## 6 Conclusions  
This study implements a three-dimensional anisotropic numerical simulation of borehole-to-surface electromagnetic responses based on an unstructured-mesh vector finite element method. By leveraging unstructured tetrahedral meshes, the method effectively discretizes small-scale structures and complex geological formations while maintaining computational accuracy and significantly improving efficiency. The accuracy and effectiveness of the proposed algorithm were validated by comparing numerical solutions of a layered isotropic model with semi-analytical solutions and by benchmarking against MARE2 DEM solutions for a two-dimensional anisotropic model. The results further underscore the advantages of incorporating resistivity anisotropy over the conventional isotropic assumption. A comparative numerical study of anisotropic and isotropic reservoirs under varying production rates revealed that the average amplitude error is 42.49%, while the average phase error is 41.54%, highlighting the critical role of anisotropy in reservoir dynamic monitoring. In a three-dimensional geoelectric model, we simulated the BSEM response for \begin{table}
\begin{tabular}{c c} \hline \hline
**Depth (m)** & **Linear Fitting Curve Slope** \\ \hline
1200 & 7.8863 \(\times\) 10\({}^{-10}\) \\
1300 & 6.94106 \(\times\) 10\({}^{-10}\) \\
1400 & 6.15028 \(\times\) 10\({}^{-10}\) \\
1500 & 5.65149 \(\times\) 10\({}^{-10}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Linear fitting curve slopes of the maximum amplitude. Figure 20: Linear fitting curve of the maximum amplitude for the transmitter located below the reservoir. anisotropic reservoirs under different production rates and conducted a detailed analysis of the effects of varying transmitter depths on monitoring performance. The results indicate that the effectiveness of reservoir dynamic monitoring varies with the transmitter position. Numerical simulations demonstrate that placing the transmitter closer to the reservoir enhances monitoring sensitivity. When the transmitter is positioned above the reservoir, the received electromagnetic signals on the surface exhibit minimal influence from the reservoir, making precise dynamic monitoring challenging. However, when the transmitter is located at the upper boundary of the reservoir, a positive correlation between the BSEM response and production rate emerges only when the production rate exceeds 20%. Future studies can incorporate actual geological data to construct more realistic models for further in-depth investigations. The findings of this study provide theoretical and practical contributions to BSEM numerical simulations and offer technical support for oil and gas development and mineral exploration. Conceptualization, B.C. ; methodology, B.C. and H.C.; software, B.C. ; validation, B.C. and S.W. ; formal analysis, M.C. and S.W. ; investigation, R.M. ; resources, H.C.; writing--original draft preparation, B.C. ; visualization, B.C. and R.M. ; supervision, H.C., M.C. and R.M. ; project administration, H.C. All authors have read and agreed to the published version of the manuscript. This research was funded by the Sichuan Provincial Natural Science Foundation Key Project \"Study on Borehole-to-Surface Electromagnetic and Ground Electromagnetic Methods for Deep Oil and Gas Exploration in Northeast Sichuan,\" grant number 25 ZNSFSC0026; the National Key Science and Technology Major Project \"Borehole-to-Surface Electromagnetic Method for Deep Oil and Gas Evaluation Demonstration,\" grant number 2024 ZD1000206; and the National Natural Science Foundation of China General Program \"Frequency Domain Borehole-to-Surface Electromagnetic Method 3D Anisotropic Forward and Inverse Modeling and Physical Simulation Study,\" grant number 41974090. Not applicable. The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.