Geophysical Interpretation of a Subsurface Landslide in the Southern Qinshui Basin

Abstract

A subsurface landslide is the key problem to a large affordable home program in the southern Qinshui Basin, China. A geophysical survey integrated with seismic refraction tomography (SRT), seismic scattered wave imaging (SSI), and electrical resistivity tomography (ERT) was performed along five profiles over the landslide body. The bedrock surface was a crucial interface, represented by a relatively high velocity, high density, and high resistivity in contrast to the unconsolidated soil and landslide material above it. Based on the most comparable geophysical features, several problems were uncovered such as a “sandwich velocity structure” in the SRT results, velocity trap in the SSI results, and rapid variations in the local topography in all three methods. Aiming to improve the comprehensive interpretation of the ERT, SRT, SSI data, the vertical gradient extremum in the ERT and SRT data and tracing the target wave group according to the velocity sensitivity in the SSI data were analyzed. Moreover, a joint interpretation of the three geophysical survey datasets as well as 32 geological wells and 73 geotechnical boreholes helped to determine one undulating bedrock surface, delineate two types of failure surface geometry (landslide surface and collapse surface), and identify three external shapes in the ex situ body (ancient river channel, landslide body and collapse body). The results showed that the integrated geophysical survey not only provided detailed evidence for the existing of landslide but also presented meaningful evidence for the sliding mechanism. These results were difficult to fully describe and to apply to understanding landslide processes. Furthermore, for near-surface landslide events, the joint interpretation of geologic, geotechnical and geophysical data was necessary to reduce problems with any single geophysical survey.