## 2 Overview of the Experiment  
The experimental site is located in the north-central Jizhong depression, which is a graben basin that developed in the North China Craton from the Mesozoic to Cenozoic. The strata, moving from the lower to the upper, in the Jizhong depression include the Archaean Eonothem, Palaeoproterozoic Eonothem, Mesoproterozoic Eathem, Upper Proterozoic (including the Changcheng and Jixian systems), Paleogene System (including the Kongdian, Shahejie and Dongying formations), Neogene System (including the Guantao and Minghuazhen formations) and Quaternary System (the Pingyuan formation) [26]. The main lithologies of the Archaean and Palaeoproterozoic Eonothems are gneiss, with a resistivity of 100 \(\Omega\)m, the Mesoproterozoic Eathem and Upper Proterozoic are carbonates, with a resistivity of 517 \(\Omega\)m, the Paleogene and Neogene Systems are mudstones and sandstones, with a resistivity between 3 and 9 \(\Omega\)m, and the Quaternary System includes gravels, sandstones and clays, with a resistivity of 20 \(\Omega\)m [27; 28].  
We chose the Beijing-Guangzhou HSR (BGHSR) as the interference source for this experiment. BGHSR runs through the entire Jizhong depression, connecting Beijing and Guangzhou with a total length of 2298 km and an operating speed of 300 km per hour. There are nearly 200 trains that operate every day from 6 o'clock to 24 o'clock local time. A train passes by every few minutes. Its traction power supply system uses 25 kV, 50 Hz single-phase alternating current (AC) to provide electric energy to the train. The basic principle is that the traction substation changes the high voltage of the grid into 25 kV AC in the contact network. The electric locomotive receives current through the sliding contact of the pantograph-catenary system, flows into the rail through the high-voltage side of the vehicle-mounted traction transformer, and then returns to the traction substation from the rail to form the current loop [29]. The instantaneous value of the traction current is close to 1000 A, which greatly increases the rail current. Ideally, the rail current will return to the traction substation through the rail. However, rail aging causes leakage onto the ground and generates strong electromagnetic noise signals [11; 23].  
In the experiment, MT data were collected at locations situated 500 m, 1 km, 1.5 km, 2 km, 3 km and 5 km from the BGHSR near Sanlipu Village, Gaobeidian City, Hebei Province (stations 1, 2, 3, 4, 5 and 6). Then, at different distances and in different directions and geological conditions, we set up two remote reference stations (stations 7 and 8) at places where human interference is less significant and acquired data simultaneously (Figure 1). The distances between stations 1 and 7 and stations 1 and 8 were 280 km and 200 km, respectively.  
The instrument used was a MTU-5A magnetotelluric acquisition system produced by the Canada Phoenix Company. A nonpolarized electrode was used to receive the electric field signal, and an inductive magnetic sensor MTC-50 was used to receive the magnetic field signal. Each station is arranged in a north-south layout for five-component acquisition. The north and the east correspond to the x and y directions, respectively, and z is perpendicular to the ground (Ex and Ey are electrical fields, and Hx, Hy and Hz are magnetic fields). The acquisition time was approximately 22 h.