Application and effectiveness of high-density electrical method in the karst ground subsidence zone of Laiwu

SUN Hongjie; WANG Zhihui; ZHAO Zhenhua; LUO Zhenjiang

doi:10.11932/karst20250609

Abstract：Frequent karst subsidence in China's carbonate regions poses a significant geological hazard that requires urgent attention.This issue often arises from excessive groundwater extraction for domestic,irrigation,industrial,and mining purposes,inducing declining water levels and sharp fluctuations that exacerbate of karst subsidence.The resultant risks include severe safety concerns and substantial property damage for residents in affected areas,posing a serious threat to personal safety and infrastructure development.Since the 1970s,karst collapse incidents have been frequent in Laiwu District,Jinan City,causing significant risks and property damage to nearby inhabitants.Therefore,focusing on Laiwu as the research area is crucial for conducting surveys of karst subsidence regions using physical exploration methods and establishing a comprehensive karst geological database.This database is instrumental in identifying vulnerable areas prone to karst subsidence and developing effective preventative and control measures.Given the higher prevalence of karst collapse risks near fracture tectonic zones,a detailed understanding of karst development features beneath known collapse areas and within fracture zones is essential for a comprehensive assessment of karst collapse characteristics. Located in the central mountainous region of Shandong Province,Laiwu is surrounded by mountains,with a broader expanse of terrain in the west,presenting a basin-like tilt from east to west.Geologically,the area can be categorized into four types:erosion-tectonic middle mountainous areas,erosion-tectonic low mountainous zones,erosion-tectonic hilly regions,and intermontane plains.The surface-exposed strata and borehole data in and around the study area predominantly consist of Paleozoic Cambrian,middle and lower Ordovician,Carboniferous,Permian,Mesozoic upper Jurassic,Cenozoic Paleocene,and Quaternary formations.The study zone is delineated into northern and southern sections.The northern section encompasses the Dawangzhuang-Zhailizhen area,while the southern section extends from Mengjiazhuang village in the east to Xiquanhe village in the south,and from Xiquanhe village in the west to Menggongqing village in the north.Topographically,the study site features a northward-protruding semicircular basin with gentle slopes descending from east to west,characterized by mild gradients in the south and steeper inclines in the north.The terrain inclines northward,eastward,and southward,converging toward the basin's center to form a horseshoe shape that opens westward,bisected by the east-west course of the Dawenhe River running through the central basin.The local climate is classified as a mid-temperate continental monsoon climate,exhibiting distinct seasonal changes with cool springs and autumns,cold winters,and hot summers.Significant variations in annual rainfall and its regional distribution are observed,with higher precipitation recorded in the southeast and northwest.The primary rivers in the study area include the Mouwenhe River and the Zhailihe River,along with the Fangxiahe River,the Simahe River(a tributary of the Mouwenhe River),and the Zhifenghe River(a tributary of the Zhailighe River).These tributaries exhibit seasonal flow patterns and short courses,experiencing rapid water level fluctuations during rainy periods and reduced flow or potential drying during dry spells. This investigation aims to evaluate the effectiveness of three geophysical exploration methods-the geo-radar,shallow seismic,and high-density resistivity-in the Laiwu karst collapse area.The main findings are summarized as follows,(1)The Schlumberger device exhibits a strong performance across four models,boasting exceptional horizontal and vertical resolution and effectively delineating the location,shape,and size of target bodies.Therefore,in the vicinity of concentrated ground collapse in Laiwu,Shandong Province,we deployed a network of Schlumberger devices for measurements,utilized RES2Dmod for data modeling,and conducted numerical simulations.(2)Several fracture tectonic zones were identified,along with karst regions characterized by low-resistance anomalies and various filling conditions.Both 2D and 3D results highlighted the method's effectiveness,particularly in detecting hidden karst formations and infill materials,thereby providing a foundational and comparative framework for engineering case analyses.(3)Multiple fracture configurations were revealed through drilling,including those intersected by all measurement lines,confirming the robustness of the high-density resistivity method in detecting such structures.This method demonstrates distinct anomaly response characteristics,delivering high precision and accuracy.(4)The Schlumberger device's excellent vertical and horizontal resolution offers significant practical value for detecting and predicting potential ground collapses in karst development regions.Rigorous verification through joint borehole assessments enhances the study's credibility,indicating that the subsidence may result from diagonal erosion-induced migration of sand and soil particles from overlying layers into adjacent karst conduits or voids,ultimately leading to collapse pit formation. In conclusion,this study offers the following recommendations,(A)In karst collapse-prone areas,regular inspections are advised to detect subsurface voids beneath roadways.(B)Expanding the scope of gravity and magnetic surveys is encouraged to better characterize the distribution of fracture tectonics across the area.