## 2 Materials and Methods  
### Research Method  
Mining activities change subsurface structures rapidly and continuously. These structural changes include changes in pressure distribution, subsidence and fault shifts. Changes in structure cause earthquakes of small to large magnitude. The hazard of microseismic activity in mining areas is correlated with several factors: pressure, rock mechanical strength and degree of rock heterogeneity [21]. Several physical parameters have been used to analyze underground mines, such as b-value, magnitude, rock stress, P-wave and S-wave velocity, etc. [5; 6; 22; 23]. Changes in velocity are an indication of changes in stress. Therefore, it is necessary to monitor velocity changes as proxies of the stress perturbations due to mining. One method that can be used to monitor the evolution of velocity is time-lapse  
Figure 1: Illustration of underground mine seismometer network design in an intrusion geological setting. The intrusion body is depicted in orange; red coloring indicates an open pit; the cave model is depicted in brown; seismometer stations are represented by the letter T; and Haulage Shaft (mining facility area) is indicated by double black lines.  
passive seismic tomography using microseismic data. Microseismic methods have been widely used in underground mining, especially in longwall mining [24; 25; 26].  
Determining the location of a microseismic event is the most important key in local tomographic studies. One of the methods that can be used to accurately locate earthquake events and describe local velocity structures is the double-difference tomography method [27]. This method can perform velocity and location inversion simultaneously [22; 23; 25; 28]. It assumes that should the distance between two adjacent earthquake sources be much smaller than the distance to the station, then the raypaths from the two sources are to be considered almost the same.  
We used the double-difference approach to assess the resolution and sensitivity of tomographic inversion in the geological setting of an inclined intrusion. This approach sharpens the results of the velocity structure near the earthquake source and produces a sharper velocity contrast than the standard tomography method [28]. [PERSON] et al. [25] performed a time-lapse double-difference tomography analysis using differential arrival time data from pairs of events in different periods. According to [PERSON] et al. [28], this can reduce some artifacts caused by differences in the distribution and number of earthquakes in a certain time interval. [PERSON] et al. [23] carried out tomography imaging of P-wave velocity in an underground mining area of the Yongshaba Deposit, Guizhou Province, China. They interpreted low-velocity anomalies in their velocity model as empty space, stress release, rock breaking and cracking. Similar results were also obtained from research in an underground nickel mining area in Ontario. An earthquake with a large magnitude causes stress release and reduces the velocity of wave propagation in the area around the earthquake [29].  
Sensitivity analysis with synthetic models is widely used in seismic tomography to assess the spatial resolution of the resultant solution [20]. The most commonly used type of synthetic reconstruction test is the CRT. CRT is a standard procedure at the beginning of tomography studies, in which synthetic resolution testing is carried out to determine the accuracy of the parameters used in inversion tomography [20]. The goal is to determine the confidence level of the tomogram obtained from the inversion results based on the distribution of sources and stations. Synthetic data of wave travel time are made from a velocity model, which, like a chessboard, has two patterns of higher and lower velocity. The area where an alternate pattern of high and low velocity can be recovered indicates the well-resolved area.