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To truly reflect changes in apparent resistivity before and after hydraulic fracturing (HF) of coal and rock masses in underground spaces, the fracturing effect should be investigated and evaluated. In this paper, experiments were conducted on true triaxial HF in rocks to investigate the apparent resistivity response law. The results show that the change in apparent resistivity was regular and uneven in the process of HF. The combined action of hydraulic cracks and fracturing water were the decisive factor in the apparent resistivity change. According to the time-sequence characteristics of apparent resistivity, the crack initiation sequence and spatial location can be reflected intuitively. The abrupt change and abnormal fluctuation in apparent resistivity near the cracks are important for crack initiation and propagation. Due to the difference in hydraulic crack shape and scope, the influence of cracks and fracturing water differs on different directions of rock. Therefore, the variation trend of apparent resistivity was different along various survey lines. After three-dimensional stress unloading, the water content in the cracks was affected by the crack opening, water storage space and drainage capacity, resulting in an obvious regional variation in apparent resistivity. The changes in apparent resistivity characteristics of true triaxial HF contain rich information about the fracture process, which is significant for guiding on-site HF and effect evaluation.

Rock breaking technique plays an important role in rock excavation engineering, such as mining, tunnel construction and hydropower engineering. Pneumatic fracturing with carbon dioxide (CO2) ice powder is potentially a safe and efficient rock breaking method, which is one of the most pertinent research topics in rock breaking. A site test of pneumatic rock breaking with CO2 ice powder was carried out on a tuff hill with a compressive strength of 80 MPa. The attenuation of peak particle velocity (PPV) induced by pneumatic rock breaking with CO2 ice powder was studied and the vibration effect on surrounding buildings was evaluated. The research results demonstrate that: (1) the pneumatic rock breaking with CO2 ice powder is greatly affected by quasi-static pressure of high-pressure gas with many macrocracks formed near boreholes; (2) with increase in distance, the PPV decreases in the form of a power function with dominant frequency less than 100 Hz, And when the charge of CO2 ice powder is 4 kg, the PPV reduced down to 21.9 mm/s at the distance of 2.8 m; (3) a simplified motion model of vibration waves propagating in slope is established, showing that the steeper the slope, the faster does the PPV attenuate; and (4) pneumatic rock breaking with CO2 ice powder is safer than traditional liquid CO2 and explosive blasting, and it can significantly improve rock breaking efficiency.

The pressure stimulated currents (PSCs) and acoustic emissions (AEs) simultaneously emitting from rocks during deformation and fracturing are gradually receiving attention for rock dynamic disaster monitoring in underground excavations (mines and tunnels). For the work described in this paper, PSC signals and AE counts were recorded simultaneously during the multi-stage loading of two types of rock samples, sandstone, and gabbro. The PSC’s and AE’s responses to stress changes are studied and compared. The AEs are analyzed to investigate the changes in microcracking and energy dissipation in the two lithologies during uniaxial compression. Using an atomic force microscope, a polarized light microscopy, and a scanning electron microscope, the PSC electrification mechanisms and their correlation with AE responses are discussed. The results show that PSC signals respond more quickly to stress changes than AEs; this is especially true for the sandstone because it has fewer microcracks than the gabbro during the early loading stages. Both the electrification mechanisms and energy dissipation are affected by differences in mineralogy and microcracking, and these differences could account for the different PSC and AE responses observed in the sandstone and gabbro. Both the PSCs and AEs increase as rock damage increases, and the PSC-AE activity can provide warnings before catastrophic rock failures. Microstructural analysis reveals that mobile dislocations and microcrack propagation are joint mechanisms for generating simultaneous PSC and AE signals. This study offers critical insights into simultaneously using PSC and AE responses for geological disaster monitoring and forecasting in deep underground mines.

In a reliability assessment of hydraulic fracturing, the efficacy of hydraulic fracturing must be evaluated, and the source properties and magnitude of micro-earthquakes induced by hydraulic fracturing in a coal mine need to be quantified. Firstly, the changing trends of the number and energy of micro-seismic events induced by hydraulic fracturing with high-pressure water were examined, and the hydraulic fracturing effect was evaluated by micro-seismic location. Then, the source parameters (seismic moment M0, source radius R, corner frequency f0, radiated seismic energy E, stress drop △σ, and apparent stress σa) were calculated using the Brune model and the grid-search method, and the relationships between M0 and other source parameters were analyzed. Finally, the source parameters between micro-earthquakes induced by hydraulic fracturing and those induced by mining were compared, and the relationship between Mw (moment magnitude) and ML (local magnitude) of a micro-earthquake induced by hydraulic fracturing was ascertained. The results show that most micro-earthquakes occur in the initial stage of water pressure increase during hydraulic fracturing. In addition, the occurrence of several micro-earthquakes does not correspond strictly to the significant decrease of water pressure; the locations of micro-earthquakes can be used to evaluate the effect of hydraulic fracturing. The source parameters of micro-earthquakes induced by hydraulic fracturing in the Xieqiao coal mine can be obtained from the Brune model and the grid-search method. Corner frequency f0 decreases with increase in seismic moment M0, source parameters of R and E, as well as △σ increase with increase in seismic moment M0, but the apparent stress σa fluctuates around a certain value; the relationships between the other source parameters and M0 of micro-earthquake induced by hydraulic fracturing are consistent with that of a micro-earthquake induced by coal mining. Considering Mw, ML1 can be better used to calculate the magnitude of micro-earthquakes induced by hydraulic fracturing. Through Mw and ML1, the magnitude of a micro-earthquake induced by hydraulic fracturing can be evaluated. Our results improve the evaluation of the hydraulic fracturing effect, the understanding of source properties, and evaluation of the magnitude of micro-earthquakes induced by hydraulic fracturing. It is found that these micro-earthquakes are within the safe controllable range, and the hydraulic fracturing process of the Xieqiao coal mine is reliable and safe.

