Study on cross-borehole resistivity CT method for detecting typical karst collapses in the Chongqing region
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摘要: 重庆地区的岩溶塌陷具有隐蔽性、突发性和难以预见的特点，常规地球物理方法难以实现精准探测。跨孔电阻率CT具有独特的观测方式和较高的成像分辨率，适用于岩溶地区的精细化隐患排查。文章在梳理和分析重庆地区岩溶塌陷演化机理、形成过程的基础上，构建了相应的电阻率模型，涵盖了暂未形成土洞、初步形成土洞和土洞进一步扩大三个阶段，并通过数值模拟获得了岩溶塌陷各演化阶段的电阻率响应特征。结果表明：跨孔电阻率CT在划分基岩界面的基础上，能够准确识别低电阻率的岩溶管道和土洞等异常体，精准地刻画异常区的空间分布，误差不超过0.5 m。实际案例表明：跨孔电阻率CT剖面能够直观反映岩层界面及隐伏塌陷的位置和范围，探测效果良好。该研究拓展了以地面电阻率法为主的传统岩溶塌陷勘探思路，为重庆地区岩溶塌陷的预警与防治提供了理论依据。Abstract:
Karst ground collapses in the Chongqing region are characterized by concealment, sudden occurrence, and unpredictability, making them difficult to detect accurately using conventional geophysical methods. Cross-borehole resistivity computed tomography (CT), with its unique observation geometry and high imaging resolution, is well-suited for the detailed detection of hidden hazards in karst regions. The evolution of typical karst collapses in the Chongqing region can be summarized into three stages: soil cave incubation, formation, and expansion. During the incubation stage, the soil above the bedrock remains intact and stable, with water levels in karst conduits fluctuating periodically due to rainfall but without significantly affecting the stability of the overlying layer. In the formation stage, activities such as tunnel construction cause a rapid drop in groundwater levels, leading to a sharp increase in negative pressure within the karst conduits. This results in increased seepage pressure and gradual loosening of the overlying soil, forming localized cavities. During the expansion stage, repeated suffusion and erosion cause the soil cave to enlarge further, the roof depth to decrease, and eventually, surface collapse occurs. Based on a review of the formation mechanisms of karst collapses in Chongqing, this study developed geophysical resistivity models for the three stages mentioned above. Finite element forward modeling and smooth inversion calculations were employed to verify the effectiveness of cross-borehole resistivity CT in detecting karst collapses at different evolutionary stages. In the models, electrode spacing was set to one meter, and borehole spacing was set to 10 m. Simulation results demonstrate that cross-borehole resistivity CT technology can clearly delineate the spatial distribution characteristics of karst conduits and soil caves at each stage, with inversion errors less than 0.5 meters. In practical applications, cross-borehole resistivity CT successfully identify the morphology of hidden collapses and their associated karst conduits, effectively overcoming the limitations of traditional surface resistivity methods in terms of vertical resolution and detection accuracy, thereby providing an efficient solution for detecting karst collapses in complex geological settings. This study expands upon the traditional methodologies for detecting karst collapses, demonstrating the significant advantages of cross-borehole resistivity CT in improving the accuracy of identifying hidden karst conduits and soil cavities. Future research should focus on refining the technical parameters of this method to enable dynamic monitoring and more precise interpretation of concealed karst collapses. Such advancements will provide critical support for the scientific prevention and mitigation of karst disasters in Chongqing, thereby contributing to the sustainable and safe development of the Chengdu–Chongqing economic circle. -
Key words:
- resistivity CT /
- karst collapse /
- resistivity method /
- numerical simulation /
- electrical prospecting
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图 1 重庆地区典型岩溶塌陷的形成演化过程示意图
Figure 1. Schematic diagram of the formation and evolution of typical karst collapses in Chongqing
图 2 跨孔电阻率CT工作示意图
Figure 2. Schematic diagram of operating cross-borehole resistivity CT
图 3 重庆地区典型岩溶塌陷演化的电阻率模型
Figure 3. Resistivity model of typical karst collapse evolution in Chongqing
图 8 钻孔间地质剖面和电阻率CT剖面
Figure 8. Geological profile and profile of cross-borehole resistivity CT
表 1 模型参数
Table 1. Parameters of the models
名称 土洞尺寸/m×m 土洞中心位置/m 土洞顶板埋深/m 岩溶尺寸/m×m 岩溶中心位置/m 模型a / / / 2.0×6.0 （10，11） 模型b 4.0×2.5 （10,6.75） 5.5 模型c 4.0×5.5 （10,5.25） 2.5 
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表 2 数值模拟基本参数
Table 2. Basic parameters of numerical simulation
模型尺寸/m×m 电极距/m 网格大小/m 测量最大重复误差/% 最大互换测量误差/% 反演最大均方误差/% 迭代次数/次 圆滑系数 10×20 1.0 0.5 3 5 3 30 100
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表 3 原始电阻率和反演电阻率数值对比表( Ω·m)
Table 3. Comparison of original and inverse resistivity values ( Ω·m)
正演/反演 岩溶塌陷阶段 模型 完整土体 土洞（土体松散区）、岩溶管道 灰岩 正演 / / 100 30 5000 反演 未形成土洞阶段 模型a 85.26~113.57（平均值：101.48） 30.28~69.25（平均值：41.99） 4799.82 ~5000 （平均值：4725.05 ）初步形成土洞阶段 模型b 79.16~118.71（平均值：98.22） 30.45~72.84（平均值：44.84） 4712.90 ~5000 （平均值：4794.06 ）土洞进一步扩大阶段 模型c 76.2~114.77（平均值：98.69） 30.05~56.08（平均值：38.56） 4636.55 ~5000 （平均值：4770.61 ）
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