## 4 Application  
### Engineering Background and Multi-Scale Grid Generation  
In this study, eight blasting events obtained by the MS monitoring system (Figure 3) in the Yongshaba mine (China) were selected for application test. In this system, there are 28 sensors: sensors T1 and T2 are triaxial sensors, sensors 1-26 are uniaxial sensors, and they are set at the 930 m level (12 sensors), 1080 m level (12 sensors), and 1120 m level (4 sensors) along the mining tunnels. The main working principles of the Institute of Mine Seismology (IMS) are shown as follows: firstly, a sensor can monitor the signal all the time. Then, the continuous signal is transported to its administering NetADC (network analogue-to-digital converter) through 4 core signal cable (uniaxial sensor) and 8 core signal cables (triaxial sensor), and the NetADC samples the signal 6000 times per second (it means that the sampling frequency of a microseismic signal that we used is 6000 Hz). After that, the NetSP (network seismological processor) does some pre-processing to the discrete signal, which comes from its administrated NetADC, including the detection of MS event signals based on threshold value. Finally, the detected discrete signal is transferred to the data exchange center, and then the data processing center. Based on microseismic signals, we can calculate microseismic event location, magnitude, focal mechanism, etc. Figure 3: Microseismic monitoring model and system structure diagram of the microseismic (MS) system installed at the Yongshaba mine. (**a**) Microsseismic monitoring model modified from [PERSON] et al. (2018). Figure 2: GSWI location errors under different velocity model smoothing scales for the blasting event 8. The red fonts correspond to the sensor id, green grid means the ground surface, dark orange line is the Jinyang road, blue lines are different mining levels, and light blue lines are the mining levels with sensor distributed. The spheres are locations of microseismic events; (**b**) System structure diagram of the MS system modified from [PERSON] et al. [53]. The green line represents 4 core optical fiber, pink line represents 4 core signal cable, and the bold pink line represents 8 core signal cable. The dip angle of ore body in the mine was about 30\({}^{\circ}\). The hanging wall of ore body is dolomite with a high stability, while crushed shale and sandstone were presented in the footwall, the rock mass of hanging wall and footwall have large differences in P-wave velocity [54,55]. After years of mining, a large number of low-velocity regions and high-velocity regions have been formed. Therefore, MS events including fault slip and caving collapse occurred frequently. It can be seen that there are high- and low-velocity anomalies, and the velocity difference can reach as much as 2000 m/s (Figure 4a). Therefore, it is very significant and necessary to carry out 3D waveform inversion-based MS location. Considering that a mine MS event is more likely to occur in the regions with large velocity contrast, the modeling space for mining area was divided into coarse-grid and fine-grid modeling regions according to the degree of velocity model variation and field mining data (dotted lines in Figure 4b show the boundary of the fine-grid range). The coarse grids with a dimension of \(20\times 20\times 20\) m\({}^{3}\) are outside the dashed lines, while the fine grids with a dimension of \(10\times 10\times 10\) m\({}^{3}\) were limited within the dashed lines, so as to reduce storage and communication time of the Green's function database. If we want to further model the wavefield database with higher frequency bands to improve location precision, finer grids can be refined in the small region near the coarse grid-based location point for new-stage MS-GSWI location. The multi-scale grid generation introduced above combining with the multi-frequency-band grid search strategy, as described in Section 2.4, can gradually optimize the location results and keep a reasonable amount of Green's function storage and computational cost.