The fracture-induced electromagnetic radiation (FEMR) method is a promising geophysical method for the monitoring and early warning of coal-rock burst disasters. At present, the time-series characteristics of FEMR data are primarily used for rock burst monitoring and early warning. However, these rock burst precursor signal identification and hazard warning methods need further development. The progress made in the field of deep learning provides a new method for the identification of rock burst warning signals and the realization of intelligent early warning. In this paper, based on a deep learning algorithm, a method for identifying rock burst precursor FEMR signals is proposed. Based on bidirectional long short-term memory recurrent neural networks, this method trains and validates the model by analyzing a large number of normal FEMR signals and rock burst FEMR precursor signals. The input of the model is the FEMR data sequence, and the corresponding output is the hazard identification result. Upon the completion of training and validation, the model can perform automatic/intelligent shock hazard precursor signal recognition and quickly and accurately provide rock burst hazard early warning without requiring parameter adjustment and manual intervention. The results obtained showed that the rock burst precursor signal recognition method based on the recurrent neural network responds well to the rock burst hazard and can capture information regarding impact hazards in advance. Therefore, it is of great significance with regard to accurate rock burst monitoring and early warning.

To evaluate the vibration effect caused by carbon dioxide phase-transition fracturing, the Hilbert–Huang transform was used to study the change of vibration energy with distance in different frequency bands. The results are drawn as follows: (1) The peak particle velocity (PPV) decreases as a power function with an increase in distance and has fallen below 25 mm/s at 2.8 m. (2) The energy of vibration signals induced by carbon dioxide phase-transition fracturing is mainly distributed at the frequency band of 10–50 Hz. With the increase in distance, the energy distribution of vibration signals falls in four phases: Propagation to high frequency (0–13.9 m); a rapid high-frequency energy decrease (20–30 m); energy fluctuation (30–47.2 m) and a stable energy distribution (larger than 50 m). (3) The proportion of the low-frequency vibration energy (0–10 Hz) increases as a result of the increase in distance (less than 8.7 m), which should be paid more attention.

Some industrial activities, such as underground mining, hydraulic fracturing (HF), can cause microearthquakes and even damaging earthquakes. In recent years, with the rapid development of mining and HF, many induced damaging earthquakes have been reported and studied. The seismic waves contain rich information of the seismic sources and wave propagation path, which are the valuable data for studying the natural and induced earthquakes. Therefore, the non-linear and time–frequency characteristics of different seismic waveforms are studied by using multifractal theory and the Hilbert–Huang transform method. In general, for both the natural and induced earthquakes, the larger the magnitude is, the longer the duration of the waveform is, and the lower the frequency is. In addition, the amplitude and frequency of natural and induced seismic wave show non-linear decreasing trend with the increase in propagation distance. Both the natural and induced earthquakes’ waveforms show clear multifractal features. The multifractal parameters Δα and Δf(α) can be used as indicators to reflect the non-linear characteristics of the seismic waveforms. The seismic waveforms’ multifractal and time–frequency characteristics recorded by the specific sensors are affected by not only the magnitudes, types and rupture mechanism of seismic sources, but also the distances from sensors to seismic sources, wave propagation path and the sensors' properties. Because the coal is soft and unconsolidated rock with developed joints and fractures, the seismic waveforms induced by mining are more complex and variable than the ones with the same magnitude induced by HF of shale and hot dry rock.

Mining operations continue to advance to greater depths to address the demand and supply of coal. However, deep mining occurs in complex and dangerous environments where mines are often subject to rockbursts induced by high-hard roof breaking (HHRB). Here, we use the #2233 working face of the Hengda coal mine as a case study to investigate the mechanism of HHRB-induced rockbursts by means of stress, electromagnetic radiation (EMR) and hydraulic support resistance. We propose an elastoplastic plate theory model to study the relationship between impact load energy and maximum roof deformation and verify the dynamic evolution of rockbursts generated by HHRB by FLAC3D numerical modeling. The results show that rockbursts are more likely to occur when the coal-rock mass under high static stress conditions is subjected to superposition of continuous dynamic stress. This is accompanied by an abrupt increase in stress, increase in hydraulic support resistance, accumulation of strain energy that is not easily dissipated and a decrease in EMR intensity. The peak dynamic stress value determines the deformation mode of the elastoplastic plate model, and the impact damage behavior of the coal-rock mass is the most apparent, whereas the maximum deformation displacement lags slightly behind the arrival time of the peak stress. The occurrence of rockbursts is accelerated by long-term and high-amplitude dynamic disturbances.

