1. Introduction
Many researchers and scientists are convinced that the use of coal as an energy carrier should be phased out as soon as possible. However, as one of the primary energy sources in the world, coal will still occupy an important position in the world’s energy structure for a long time to come. With the gradual depletion of high-quality shallow coal resources, coal mines have gradually entered the stage of deep mining. Generally, “deep mining” means that the mining depth is below 600 m. Deep mining faces the objective reality of high ground temperature, high ground pressure, high gas content, and low coal seam permeability compared to shallow coal mines [
1]. Research shows that the airflow temperature of the mining face increases by about 1.5 °C for every 100 m increase in mining depth [
2,
3]. According to Provisions on Geothermal Survey for Geological Exploration of Coal Resources, deep wells below 1000 m in China are generally in Grade I or II heat damage zones [
4]. The mining depth is proportional to the original rock stress and tectonic stress. The vertical initial rock stress is 20 MPa when the mining depth is 800 m and increases to 27 MPa when the mining depth is 1000 m, which far exceeds the compressive strength of the general engineering rock mass [
5]. At high temperatures and pressures in the deep coal mine, coal rocks become more brittle and are more prone to instability under external forces. Even coal rocks with good tensile and compressive resistance under normal conditions will deteriorate in terms of the strength and toughness of the rock mass due to changes in the force field and other factors under very deep conditions, resulting in deformation [
6]. At the same time, at least 80% of the mines in China have high gas content and poor permeability; low gas permeability pressure, permeability, and saturation; and strong non-homogeneity. In a deep high-pressure environment, coal permeability is seriously insufficient. Many free gas substances are compressed and hidden in coal and rock cavities, accumulating enormous kinetic energy in the coal. In mining and driving, a large amount of gas energy stored in the underground coal seam will instantly burst out when it reaches a certain threshold, resulting in a coal and gas outburst disaster/accident. In general, the risk of coal and gas outburst accidents is directly proportional to the mining depth of a coal mine [
7]. After the mine enters the deep mining zone, due to changes in the geological conditions of the coal seam, in situ stress structure, gas stability, and other factors, a mine that has not had coal and gas outbursts or has had soft coal and gas outburst accidents in the shallow mining stage is very likely to have severe coal and gas outburst accidents. Therefore, the risk assessment of coal and gas outbursts in deep coal mines is an essential new topic in this critical field, which has fundamental scientific significance and broad application prospects.
Scholars’ research on coal and gas outbursts mainly involves their mechanism, influencing factors, evaluation, prediction, and risk prevention. Among them, the mechanism and influencing factors of coal and gas outbursts are mainly based on sensitive indicators. These indicators are related to gas adsorption [
8], geological structure [
9], geological stress [
10], gas pressure [
11], temperature [
12], soft coal seams [
13], moisture content [
14], porosity [
15], gas diffusion [
16], deformation [
17], the critical value [
10], permeability [
18], seams [
19], etc. The research methods of coal and gas outburst assessment are mainly numerical simulations and modeling, which have mainly focused on the gas management and gas outburst risk assessment of Shimen coal uncovering and the tunneling face [
20]. Related research mainly includes traditional evaluation based on subjective and objective weighting and intelligent evaluation based on machine learning.
