## 3 Numerical Simulation of CSAMT
### Forward Modeling of the Jinding Lead-Zinc Geophysical Model  
The coupled finite element-infinite element method was used for the following numerical simulation and forward modeling calculation parameters:  
In the x direction, the subdivision area ranges from \(-3000\) to 15,000. Within the survey line area, the grid size is set to 50 m, resulting in a total of 121 nodes.  
In the y direction, the subdivision area ranges from \(-3000\) to \(3000\). Within the survey line area, the grid size is set to 50 m, resulting in a total of 63 nodes. Near the field source and the target body, the grid subdivision is relatively dense.  
In the z direction, the subdivision area ranges from \(-3200\) to \(3000\), with a total of 36 nodes. The air region spans from \(-3200\) to \(0\) and is subdivided into six layers. Due to the rapid attenuation of the field in underground media, the grid subdivision size is smaller, and the grid size is set to 25 m.  
Frequencies: 8192, 4096, 2048, 1024, 512, 256, 128, 64, 32, 16, 8, 4, 2, and 1 Hz, totaling 14 frequencies. The average calculation time for each frequency point is approximately 240 s.  
To demonstrate the advancement of the coupled finite element-infinite element method proposed in this paper. Table 2 shows the computational data of the traditional large-scale finite element method and the finite element-infinite element coupling method(computation frequency: 256 Hz). In Table 2, the coupled finite element-infinite element method has a small discrete area, accounting for only 1% of the traditional finite element method. The total number of degrees of freedom is reduced by 40%. In terms of memory, the finite element-infinite element method reduces memory usage by 57%. In terms of computation time, the finite element-infinite element method improves the calculation speed by 45%. Therefore, the coupled finite element-infinite element method has obvious advantages such as a small discrete area, fewer computational nodes, and a faster solution speed compared to the traditional large-scale finite element method.  
The numerical simulation results are presented in Figure 7. In Figure, from left to right, the vertical source-receiver distances of the five survey lines are 9600 m, 9800 m, 10,000 m, 10,200 m, and 10,400 m, respectively (Figure 3). From the apparent resistivity pseudo-section map, it can be observed that the CSAMT 3D forward modeling results exhibit a significant response to the low-resistivity dome structure of the Jinding lead-zinc deposit, providing a clear reflection of the structure's morphology. However, due to the boundary effects, the boundary between the dome structure and the high-resistivity limestone is relatively blurred. In addition, due to the low-resistivity shielding effect of electromagnetic methods, the relatively high-resistivity lead-zinc ore bodies show little response in the simulation. Based on the numerical simulation results, it can be seen that when exploring high-resistivity target bodies in low-resistivity surrounding rocks, the CSAMT electromagnetic response is weak, and the numerical effect is not significant. However, when exploring low-resistivity structures such as faults and fracture zones in high-resistivity surrounding rocks, the effect is very pronounced. Therefore, it is feasible to conduct CSAMT exploration research in the Jinding lead-zinc mining area to identify low-resistivity faults, dome structures, and other features within the ore deposit.  
Additionally, the forward modeling parameters used in the 3D CSAMT numerical simulation, such as grid size, vertical source-receiver distance, frequency band range, survey line length, and the three-dimensional size and location of the target body, can assist us in designing the field CSAMT acquisition parameters to achieve good exploration results.