill of a karst cave, cross-hole computer tomography (CT) of radar and seismic method is employed. The results show that the cross-hole radar CT yields superior results over the cross-hole seismic CT in characterizing a shallow soil layer and delineating the karst cave boundary. The geology of the test area is characterized by combining these geophysical findings and the drilling results. These findings provide useful information for planning underground engineering projects in urban areas.