断层－充水溶洞上不同装置三维高密度电阻率法正演模拟响应特征分析

高阻碳酸盐岩中的低阻断层和充水溶洞是岩溶区地下水勘查的主要目标体。根据高阻中找低阻的原则，建立碳酸盐岩类裂隙溶洞水赋存模式的断层－溶洞地电模型，利用RES3D软件进行温纳、施伦贝格、偶极－偶极三种采集装置的正反演模拟计算，从三维反演结果、剖面、电测深曲线三个维度，对比分析不同装置下断层－溶洞目标体的地电响应特征和规律。结果显示:(1) 探测深度内，温纳、施伦贝格、偶极－偶极装置可有效识别断层及其上盘三倍于电极距规模的充水溶洞，无法分辨断层下盘二倍于电极距规模的充水溶洞；(2) 相同模型和观测条件下，偶极－偶极装置对目标体的识别能力最强，其三维反演结果可识别溶洞下边界，剖面中形成左凸低阻圈闭异常，不同测深点的曲线类型、拐点、极值点与模型设计最为贴近，且拐点对应岩性界面，极值点位于地质体的中心深度。该工作对野外观测方式的选取和地质解译有一定指导意义。 

Response characteristics of forward modeling of 3D high-density resistivitymethod on different devices in the fault-water-filled cave

Low blocking layers and water-filled caves in high resistivity carbonate rocks are the main target bodies for groundwater exploration in karst areas. Three-dimensional imaging by high-density resistivity can obtain rich and high-precision geological anomaly information in any direction in the whole underground space of the survey area through multi-line and multi-direction observation, which is one of the most commonly used methods for water exploration of carbonate fissure caves. According to the principle of finding low resistance in high resistance, the water occurrence model of carbonate fissure karst cave in overlying karst areas is simplified into a combined geoelectric model of fault-karst cave, and the physical property parameters of various bodies are assigned according to the resistivity characteristics of common media in the karst area of south China. The specific parameters of the model are as follows: the surface overlying layer is cultivated soil with a thickness of 3 m and a resistivity of 100 Ω·m. In the fractured limestone with an underlying resistivity of 4,000 Ω·m, a tensile water-rich fault, with a dip angle of 78° and a width of 10m, is developed with a resistivity of 800 Ω·m. Water-filled cave 1, with a diameter of 15 m and a central burial depth of 17.5 m, is developed in the hanging wall of the fault, and Water-filled cave 2, with a diameter of 10m and a central burial depth of 15 m, is developed in the footwall. The resistivity of both caves is 25 Ω·m. Given the burial depth, resolution, working efficiency and other factors of the target body, the mesh of the 3D model is divided into x×y=50×30 electrodes. The number of layers is 12; the electrode distance is 5m; the measurement range of x direction is 0-245 m; the measurement range of y direction is 0-145 m. RES3D software has been used to perform forward and inverse simulation calculation of Winner, Schlumberger and Dipole-dipole acquisition devices. Inversion results within the range of detection depth are displayed in a three-dimensional manner, and XY horizontal slices are displayed on the model interface (Z=0 m, 3 m, 10 m, 25 m, and the maximum detection depth). To further explore the response characteristics of the model on different devices, XZ profile① across the karst cave and fault area (y=67.5 m), XZ profile② far away from the karst cave (y=7.5 m), and the sounding curves of different points (Ground sounding point I of Karst cave 1, Ground sounding point II of Karst cave 2, Ground sounding point III of Karst cave, and Ground sounding point IV far from the karst cave area) are extracted. The electrical characteristics of the three devices at and away from karst caves, the development depth and boundary of Karst caves 1 and 2, the inclination of faults, the burial depth at the top, the horizontal width, the inclination of the dip and the boundary of the hanging wall are analyzed. Through the three-dimensional forward and inverse calculation of the fault-water-filled karst cave, combining with the typical profile and electrical sounding curve, we analyze the response characteristics and laws of the target body in different devices and draw the following conclusions, (1) Within the detection depth, Winner, Schlumberger, Dipole-dipole devices can effectively identify the fault and the cave of the upper wall three times the size of electrode distance of Cave 1. The target body is a concentric circle of low resistance trap anomaly with green ribbon and color gradient. These three devices cannot distinguish the fault footwall of two times the size of the electrode distance of Cave 2. (2) Under the same model and observation conditions, the Dipole-dipole device has the strongest recognition ability for the target body. Three-dimensional inversion results can identify the lower boundary of the cave, and the left convex low resistance trap anomaly is formed in the profile. The curve types, inflection points and extreme points of different sounding points are closest to the model design. Inflection points correspond to the lithology of interface. And extreme points are located at the central depth of the geological body. Therefore, when using the three-dimensional high-density resistivity method for water exploration in the karst area of south China, we can prioritize Dipole-dipole observation method with the careful consideration of selecting the electrode distance. This study is of guiding significance for the selection of field observation mode and geological interpretation.