**3. The Field Data**

The passive seismic data acquisition area is located in a mining area in Sichuan (as shown in the blue square in Figure 1a), where Triassic feldspar quartz sandstone, Jurassic marl-dolomite-feldspar quartz sandstone, Cretaceous purplish red mudstone-siltstonesandstone, and Quaternary strata are exposed. Figure 1b shows a diagram of the geological structure of the mining area. The main structural direction of this area is north–northeast, followed by nearly east–west. Magmatic rocks in the mining area are mainly gabbro and diabase from the late Jinning period, which are exposed as rock masses and dikes of different sizes. The wall rock alteration is developed, among which the wall rock alteration related to mineralization mainly includes chloritization, silicification, and carbonization.

We arranged a two-dimensional passive seismic survey line, P18-P18, at the periphery of the mining area (as shown by the red dotted line in Figure 1b). The passive seismic data were continuously recorded for 4 days with a 1-millisecond sample interval using 120 node-type single-component geophone I-Nodal at 1-240 Hz natural frequency. The distance between each geophone was 12.5 m, and the total length of the survey line was 1.5 km. In order to avoid surface wind noise, the geophones were buried at depths of about 20 cm.

The maximum elevation difference of the terrain in the test area was 100 m, and the slope was gentle. The surrounding areas were sparsely populated and underdeveloped, with one or two village-level roads. There were many rivers nearby, and there were mining activities about 4 km northwest. Comprehensive analysis showed that the noise in this area mainly came from rivers, traffic, and mining activities, and mining activities may be the main source of prominent noise events. Figure 2 shows the daily noise data recorded by a geophone.

**Figure 1.** Location of the passive seismic survey line. (**a**) Location of mining area where data were collected (blue square); (**b**) geological structure of the mining area. The red dotted line P18-P18 indicates the passive seismic survey line.

**Figure 2.** Typical ambient noise data recorded by a geophone in the survey area.

There was very little noise from night to morning (0–6 h), with relatively low amplitude and stable change, whereas during the day, due to activities such as surface mining, the noise amplitude was relatively high and changed drastically, including some prominent high-amplitude noise events (red arrows). Therefore, in order to satisfy the hypothesis of a theoretical wavefield as much as possible, the original field data were processed.
