Response of 3D high-density electrical method to the interface of rock (soil) layer and karst collapse

Karst ground collapse occurs frequently in karst areas in China, which can easily lead to a series of hazards such as road deformation, house destruction and pipeline rupture. Karst collapse is uncertain, hidden and sudden, which poses a great threat to people's lives and property. Moreover, karst collapse is potential to facilitate the connection between surface water and groundwater, which may lead to the intermingling of these two water sources and subsequently contribute to groundwater contamination. Therefore, karst survey is of great significance to urban planning and disaster prevention and reduction. However, within the complex urban environment, accurately depicting karst morphology presents a significant challenge for geophysical exploration. Linyi City of Shandong Province, located in the west of the Tanlu Fault Zone, is one of the important temperate areas of karst development in the world. This city is especially characterized by soluble carbonate strata of Cambrian system and Ordovician system. The survey area is located in the monoclinic hydrogeological unit of Linyi, where karst fissure water is developed. In the past, the disorderly exploitation of groundwater led to frequent flowing of groundwater and many occurrences of karst collapse. In this study, we carried out 3D high-density electrical surveys in the study area, and laid 14 lines for high-density electrical survey at intervals of 12 m between each measuring line and 2 m between each measuring point. We also adopted the Wenner device, setting 25 as the maximum isolation coefficient. In this study, we firstly analyzed the physical properties of rocks in the study area, the results of which provided a physical basis for us to utilize the high-density electrical method for dividing stratigraphic boundaries and detecting karst. Subsequently, we established a geoelectric model and a 3D geoelectric structure of the study area to analyze the response characteristics of 3D high-density electrical method. Afterward, we selected some engineering exploration holes for verification and repeated deduction to improve the accuracy of simulation results. Taking P10 section as an example, we drilled holes at Point 127 for verification. According to geological records, a karst cave with a thickness of 4.1 m was found at the position of 24–28.1 m, which was basically consistent with the predicted results.

The results show as follows. (1) Based on the combination of forward and backward numerical simulation, the inversion results are believed to be reliable. (2) The 3D high-density resistivity method can provide a substantial amount of data and rich geoelectric information. It clearly reflects the interface between Quaternary strata and limestone, and offers an intuitive visualization of karst caves, making it highly effective in engineering site selection. (3) The analysis indicates that the karst drawdown funnel in Linyi City shifted from Lihang–Hougangtou in 2003 to Qijiazhuang village in 2007, during which karst collapse occurred frequently. Since 2012 after the nearby karst groundwater well that was overly exploited was closed, no karst collapse has occurred again. However, the early karst development left behind some caves, so thorough surveying should be conducted during urban construction. In conclusion, in cases where there are significant differences in physical properties, the high-density resistivity method can effectively characterize the interface and karst features in detail. It can also encompass extensive geoelectric information. Moreover, the 3D visualization of inversion results can enhance the clarity of the findings. This approach provides a solid foundation for subsequent engineering construction.