During the process of coal road excavation, various interference signals, induced by environmental noise, drilling, and scraper loader, will affect the risk assessment of coal and gas outburst using acoustic emission (AE) and electromagnetic radiation (EMR) monitoring technology. To distinguish between different interference signals and danger signals, discrete wavelet transform (DWT) was used to decompose and reconstruct signals, and continuous wavelet transform (CWT) was used to obtain the time-frequency plane. The research results show that: (1) interference signals generally exhibit fluctuating changes within small ranges; in comparison, the intensity of AE and EMR signals caused by coal and rock fracture is found to continuously rise for a long period (longer than 2 h). (2) Different interference signals and danger signals differ significantly in their time-frequency plane. (3) Through decomposition and reconstruction of original signal, obvious precursor information can be found in the time-frequency plane of reconstructed signals.

Determining the location of the rupture source is the greatest advantage of the acoustic monitoring method, whose accuracy has a significant impact on monitoring the stability of complex rock mass environment. The traditional location method uses equivalent velocity to determine the source parameters generally, but equivalent velocity is a function relating to source and sensor position, and medium parameters, etc. For a complex rock mass environment, there might exist relatively large error using equivalent velocity to determine the source parameters. To improve the source location accuracy, the source location method based on Dijkstra’s algorithm (SLMD) is studied in this paper, which can take full advantage of the velocity information in a complex rock mass environment. Based on SLMD location characteristics, a new source location method (Average Source Location Method, ASLM) is proposed. Combined with two-dimension plane layered models, it is inferred that the location accuracy of ASLM is evidently higher than SLMD. Even if SLMD with high-density grids is used, ASLM still has pronounced advantage of location accuracy and time complexity. For further validation of obtained results through ASLM and conclusions inferred, complex rock mass environment models are used in the laboratory experiment. The research results show that ASLM has a more evident advantage over the Simplex method and Double-Difference method under complex rock mass environments. This research provides a new source location method for the determination of the source parameters, which has significant theoretical and practical meaning for monitoring the stability of complex rock mass environment.

The differences between mechanical properties and acoustic emission (AE) and electromagnetic radiation (EMR) characteristics of natural coal samples and saturated coal samples were analyzed by performing indirect tensile experiments. The experimental results show that coal samples go through four stages: compaction, elastic deformation, plastic deformation and failure. There is good correspondence between AE and EMR signals and the damage to coal samples. Under the action of water, tension strength of samples is reduced, while the plasticity is enhanced; also, the softening coefficient of tensile strength becomes 0.65. The saturated coal samples have a longer plastic stage and a more obvious AE quiet period. The damage to natural coal samples is tension damage, while that of saturated coal samples is due to tension and shear damage, which is more sufficient and irregular. EMR is still remarkable when AE is in a quiet period, and EMR is better for the precursor of rupture. Water weakens the generation and propagation of AE and EMR signals, especially in the earlier stage. AE and EMR damage factor D, defined by AE and EMR counts, has a better description of the damage degree in the indirect tensile process. This study is of great significance for research on the damage mechanism of water-bearing coal, the stability monitoring of water-bearing coals in the actual engineering process and the effect evaluation of hydraulic flushing.

In this study, we established a dynamic ejection coal burst model for a coalmine roadway subject to stress, and held that the stress concentration zone at the roadway side is the direct energy source of this ejection. The formation and development of such burst undergoes three stages: (1) instability and propagation of the cracks in the stress concentration zone, (2) emerging of a layered energy storage structure in the zone, and (3) ejection of coal mass or coal burst due to instability. Moreover, we figured out the initial strength of periodic cracks is parallel to the maximal dominant stress direction in the stress concentration zone and derived from the damage strain energy within the finite area of the zone based on the Griffith energy theory. In addition, we analyzed the formation process of the layered energy storage structure in the zone, simplified it as a simply supported restraint sheet, and calculated the minimum critical load and the internally accumulated elastic energy at the instable state. Furthermore, we established a criterion for occurrence of the coal burst based on the variational principle, and analyzed the coal mass ejection due to instability and coal burst induced by different intensity disturbances. At last, with the stratum conditions of Junde Coalmine as the model prototype, we numerically simulated the load displacement distribution of the stress concentration zone ahead of the working face disturbed by the main roof-fracture-induced dynamic load during the mining process as well as their varying characteristics, and qualitatively verified the above model. Keywords: Coal and rock burst, Evolution model, Dynamic ejection, Stress concentration, Layered energy storage structure

Previous studies indicate that an electric potential (EP) signal is generated during the loading process of coal and that the EP response is related to the damage evolution. When coupled with gas, EP changes the pore structure and mechanical properties of a coal mass, promoting crack generation and growth and accelerating damage evolution. To study the EP response characteristics and investigate the damage of gas-bearing coal, a triaxial test was carried out with a gas-controlled confining pressure, and multiple types of data were measured and analyzed. The results show that with the change in stress, the EP response increases and fluctuates. This response reflects the stress and reveals the damage evolution, which could be verified with the variation in the acoustic emission response. For the mechanism analyses, the failure of the sample is caused by crack expansion and propagation under the coupling action of stress and gas. Consequently, microscopic charge separation and electron emission are the dominant mechanisms controlling the EP response. Furthermore, the constitutive damage equation of gas-bearing coal is established based on the EP response in view of continuous damage theory and the stress intensity distribution hypothesis. The calculation results of damage and stress based on the EP response are utilized for verification; the results indicate that the damage expressed by the EP response is reasonable and useful. This finding is helpful for understanding the damage evolution mechanism of gas-bearing coal.

