Siyuan Gong

Quantitative analysis of seismic velocity tomography in rock burst hazard assessment

In order to quantitatively evaluate the relationship between the tomographic images of P wave velocity and rock burst hazard, the seismic velocity tomography was used to generate the P wave velocity tomograms during the retreat of a longwall panel in a coal mine. Subsequently, a novel index (bursting strain energy) was proposed to characterize the mining seismic hazard map. Finally, the structural similarity (SSIM) index in the discipline of image quality assessment was introduced to quantitatively assess the relation between the bursting strain energy index images and the tomographic images of P wave velocity. The results show that the bursting strain energy index is appropriate for quantitative analysis and seems to be better for expressing the mining seismic hazard than the conventional map. The SSIM values of the future bursting strain energy compared with the P wave velocity and the current bursting strain energy reach up to 0.8908 and 0.8462, respectively, which illustrate that the P wave velocity and the bursting strain energy both are able to detect the rock burst hazard region. Specifically, seismic velocity tomography is superior to the bursting strain energy index in the detection range and the precision and accuracy of detection results.

Evolution of Stress Concentration and Energy Release Before Rock Bursts: Two Case Studies from Xingan Coal mine, Hegang, China

Since the first recorded rock burst occurred in England in 1738, more than 20 countries have reported rock bursts (Kabiesz and Makowka 2009; Ortlepp and Stacey 1994; Patynska and Kabiesz 2009; Patynska 2013; Uszko 2009), including Germany, South Africa, Poland, the Czech Republic, Canada, Japan, France, etc. In China, rock bursts have become a common safety issue in underground coal mining. The number of coal mines experiencing rock bursts has increased annually (Jiang et al. 2010; Li et al. 2015). To date, 142 coal mines in China have suffered rock bursts which resulted in large economic losses and heavy casualties. For instance, the rock burst on 15 March 2013 in Junde Coal mine, Hegang City, caused the closure of a 200 m gateway, trapped 24 people, and killed four (Lu et al. 2015). The rock burst on 3 November 2011 in Qianqiu Coal mine, Yima City trapped 75 people underground and killed 10 people (Li et al. 2015). The rock burst on 14 February 2005 in Sunjiawan Coal mine, Fuxin City, caused a serious gas explosion and killed 214 people (State Administration of Work Safety, State Administration of Coalmine Safety 2005). The temporal and spatial evolution of mining-induced tremors reveals the process of initiation, development, and expansion of micro-fractures inside the coal-rock mass together with energy accumulation and release. This process may develop to cause either a tremor or a rock burst (Li et al. 2014; Wang et al. 2013). Tremors, rock bursts, or both are more likely to be induced in high-stress regions and the energy released therein is much higher. Therefore, rock burst risk can be evaluated by locating high-stress, and high-energy, regions. Microseismic (MS) monitoring is applicable when detecting the location and energy of mining-induced tremors. Recently, seismic velocity tomography (SVT) has been widely used for inference of high-stress distribution zones in underground mines by introducing seismic signals received by MS monitoring systems. For instance, Luxbacher et al. (2008) and Hosseini et al. (2012, 2013) conducted SVT by introducing mining-induced seismic signals, and found that high-velocity regions agreed well with high-abutment stress regions as predicted by numerical modelling, both of which were observed to redistribute as the coalface advanced. Dou et al. (2012), Banka and Jaworski (2010), and Lurka (2008) conducted SVT at regular time intervals during longwall mining, and found that rock bursts, or strong seismic events (i.e. tremors in underground mining), mainly occurred in high-velocity regions. Meanwhile, the bursting strain energy (BSE) index, which views rock bursts as a process of energy accumulation and release in the coal-rock mass as mining activities disturb the in situ stress field, was proposed here to characterise the spatial distribution of tremors [refer to Cai et al. (2015) for more details]. It was found that the & Si-yuan Gong libcumt@163.com

Focal mechanism caused by fracture or burst of a coal pillar

As a regional, real-time and dynamic method, microseismic monitoring technology is quite an appropriate technology for forecasting geological hazards, such as rock bursts, mine tremors, coal and gas outbursts and can even be used to prevent or at least reduce these disasters. The study of the focal mechanisms of different seismic sources is the prerequisite and basis for forecasting rock burst by microseismic monitoring technology. Based on the analysis on the mechanism and fracture course of coal pillars where rock bursts occur mostly, the equivalent point source model of the seismicity caused by a coal pillar was created. Given the model, the seismic displacement equation of a coal pillar was analyzed and the seismic mechanism was pointed out by seismic wave theory. The course of the fracture of the coal pillar was simulated closely in the laboratory and the equivalent microseismic signals of the fractures of the coal pillar were acquired using a TDS-6 experimental system. The results show that, by the pressure and friction of a medium near the seismic source, both a compression wave and a shear wave will be emitted and shear fracture will be induced at the moment of breakage. The results can be used to provide an academic basis to forecast and prevent rock bursts or tremors in a coal pillar.

Tomographic imaging of high seismic activities in underground island longwall face

Anomalous information identification is a key issue for seismic hazard prevention in underground mining. Velocity tomograms can image the stress redistribution around coal face and provide better understanding of strata failure mechanisms. In this paper, based on microseismic events recorded during mining operation, passive tomographic imagings have been presented to assess strong tremor hazard and locate high seismic activity zones around an island coal face under super-thick strata. The zones of high velocity or velocity gradient anomalies have been found to correlate well with the distribution of strong tremors, indicating that velocity tomography is feasible for seismic hazard assessment and risk region division in underground mining.

Spatio-temporal assessments of rockburst hazard combining b values and seismic tomography

A better understanding of rockburst precursors and high stress distribution characteristics can allow for higher extraction efficiency with reduced safety concerns. Taking the rockburst that occurred on 30 January 2015 in the Sanhejian Coal Mine, Jiangsu Province, China, as an example, the mechanism of rockburst development in a roadway was analysed, and a combined method involving b values and seismic velocity tomography was used to assess the rockburst in both time and space, respectively. The results indicate that before the rockburst, b values dropped significantly from 0.829 to 0.373. Moreover, a good agreement between a significant decrease in b values and the increase of the number of strong tremors was found. Using seismic tomography, two rockburst risk areas were determined where the maximum velocity, maximumxa0velocity anomaly and maximumxa0velocity gradient anomaly were 6xa0km/s, 0.14 and 0.13, respectively. The high-velocity regions corresponded well with the rockburst zone and large seismic event distributions. The combination of b values and seismic tomography is proven to have been a promising tool for use in evaluating rockburst risk during underground coal mining.