## 5 Conclusions  
This study systematically investigates borehole-to-surface electromagnetic method (BSEM) imaging technology, focusing on 3D forward algorithms, 3D geoelectric model construction, and field application scenarios. By developing frequency-domain 3D forward modeling for BSEM imaging and integrating forward simulation and surface response analysis, this research validates the effectiveness and reliability of this technology. The key conclusions and insights derived from this study are summarized as follows:  
* Based on the developed frequency-domain borehole-to-surface electromagnetic 3D forward algorithm, this paper discusses the optimal method for simulating the transmitter source and conducts correctness verification of the program. A simulation method for constructing complex line sources using the borehole trajectory data of actual wells is proposed. By decomposing highly deviated wells and horizontal wells into vertical and horizontal line sources, the influence of different observation modes with varying transmitter source (B-pole) positions on surface responses is studied, proving that placing the B pole along the borehole trajectory extension direction is the optimal observation mode. * The surface responses of line sources at a certain depth underground are studied, verifying the effectiveness of the frequency-domain borehole-to-surface electromagnetic observation system, and it is concluded that the optimal surface receiving data is the _Ex_ and _Hy_ components. For fracturing operations, this study verifies the effectiveness of borehole-to-surface electromagnetic method monitoring through borehole-to-surface observation modes and forward numerical simulations. In the simulation of fracturing operations in complex undulating formation models, the fracturing fracture bodies are set, proving the high observation accuracy of surface differential responses of the frequency-domain BSEM method. * This study verifies the high sensitivity of the frequency-domain BSEM method to monitor changes during the fracturing process through forward simulation cases. By  
Figure 14: Simulation results of _Ex_ surface response for the 28 th section fracturing: (**a–c**) frequency: 1.25 Hz, (**d–f**) frequency: 2.5 Hz. selecting surface electromagnetic field component data at appropriate monitoring frequencies, the development of fractures and fracturing effects can be understood in real time. These research results provide a scientific basis and practical guidance for the setup of borehole-to-surface electromagnetic systems in different scenarios. In summary, the application of BSEM imaging in dynamic monitoring of actual wells--including water or gas injection, gas storage reservoir operations, and hydraulic fracturing--demonstrates efficiency in subsurface dynamic characterization, highlighting its broad prospects for deep borehole monitoring [55; 56; 61]. Writing--original draft, validation, methodology, and formal analysis, Q.Y. ; writing--review and editing, funding acquisition, and conceptualization: Q.Y., M.T. and J.Y. ; review and editing, analysis, and interpretation: Y.Z., B.W., X.T., H.Z. and P.D. All authors have read and agreed to the published version of the manuscript. This work was supported in part by the National Key Research and Development Program of China (grant No. 2023 YFC3008901), the National Natural Science Foundation of China (grant Nos. 42230811, 42430810, 42174149), the National Science and Technology Major Project (grant Nos. 2024 ZD1000403, 2024 ZD1002703), and the Director's Fund of the Marine Oil and Gas Exploration Engineering Research Center (2024). Not applicable. The raw data supporting the conclusions of this article will be made available by the authors on request. The authors would like to thank China University of Mining and Technology for providing the facilities required for this research. The authors would like to thank Sinopce Research Institute of Petroleum Engineering Co., Ltd. for providing the model data required for this research.