3D thermo-mechanical models have been constructed to explore the influence of pre-existing faults on the temporal-spatial distribution of high-grade amphibolite-granulite facies metamorphic rocks during extension. The different dip amounts of three pre-existing faults (refer to the Bole-Nangodi shear zones, the Jirapa shear zone and the Bulenga shear zone in the study area) are studied in models. The results show the lower crust exhumes occur at a relatively low rate prior to attaining a Stretching Factor = 4.2%. The partially molten lower crustal rocks tend to move (up to 20 km) towards the center of the model and focus exhumation in regions where pre-existing faults intersect. The high-strain corridors in models are used to understand the loci of exhumation in the Bole-Bulenga domain of NW Ghana. Accordingly, in the eastern and western parts of the high-grade rock corridors in NW Ghana, partially molten rocks exhumed from the lower into middle-upper crustal levels are interpreted to have been dominantly facilitated by the km-scale high-strain corridors. In the central part of the Bole-Bulenga domain, the high-grade rocks are interpreted to have been exhumed because of a coupling between two mechanisms: (1) The exhumation of partially molten rocks between the Jirapa and Bole-Nangodi faults increases in spatially due to the reduction in space from north to south. (2) The exhumation of lower partially molten rocks in the central part, as a result of inherited orthogonal (E-W) structures.

The EP (electric potential) signals can be generated during the deformation and fracture process of coal and rock mass. Meanwhile, the EP response is closely related to its stress state and damage evolution, which is expected to be used in monitoring and coal and rock dynamic disaster hazards. Based on this, this paper developed an EP monitoring device for mining to continuously monitor the temporal response characteristics and spatial distribution of coal seam internal EP signals in real time. Further, the experimental tests were carried out, whose results showed that the device has high monitoring sensitivity and little error for the EP signals and can reveal the loading state and damage degree of the coal and rock specimens during the deformation and fracture process. Moreover, the tests and application of EP monitoring were carried out during mining activities in the field. The results showed that the EP signals fluctuate during the coal mining stage and remain relatively stable during the maintenance stage. When the abnormal mining stress or the coal cannon phenomenon occurs, the intensity of EP signals increases rapidly and fluctuates violently, which has precursory response information for the hazards of dynamic disasters. Considering the advantages of sensitive response and nearly non-destructive monitoring, the study results can provide key monitoring equipment and research basis for field testing the EP signals during the mining process, to monitor and forecast the hazards of coal and rock dynamic disasters.

Electromagnetic radiation (EMR) anomalies preceding or accompanying coal and rock dynamic disasters have been reported for many years, but the mechanism that produces these anomalies is still unclear. To investigate the role that charge separation plays in these electromagnetic (EM) anomalies, uniaxial compression experiments on three different brittle materials: coal, marble, and polymethyl methacrylate were conducted. The results of the experiments indicate that EMRs are observed in piezoelectric and nonpiezoelectric materials but EM anomalies are only observed when abrupt stress drops occur. The EMR energy rate increases with the magnitude of the stress drop, and those two variables are related by a polynomial function. The charge separation responsible for the anomalies occurs on the newly generated cracks when chemical bonds are broken. A charge distribution model has been established for this phenomenon. Based on this model, the coupling relationship between EM energy density[Formula: see text] and charge density [Formula: see text] has been determined by theoretical analysis. According to this analysis, [Formula: see text] is proportional to [Formula: see text]. In addition, the relationship between [Formula: see text] and surface energy has been established, and it indicates that [Formula: see text] increases with (1) the stress to which the surrounding material is subjected and (2) the length of newly generated cracks. Research findings reported should provide a more detailed understanding of the EM anomalies when coal and rock dynamic disasters occur and provide guidance for developing warning strategies for mine safety.

To distinguish various blasting and mine microseismic (MS) waveforms, the time–frequency characteristics and non-linear characteristics of mine MS waveforms caused by mining activities and different blasting waveforms (loosening blasting, pre-splitting blasting and deep-hole blasting) are analyzed using fast Fourier transform (FFT), smoothing pseudo-Winger-Ville distribution (SPWVD) and multifractal method. The results indicate that (1) Time–frequency spectrum of SPWVD can show the dynamic change of energy with time and instantaneous frequency. For mine MS waveforms, the energy and instantaneous frequency show a gradual attenuation trend with time. The larger the fracture strength of coal and rock caused by blasting, the higher the energy distributed in low frequency zone. (2) The dominant frequency obtained by FFT can divide the four types of waveforms into two types. The dominant frequency of loose blasting and deep-hole blasting waveforms are greater than 135 Hz, while that for pre-split blasting and mining MS waveforms are less than 135 Hz. (3) The loose blasting and deep-hole blasting waveforms can be distinguished by duration. The duration of loose blasting waveforms are less than 220 ms, while that for deep-hole blasting waveforms are greater than 220 ms. (4) Multifractal parameters (Δα and Δf(α)) can reflect the difference of local fluctuation intensities and heterogeneities of different waveforms and be affected by dominant frequency, duration and post-peak attenuation. The Δα and Δf(α) of pre-split blasting waveforms are less than 0.78 and − 0.2, respectively, while that for mine MS waveforms are opposite.

