Detection of Beaded Karst Caves in Subway Works by Mixed-Source Surface Wave Survey: A Case Study

Abstract

Karst cave causes geological disasters in tunneling engineering, which brings great risks to subway construction. The microtremor survey has become one of the main methods for karst cave detection in urban areas due to its convenience and great detection depth. However, the microtremor survey lacks high-frequency signals, resulting in poor detection accuracy on the shallow layer. An advanced method called mixed-source surface wave (MSW) method is proposed by imposing active sources during the continuous passive surface wave survey. MSW is applied to the detection of beaded karst caves (i.e. two or three karst caves at different depths in the same location) in Ji'nan, China. The influences of ambient noise, array type, and other factors on the detection accuracy were studied. Experimental results show that the detection depth and accuracy of MSW are much better than those of passive surface wave method. The inversion accuracy of linear array perpendicular to road is better than the triangle array and L-shaped array. The accuracy of the linear array along the road is worst. The detection accuracy of MSW is similar to that of combined active–passive surface wave methods. However, the basic principle of MSW is easier to understand, and its operation is more convenient. The two-dimensional measurement results show that the MSW method can well reflect the size and location of the beaded karst cave and provide important references for the practical engineering.