Progressive Fine Integrated Geophysical Method for Karst Detection During Subway Construction

Various geological disasters can easily occur in tunnels that pass through a limestone stratum. The fine detection of the karst system is significant for safe construction. In this paper, a progressive integrated geophysical method is proposed for fine detection of the karst area of an urban subway. The method combines ground-penetrating radar (GPR), transient electromagnetics (TEM), cross-hole electrical resistivity tomography (ERT), and 3D laser scanning to provide quantitative and integrated detection imaging in a shallow karst system. First, 3D images of reflected waves and the apparent resistivity in the study area are determined using GPR and TEM methods, and the key development of abnormal karst bodies along the mileage direction is delineated. Meanwhile, the TEM slice combination results intuitively show the low-resistance anomaly's distribution and development direction. The main water channel is inferred within the detection range. Second, cross-hole ERT exploration is performed to realize the type of judgment and accurate positioning of karst caves. Finally, 3D laser scanning is conducted to obtain the 3D spatial orientation and size of the large-scale caves identified or those that could have a large influence on metro tunnel construction. Geophysical survey results provide a reference for the design and parameter selection of the cave grouting treatment scheme. This method has been effectively applied in a shield section of the R1 line of the Jinan Metro.