In order to provide an effective theoretical basis for the location of microseismic source, active seismic exploration technology, and internal medium structure inversion of coal mine in different areas, we analyze the propagation law of microseismic wave in different medium accurately and intuitively. In this paper, we first studied the finite difference method suitable for the analysis of wave field characteristics of coal and rock medium in a coal mine. Then, we established the typical models of coal-rock combination multi-layer medium, fault-bearing coal-rock medium, coal-rock dual-phase medium of water-bearing and gas-bearing and stochastic medium model, and random medium model. Finally, we obtained the wave field snapshot and single shot recording of different medium model. The research shows that the high-order finite difference method can accurately obtain P and S waves field characteristics under different media conditions. The wave field characteristics of different medium models are different. In the coal-rock combination multi-layer model, the transmitted wave, transmitted converted wave, reflected wave, and reflected converted wave are generated on the different medium interfaces. In the fault model, not only the transmitted wave, transmitted converted wave, reflected wave, and reflected converted wave but also the refracted and diffracted waves are produced at fault area. In the coal-rock dual-phase medium of water-bearing and gas-bearing model, three types of waves are excited by the source, which are fast longitudinal (P) waves, transverse (S) waves, and slow longitudinal (P) waves. However, the propagation of slow longitudinal wave is different in these models. Under the stochastic media model, there are very complex scattering waves. The snapshots at different time further describe the wave field’s dynamic characteristics vividly, which establish the theoretical basis of the analysis of microseismic waves. Based on the analysis of the wave field characteristics of coal and rock in the mine, it can provide theoretical support and basis for the active seismic exploration and internal medium structure inversion of coal mine in different areas. What’s more, a more practical wave velocity model can be established to provide an effective theoretical basis for the location of microseismic source. When there is a disaster in the coal mine, we can determine the exact location of the source as soon as possible and take corresponding measures.

Rockburst is a serious threat to the safety and efficiency of coal-mine production in China. Accurate monitoring and prediction of rockburst is an ongoing challenge in coal mining. Electromagnetic radiation (EMR) technology is a real-time and non-contact prediction method for changes in the mechanical parameters of coal or rock. In this paper, uniaxial compression testing of coal and rock samples was carried out, EMR data were collected and analysed from deformation to failure. The rescaled range (R/S) analysis method was applied to study the Hurst index of the EMR signals during the uniaxial compression test. The results showed that the higher the degree of deformation and failure, the larger the Hurst index. On the basis of the experimental and theoretical results, the Hurst index of the EMR signal during the ‘8.11’ rockburst in the Yuejin coal mine was analysed. The results showed that the Hurst index was low before the rockburst, and increased to 0.9 in the pre-rupture stage, then decreased 1–2 days before the rockburst. This suggests a correlation between EMR and the conditions leading to a rockburst event. Therefore, further study on the characteristics of EMR can contribute to development of early-warning technology for rockburst.

High temperature causes thermal damage to rock; the macrofracture and microfracture of rock can be produced under the action of temperature treatment. Under the influence of high temperature, the surrounding rock of deep underground engineering will suffer instability damage and cause serious harm to the people. In order to use the electromagnetic radiation (EMR) technology (a non-contact geophysical method) for evaluating the thermal stability of rock in underground thermal engineering applications, we established the EMR testing experimental system of rock under the action of a continuous heat source. The variation of EMR signals of rock under different temperatures was tested, and the EMR signals generates during the process of rock thermal deformation and thermal fracture, which were later analyzed. Under the action of a continuous heat source, the rock materials produced EMR signals with three kinds of frequencies. With the increase of rock temperature, the variation trends of EMR signals varied from the slow growth rate to the rapid growth rate, EMR signals can be divided into five stages. The increase of EMR signals is positively correlated with temperature, the Hurst exponent was higher than 0.7. The thermal stress was responsible for thermal deformation and fracture, thus generating the EMR signals. The research results have important guiding significance for the application of EMR technology to the evaluation of rock thermal stability.

Aiming at promoting thin coal seam pressure relief gas extraction, as well as delimiting the protection range of the low-protective layer in thin coal seam, in this paper, we simulated the changing process of the protective layer stress field and deformation field during the low-protective layer mining and obtained the characteristics of the displacement as well as the stress varied characteristics of the overlying strata in the low-protective layer mining. According to the protection layer mining stress relief and the deformation protection criterion, the tendency and trend of the protection layer protection scope were determined, and the theoretical calculation results were basically consistent with the direction protection angle of the protected coal seam. Taking the Dahe (DH) coalmine as an example, the floor rock roadway as the gas drainage roadway to extraction gas as well as the scheme of gas extraction and pressure relief in the upper and lower layers of the protective layer were implemented. The effect of gas control scheme showed that the working gas concentration reached the mining requirement. We finally established the low-protective layer in thin coal mining integrated technical scheme of protection scope and pressure relief gas extraction.

