## 6 Discussion and Conclusions  
BGHSR noise has a fixed shape in the time domain waveform, and the amplitude is very large, which can be observed from the electromagnetic field. The interfering signals between each station or between all components of the same station are highly correlated, but the polarization directions are different. These features can provide a basis for removing the noise of HSR.  
From the definition of CSAMT, the high-frequency band of the HSR noise signals above 10 Hz is in the far field, which approaches the MT assumption of quasi-homogeneous fields and is included as part of the MT signals. Therefore, the signals in this frequency band are enhanced [23]. The mid-frequency band (10-0.01 Hz) enters the transition zone and near zone, causing the field source effect. There is no HSR noise in the low-frequency band, and the MT signals are restored to quasi-homogeneous fields.  
When station 7 or station 8 is used as an RR, the near-field interference is not effectively removed. In the frequency band near 0.1 Hz, the so-called \"dead band\" [42; 43], it is likely that the natural field signal in this frequency band will be weak, and the denoising ability of RR is also reduced, resulting in unreliable data.  
We analyzed the noise signals of the BGHSR in the time domain and frequency domain and found that they are mainly impulse, charging-discharging and triangular-wave noises. The distance of the interference exceeds 5 km, and the interference frequency band is mainly in the range of 10-0.01 Hz.  
The MRR method [34; 35] is better than the single-RR method [15; 33], since it can remove human interference more effectively and obtain high-quality MT data.  
Although the MRR method has an ideal effect in removing interference, it is not a panacea. When one data point in the remote reference data is not good or has less coherence with the station data, the result may be inferior to the single remote reference. In the future, we will consider how to select remote reference data of different frequencies, and the remote reference data frequency bands with poor denoising effects will not be involved in the calculation so as to improve the noise removal effect. The farther away the noise sources are, the smaller their impact on the data is. Therefore, it is also very important to arrange instruments in the field as far away as possible so as to avoid noise sources.  
Conceptualization, G.W.; data curation, W.W. and W.Z.; formal analysis, Y.Z.; funding acquisition, Y.Z.; investigation, Y.M., W.W., W.Z. and A.C.; methodology, G.W., Y.M. and Y.L.; project administration, G.W. and Y.M.; software, Y.L. and W.W.; validation, W.Z.; writing--original draft, G.W.; writing--review and editing, D.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key R&D Program of China (No. 2018 YFE0208300), China Geological Survey Project (DD20190556, DD20221639 and DD20230298) and the national nonprofit institute research grant of the Chinese Academy of Geological Science (AS2019J01).  
Not applicable.  
The authors thank the editors and anonymous reviewers who provided constructive comments to improve the clarity of this paper.  
The authors declare no conflict of interest.