# Research on Grouting Dynamic Monitoring Based on Borehole-Tunnel Joint Resistivity Method
## 3 Resistivity Monitoring
### Borehole-Tunnel 3D Cross-Array Layout  
The borehole electrode spacing was determined through 3D numerical simulations and field calibration tests, taking into account the electric field gradient error and computational time consumption. A uniform spacing of 20 m was ultimately adopted. The monitoring focused on the F1 fault grouting zone. Two planar monitoring lines (Lines 1 and 2) were established in the coal face roadways to form a two-dimensional monitoring network. Line 1, deployed in Roadway 1, included 38 electrodes with 20 m spacing, spanning 1000 m. Line 2, in Roadway 2, consisted of 32 electrodes spaced 20 m apart over 640 m. To capture the three-dimensional diffusion of grout within fracture networks--unattainable with 2D configurations--a borehole monitoring line (Line 3) was added. Line 3, coupled to the borehole wall via drilling mud, incorporated 17 electrodes at 20 m intervals, covering 340 m. The hybrid 2D-3D system layout is illustrated in Figure 3.  
A three-tiered acquisition system was employed: Line 1 served as the transmitter while Lines 2 and 3 acted as receivers, enabling cross-transmission data collection to enhance spatial resolution and fracture characterization. Figure 3: Schematic diagram of the tunnel–borehole 3D cross-line layout. Figure 2: Schematic diagram of the coal mine working face resistivity monitoring system. The monitoring electrodes are strategically deployed in Working Face Entries 1 and 2. To optimize monitoring efficacy, steel electrodes must be driven into the floor strata. Electrode installation follows these specifications:  
1. Spatial configuration: Positioned along the retreating side of entries, commencing from the face line towards the portal direction. 2. Geometric parameters: 30 cm length electrodes spaced at 20 m intervals, achieving \(\geq\)0.2 m embedment depth while maintaining \(\geq\)0.1 m protrusion above floor surface (Figure 4). When deploying cables in boreholes, the process involves three key technical considerations:  
1. Electrode Configuration The first step is to determine the connection method of the electrode string within the borehole. Exposed copper cylinders (with diameters larger than the cable's outer diameter) can be installed at fixed intervals along a multi-core cable to serve as grounding media. Coupling with the borehole wall is achieved through water or mud injection. 2. Cable Parameter Optimization Critical parameters--such as cable length, diameter, and spacing between electrodes--must balance detection range, data density, and deployment feasibility. Simulations and field tests indicate that an electrode spacing of 10-20 m optimizes data resolution and material efficiency, avoiding excessive core counts while maintaining detection accuracy. 3. Performance Requirements The borehole cable must meet stringent specifications for pressure resistance, waterproofing, and tensile strength to withstand deployment challenges. A schematic of the borehole monitoring cable is illustrated in Figure 5.  
The key challenge in downhole electrical monitoring technology lies in effectively deploying multi-core cables within deep boreholes. Current directional boreholes in coal mine roofs and floors typically have significant depths ranging from 300 to 800 m, rendering conventional manual shallow-hole pushing methods impractical and necessitating the development of specialized cable deployment techniques capable of reaching over 500 m. Drawing from existing long-hole cable installation technologies, the hydraulic propulsion system used in wireline coring demonstrates particular suitability. This optimized solution employs \(\varphi\)73 mm large-bore flush-joint drill pipes as delivery channels with modified auxiliary equipment, incorporating three critical innovations: The flush-joint wireline coring  
Figure 4: Cross-sectional schematic of entry electrode array.