Shale gas reservoir is a typical type of unconventional gas reservoir, primarily because of the complex flow mechanism from nanoscale to macroscale. A triple-porosity model (M3 model) comprising kerogen system, matrix system, and natural fracture system was presented to describe the multispace scale, multitime scale, and multiphysics characteristic of gas flows in shale reservoir. Apparent permeability model for real gas transport in nanopores, which covers flow regime effect and geomechanical effect, was used to address multiscale flow in shale matrix. This paper aims at quantifying the shale gas in different scales and its sequence in the process of gas production. The model results used for history matching also showed consistency against gas production data from the Barnett Shale. It also revealed the multispace scale process of gas production from a single well, which is supplied by gas transport from natural fracture, matrix, and kerogen sequentially. Sensitivity analysis on the contributions of shale reservoir permeability in different scales gives some insight as to their importance. Simulated results showed that free gas in matrix contributes to the main source of gas production, while the performance of a gas shale well is strongly determined by the natural fracture permeability.

This paper presents an electromagnetic radiation-based method to qualitatively assess mine-induced stress field at the longwall face. We first analyze the correlation between electromagnetic (EM) signals and stress, and then measure the signals and drilling bits in both the strike and dip directions at the 11702 working face of Nuodong Coal Mine, China, which indirectly verifies this method. Results show that there is a positive correlation between EM intensity and stress. In the same coal seam, the frequency of EM signals is inversely proportional to the extended size of the cracks inside the coal rock, and the dominant frequency (DF) of the EM signal is negatively related to external stress. Test results in Nuodong Coal Mine show that the EM intensity and DF can well reflect a stress relief zone, concentration zone and original zone. In addition, through field test and theoretical calculation, we can ascertain that the crack propagation length is from several centimeters to greater than 20 cm. The stress magnitude inside the coal rock can be qualitatively evaluated by using the correlation of the EM signal intensity, frequency and stress.

A rockburst is a dynamic disaster that occurs during underground excavation or mining which has been a serious threat to safety. Rockburst prediction and control are as important as any other underground engineering in deep mines. For this paper, we tested electromagnetic radiation (EMR) signals generated during the deformation and fracture of rock samples from a copper mine under uniaxial compression, tension, and cycle-loading experiments, analyzed the changes in the EMR intensity, pulse number, and frequency corresponding to the loading, and a high correlation between these EMR parameters and the applied loading was observed. EMR apparently reflects the deformation and fracture status to the loaded rock. Based on this experimental work, we invented the KBD5-type EMR monitor and used it to test EMR signals generated in the rock surrounding the Hongtoushan copper mine. From the test results, it is determined the responding characteristics of EMR signals generated by changes in mine-generated stresses and stress concentrations and it is proposed that this EMR monitoring method can be used to provide early warning for rockbursts.

Hydraulic flushing in gas predrainage is widely used, but the hydraulic-flushing effect is evaluated in a traditional way, by determining the desorption volume, moisture content, gas drainage rate and other conventional indices. To verify the rationality and feasibility of the multielectrode resistivity method in the evaluation of coal-seam hydraulic flushing and to research the spatio-temporal evolution of apparent resistivity during hydraulic flushing, a field test was conducted in 17# coal seam at Nuodong Mine, Guizhou. During hydraulic flushing, four stages were defined according to the variation in coal rock resistivity with time, namely, the preparation stage, the sharply decreasing stage, the rapidly increasing stage and the steady stage. The apparent resistivity of the coal rock mass is affected mainly by its own degree of fragmentation and flushing volume. A more serious rupture and a greater flushing volume yield a smaller apparent resistivity during the sharply decreasing stage and a higher resistivity during the stable stage. After three months of gas predrainage, the residual gas content and the gas pressure at different points in the expected affected area decrease below the critical value. Changes in the residual gas content and gas pressure at these points are consistent with the apparent resistivity, which validates the rationality and feasibility of the multielectrode resistivity method in evaluating coal-seam hydraulic flushing.

To assessment the source parameters and slip distribution on the rupture surface of a rock burst event in Qianqiu coal mine, and verify the accuracy of the source parameters and slip distribution on the rupture surface, four rock burst events in Qianqiu coal mine were selected for source analysis. Firstly, the source parameters of four rock burst events in Qianqiu coal mine were obtained based on the Brune model. Then, the rupture scale and average slip on the rupture surface of seismic source were identified by using the established relationship between rupture scale and moment magnitude. Finally, based on the k2 rupture-slip model and the obtained source parameters, the slip distribution on the rupture surface was inverted to reveal the slip-rupture process of the source, and the accuracy of the seismic source parameters and the slip distribution on the rupture surface was verified based on the in-situ rupture-slip behavior. The results show that: the source parameters can be calculated using the Brune model and grid search method; the rupture scale and average slip on the rupture surface were obtained by establishing the relationship between the moment magnitude and the rupture scale of the source; based on the k2 rupture-slip model and the scale of rupture, the slip distribution on the rupture surface of a microseismic event in a coal mine can be deduced by inversion. The calculated source parameters and slip distribution of four rock burst events are consistent with the actual records of such events in a coal mine, which further verifies the accuracy of the source parameters and slip distribution of rock burst events. These results can be used to quantify and evaluate the rupture properties, scale and process of a rock burst, and provide reference for further study of the evolution process and risk assessment of rock burst events in coal mine.

