# Applications of Microseismic Monitoring Technique in Coal Mines: A State-of-the-Art Review
## 3 Interpretation of Mining-Induced Microseismicity
### Stress Inversion  
Deep coal mining inevitably leads to stress adjustments in the surrounding rocks: the abutment stress generally increases from the virgin stress regime with the progress of excavation and decreases to residual stress when the highly stressed coal mass exceeds its strength limit [2]. The redistributed stress is influenced by numerous contributing factors, e.g., the mechanical properties of coal/mass, the geometric characteristics of overlying strata, mining height and intensity, and geological structures. The stress around the longwall working face can be measured by stress meters embedded in the coal/rock masses [80]. However, describing the stress distribution using limited data from a few stress meters is exceedingly difficult. Understanding the high stress concentration zone is critically important to evaluate the risk of mining-induced instabilities. Seismic velocity tomography is an effective method for stress evaluation, which can be used to identify the impending coal/rock failure in coal mines.  
Rock deformation generates the ultrasonic P-wave and S-wave with different transmit velocities [81]. The variation of seismic wave velocity is often utilized to characterize the stress-level changes caused by mining [82]. Seismic velocity tomography is dependent on the relation that the seismic wave velocities in the rock medium (divided into discrete units) equals the transmission distance divided by the traveling time, as follows [83; 84]:  
\[V=\frac{L}{T}\to VT=L \tag{2}\]  
\[T_{i}=\int_{L_{i}}\frac{dL}{V(x,y,z)}=\int_{L_{i}}S(x,y,z)dL \tag{3}\]  
\[T_{i}=\sum_{j=1}^{M}d_{ij}S_{j}\;(i=1,\cdots,N) \tag{4}\]  
where \(V(x,y,z)\) is the seismic ray velocity, \(L\) denotes the transmission distance of the seismic ray, \(T\) denotes the traveling time of the seismic ray, \(S(x,y,z)\) denotes the slowness, \(d_{ij}\) is the distance of the \(i\)-th seismic ray through the \(j\)-th rock unit, and \(N\) and \(M\) are the numbers of the seismic ray and rock units, respectively.  
[PERSON] et al. [85] characterized the ability of seismic velocity tomography concerning the detection of geological discontinuity, the identification of stress change, and the assessment of rockburst hazard by applications of active and passive seismic velocity tomograms in longwall mining panels of the Yuejin coal mine, and a conclusion was drawn that the active seismic velocity tomography detects the geological discontinuity accurately, while the passive seismic velocity tomography works better in the cases of identifying the stress change and assessing the rockburst risk. [PERSON] et al. [81] applied compressional wave velocity tomography to invert the stress regime consisting of static and dynamic stresses and found that a sharp increase in the stress concentration factor is the stress precursor of the rockburst in the Junde coal mine. [PERSON] et al. [86] used seismic velocity tomography to recognize the high-stress region in the Xing'an coal mine, and they further employed large-diameter hole and deep-hole blasting as countermeasures to mitigate the rockburst risk in the high-velocity region (red region in Figure 6).