## 3 Detection of Coal Burst Risk Active-Passive Seismic CT
### Implementation of Active-Passive CT  
#### 3.2.1 Active-Passive Seismic Wave CT Inversion at the Beginning Stage  
According to the on-site conditions of LW23908, before the workface excavation, the geophone receivers and the artificial blasting points were all arranged in the roadways, a total of 22 geophones, of which 12 were installed in the headgate, and the others in the tailgate were used with an interval of 25 m, and two substations were placed in the chambers of the two entries, respectively. Blasting is designed as the active source in both entries, also with an equal interval of 25 m and 300 g explosive charges were used at each point. To increase the seismic ray's coverage area and inversion accuracy, the mining tremors were also introduced and selected for the inversion with energy greater than 1000 J, mostly in the range of \(10^{3}\) to \(10^{5}\) J, as shown in Figure 4. All the geophones will move forward as the mining advances to a certain distance, and the sources used for inversion are mainly mining tremors and a few actively blasting as auxiliary; therefore, we can get the distributions of the seismic wave during the excavation at different periods to understand the evolution of the stress state with a fairly high accuracy.  
As KJ 470 cannot calculate the energy of the tremors, so, first, the mining-induced tremors were located and calculated energy by the SOS microseismic system using the constant velocity model. When we obtained the basic information about the mining-induced tremors, we chose file directories of those with relatively high energy and clear P-wave arriving time. Then, in KJ470, we combined the blasting tremors and mining tremors to calculate the velocity and evaluate the reliability based on the first inversion;  
Figure 4: Schematic layout of active–passive detection.  
Figure 3: The schematic diagram of the dual-source CT system.  
the obtained velocities were used to calibrate the locations of the mining-induced tremors and analyze the errors, and the new locations were used as inversion sources; this process continued to loop, and ultimately, an optimal speed solution can be obtained. The location error of the mining tremors in LW23908 was about 10 m on average, based on the deep hole blasting verification.  
From the time the mining begins, the first weighting of the main roof is called the beginning stage of the longwall face. The first active-passive CT detection was carried out on 18 March 2022, when the LW23908 had mined 24 m; a total of 24 blasting points and more than 70 mining tremors with energy exceeding 1000 J were used as the sources, no matter whether they were in the roof strata or in the coal seam. Figure 5 shows the ray grid generated by all the effective and reliable seismic events. The inversion grid was 10 m times 10 m. Finally, we obtained a 3D velocity distribution, but we only focused on the velocity in the coal seam, so we sliced the P-wave velocity in the coal seam to determine the high-stress areas, which were high rockburst areas correspondingly. Moreover, Figure 6 shows the slice of the P-wave velocity in the coal seam, and the high-velocity areas can be identified, which represent the relatively high-stress areas based on the positive correlation between each other [33; 34; 35; 36]. In the beginning phase of the mining before the first weighting of the main roof, the high-stress area is located around the workface as the result of the stress redistribution and concentrated on the pillars. The envelope of the high seismic velocity is consistent with the theoretical analysis. The inversed velocity also indicates that the front and side abutment pressures of LW23908 are about 100 m and 30 m, respectively.