Using a portable microseismic monitoring instrument, we monitored blast waves at different distances from the blast center and analyzed their spectra, wavelet packet energies, and fractal characteristics. The results show that with increasing blast–center distance, the main frequency bands become narrow and the main frequency decay is governed by a power law (f = 424.98 l −0.72). The energy of the wavelet packet is concentrated within the main frequency; energies associated with high frequency bands decay quickly, whereas for the low frequency bands (low to 78 Hz), the percentage of the total energy in these bands presents a rising trend of around 42% and 71% at blast–center distance 30 m and 110 m, respectively. In addition, the correlation dimension D of the blast wave tends to decrease with increasing blast–center distance; the correlation dimension D is 3.55 at distance 30 m and decreases to 1.26 at distance 110 m. The blast waves frequency spectra, wavelet packet energies, and fractal characteristics are inherent unity. These results provide an important basis for further study on the propagation law of blast waves in coal mines.

An accurate and non-destructive evaluation method for the hydraulic measure impact range in coal seams is urgently needed. Aiming at the application demands, a theoretical study and field test are presented using the direct current (DC) method to evaluate the impact range of coal seam hydraulic measures. We firstly analyzed the law of the apparent resistivity response of an abnormal conductive zone in a coal seam, and then investigated the principle of non-destructive testing of the coal seam hydraulic measure impact range using the DC method, and used an accurate evaluation method based on the apparent resistivity cloud chart. Finally, taking hydraulic fracturing and hydraulic flushing as examples, field experiments were carried out in coal mines to evaluate the impact ranges. The results showed that: (1) in the process of hydraulic fracturing, coal conductivity was enhanced by high-pressure water in the coal seam, and after hydraulic fracturing, the boundary of the apparent resistivity decrease area was the boundary impact range. (2) In the process of hydraulic flushing, coal conductivity was reduced by holes and cracks in the coal seam, and after hydraulic flushing, the boundary of the apparent resistivity increase area was the boundary impact range. (3) After the implementation of the hydraulic measures, there may be some blind zones in the coal seam; in hydraulic fracturing blind zones, the apparent resistivity increased or stayed constant, while in hydraulic flushing blind zones, the apparent resistivity decreased or stayed constant. The DC method realized a comprehensive and non-destructive evaluation of the impact range of the hydraulic measures, and greatly reduced the time and cost of evaluation.

A rock mechanics test system and an acoustic emission (AE) test system have been employed to conduct AE event location experiments on sandstone samples under uniaxial compression. According to the single-link cluster (SLC) method, an SLC structure is set up. Each single link is defined as a spatial vector, which can be calculated based on the spatial coordinates of the nearest neighbor of a certain AE event minus the spatial coordinates of this event. Based on the basic theory of fractal geometry, a spherical surface-covering method is proposed herein to analyze the fractal property of spatial vector direction in an SLC structure. To study the change in fractal dimension over time, a sliding event window is used. The results show that the window length and slide step length have little effect on the change in trend of fractal dimension. Before buckling failure of a rock sample, the fractal dimension curve shows a clear downward trend. The reason for the change in fractal dimension is related to the generation mechanism of AE events. Therefore, a clear decrease in fractal dimension can be taken as precursor to rock buckling failure under uniaxial compression. This research provides a new method for studying the evolution process of rock buckling failure and has important significance regarding the monitoring of rock stability and early warning of rock burst.

To investigate the source properties and focal mechanism of coal and sandstone specimen rupture under uniaxial loading, acoustic emission (AE) testing of sandstone and coal specimen rupture under uniaxial loading was conducted. The spatio-temporal evolution of AE parameters in the rupture process was determined through the AE testing of coal and sandstone specimen rupture under uniaxial loading. The source parameters of sandstone and coal specimen rupture were then calculated and analyzed using the Brune model and grid-search method. Finally, the focal mechanism of sandstone and coal specimen rupture was deduced by inversion and compared based on source parameters and moment tensor inversion. The results show that: the rupture morphology of sandstone specimens differs from that of coal specimens; the source displacement spectra of sandstone and coal specimens under uniaxial loading are in good agreement with the Brune model. The corner frequency of coal and sandstone specimens decreases linearly with the increase of seismic moment; source radius and energy increase linearly with increasing seismic moment; the stress-drop fluctuates around a certain value. For events of the same magnitude, the corner frequency, energy, and source radius of sandstone specimens at rupture are greater than those of coal specimens at rupture, but the stress-drop in sandstone specimens (at rupture) is smaller than that occurring in coal specimens. In the middle and early stages of rupture, the coal and sandstone specimens mainly undergo shear failure. Shear-tension and shear-compression failures occur in the later stage of rupture. These results can be used to quantify and evaluate the source properties and focal mechanism of small-scale specimens at rupture.

Source location is the key step of microseismic monitoring. The monitored microseismic signals are often mixed with unrecognized blasting interference signals, which affect the results of microseismic monitoring and location. Therefore, it is very important to extract the characteristic parameter information to identify the microseismic signal and blasting signal. The rupture processes during micro-earthquakes and blasting events in a coal mine are complicated and have different characteristics, which seriously affects the safety of coal mine. To quantify the seismic source properties of micro-earthquakes and blasting events in coal mines, and further distinguish between micro-earthquakes and blasting events, the source parameters of micro-earthquakes and blasting events in coal mine were obtained, and the differences between micro-earthquakes and blasting events were investigated by comparing the characteristics of their respective source parameters. Firstly, the corner frequency f0, spectral level Ω0, seismic moment M0, source energy E, source radius R, stress drop △σ, apparent stress σa, and magnitude (Mw and ML) of micro-earthquakes and blasting events in Qianqiu coal mine were calculated using the Brune model and grid-search method; then, the relationship between seismic moment and the other source parameters was analysed, the differences between micro-earthquakes and blasting events were studied based on the above source parameters, and the automatic identification model of micro-seismic and blast signals thus established; finally, the local magnitude was calculated and the relationship between moment magnitude and local magnitude analysed. The magnitudes of micro-earthquakes and blasting events are determined based on the calculated Mw and ML. The above analysis can be used to determine and quantify the seismic source properties and scale of micro-earthquakes and blasting events, and further distinguish micro-earthquakes from blasting events effectively to ensure the accuracy of source location and allow implementation of effective control measures.

Rockburst is a complex coal and rock dynamic disaster, which poses a serious threat to safe and efficient mining activities. With the increase in the mining depth and strength, rockburst becomes more and more serious. Coal rock contains different types and sizes of structural surface, and its bucking failure is the result of crack propagation and coalescence. In this paper, based on the elastic–plastic mechanics and damage mechanics, we established the elastic–plastic-brittle mutation rockburst model of coal rock with structural surface, introduced the concept of the coal rock volume energy potential function, and researched the relationship between energy accumulation and dissipation during the coal rock dynamic deformation and rupture process to obtain the rockburst energy condition. In addition, the volume element-region-system model of rockburst was established to explain the rockburst evolution process very well. In the external disturbance or high stress state, the coal rock system volume element is destroyed and quickly releases energy to the adjacent volume element. When the adjacent volume element cannot absorb this energy, it is destroyed, and the energy is released quickly. This energy release forms a chain reaction, and the local coal rock is unstable. Eventually, the entire coal rock system is unstable, and rockburst happens. Based on the above theory, the microseismic (MS) and electromagnetic radiation (EMR) characteristics of coal rock in the rockburst evolution process were analyzed. The significance of the study lies in both theory and practice for deeply understanding the rockburst mechanism, as well as for accurate monitoring and early warning.

Coal failure behavior under dynamic loading is significant for dealing with the failure issues encountered in underground coal mines. In this study, the crack propagation and internal fracture process of coal under dynamic loading were investigated by a split Hopkinson pressure bar (SHPB) experimental system and numerical approach, respectively. The experimental and numerical results show that the coal crack propagation and internal fracture surface manifest significant multifractal features. Further, multifractal analysis suggests that the multifractal feature becomes more and more significant during the dynamic loading. The formation mechanism of the multifractal features was further discussed, and the result demonstrates that the crack propagation path within coal is essentially a multifractal structure, thus causing the total fracture behavior of coal to show multifractal features under dynamic loading. Moreover, the multifractal spectrum parameter [Formula: see text] was proved to be closely related to the brittle fracture property of coal, which transpires that the multifractal feature is feasible to evaluate the coal brittleness under dynamic loading and also applicable to predict the coal failure risk in underground coal mines.

Coal and gas outburst disaster of the No. B-1 Coalbed, Henan, China, have lasted for nearly 60 years, and the threat will become more and more serious as mining depths continuously increase. However, coal and gas outburst characteristics and factors of the coalbed have not been studied in detail. To effectively prevent and control coal and gas outburst, we analyzed the type, intensity, location and precursors of coal and gas outburst occurring in the No. B-1 Coalbed. Moreover, the effects of geological conditions (burial depth, faults, folds, coalbed thickness and dip) and mining disturbances on coal and gas outburst were studied. The results showed that these outburst accidents were mostly typical, small-sized and medium-sized outburst, which mainly occurred at the coal roadway working faces. There were many precursors such as blasting sound, changes in coal structure, and abnormal gas emission prior to the accidents. Within a burial depth of 500 m, the average outburst intensity had a stronger correlation with the burial depth, which was more obvious at a burial depth of 301∼400 m, and less obvious at a burial depth of 401~500 m. However, the distribution of these outbursts barely changed within the burial depth. Up to 99.15% of coal and gas outburst occurred at faults, folds, and areas with changes in coalbed thickness and dip. Up to 68.25% were induced by blasting and 18.04% occurred during shelving, coal-shoveling and no operation. The No. B-1 Coalbed was characterized by delayed outbursts. Moreover, specific suggestions were recommended for the prevention and control of coal and gas outburst. The study is useful to government regulators and staffs engaged in the prevention and control of coal and gas outburst.