The traditional evaluation mainly includes subjective and objective weighting methods and the combination of subjective and objective weighting. Standard subjective evaluation methods include questionnaires, expert interviews, expert scoring, analytic hierarchy process, etc. Traditional objective evaluation methods include the entropy weight method, gray theory, rough set theory, fuzzy theory, set pair analysis, extension analysis, etc. Given the advantages and disadvantages of subjective and objective evaluations, the combination of subjective and objective assessments is gradually being recognized. This approach mainly includes the combination of two or more methods, such as hierarchical analysis, gray correlation analysis, distance discriminant analysis, the entropy method, gray target theory, object element analysis, extension evaluation, fuzzy theory, K-means cluster analysis, principal component analysis, accident tree evaluation, and the rough set method [
21,
22,
23,
24,
25]. With the increasing maturity of computer technology, deep learning, machine learning, and intelligent algorithms have been gradually applied to disaster evaluation and prediction. The commonly used intelligent evaluation algorithms include artificial neural networks [
26] (hereafter referred to as ANNs) and support vector machines [
27] (hereafter referred to as SVMs). Although ANNs have robust learning and computing abilities and unique advantages in dealing with complex system evaluation, ANNs require a large amount of data, and coal and gas outbursts are typically high-dimensional, nonlinear, small-sample problems. The so-called high dimensionality means that deep coal and gas outbursts are not determined by a single or small number of factors but are the result of the joint action of many factors, such as the physical properties of coal, gas endowment characteristics, mining depth, etc. It is a typical complex brittle system, and the elements do not simply follow the linear development law but present specific nonlinear characteristics. In terms of the simplicity of the object involved, the simpler the problem, the fewer the samples needed to explain it theoretically. Otherwise, a large amount of data is required. When the problem’s simplicity does not correspond to the amount of sample data, and the amount of sample data is small, it is a small sample. A coal and gas outburst is a typical small-probability event, and the related data collection is inherently inadequate. Meanwhile, the high-dimensional characteristics of coal and gas outbursts in deep coal mines further strengthen the small-sample property. Therefore, SVM has a natural advantage in dealing with coal and gas outbursts in deep coal mines. At present, SVM is widely used in the evaluation and prediction of coal and gas outbursts, and relevant scholars have made many improvements on the basis of the traditional SVM, such as adaptive SVM [
28], rough set–support vector machine (RS-SVM) [
29], rough set–clone selection algorithm–support vector machine (RS-CSA-SVM) [
30], complete chaotic particle swarm optimization–support vector machine (CCPSO-SVM) [
31], quantum genetic algorithm–least-squares support vector machine (QGA-LSSVM) [
32], NN-SVM [
33], fruit fly optimization algorithm–support vector machine (FOA- SVM) [
34], etc.
Adaptive SVM is a risk prediction model obtained using the adaptive genetic algorithm to improve the time domain exponent a, penalty coefficient C, and kernel function parameter σ of the fuzzy support vector machine, which may be suitable for the complex characteristics of coal and gas outbursts. To reduce the complexity of SVM implementation, RS-SVM was developed, which is a model that reduces the data of coal and gas outburst risk data through a rough set, extracts the core discriminant index, and performs risk discrimination. RS-CSA-SVM is an SVM parameter vector optimized by a clone selection algorithm based on RS-SVM and is an improved algorithm to improve the efficiency and accuracy of the model operation. CCPSO-SVM is a prediction model that uses the multifractal spectrum of the time series of dynamic changes in coal gas emission in front of the mine face as the characteristic index, and it uses the improved completely chaotic particle swarm optimization algorithm and the minimum classification error rate criterion of the test set sample set to select and optimize the SVM parameter vector. QGA-LSSVM is a kind of coal and gas outburst prediction model that uses a quantum genetic algorithm to optimize the parameters of LSSVM, makes full use of the characteristics of the quantum genetic algorithm for parameter optimization, and optimizes the penalty parameter C and kernel parameter σ of LSSVM to improve the prediction accuracy and global search ability of LSSVM. The Drosophila optimization algorithm carries out the global optimization of SVM, and the FOA algorithm finds the optimal combination of each parameter of the support vector machine. The prediction model of FOA-SVM is established to solve the empirical dependence of the setting of each parameter in SVM and the problem of a significant network error to improve the performance of risk prediction. NN-SVM is a discriminant model obtained by pruning the training set and then using SVM to train each sample according to the similarities and differences between each sample and its nearest neighbor. The common feature of the above models is that they entirely rely on the advantages of SVM in solving problems with small samples, high dimensions, and complex nonlinearities and optimize the training parameters of SVM with the help of various intelligent algorithms to save training time and improve training accuracy.
Therefore, the key to studying the risk assessment of coal and gas outbursts in deep coal mines is determining how to solve the complex data-processing problem of coal and gas outbursts in deep coal mines based on the features of deep coal mines. In this paper, according to the characteristics of the small sample, high dimensionality, and nonlinearity of deep coal and gas outbursts, an intelligent algorithm and deep learning are effectively fused, and an improved quantum particle swarm optimization support vector machine (IQPSO-SVM) for deep coal and gas outburst evaluation is proposed. This method is mainly based on the fact that deep coal and gas outbursts are small-probability events and naturally have the characteristics of a small sample. As a commonly used machine learning classification algorithm, SVM has particular advantages in solving small-sample, high-dimension, local-minimum, and nonlinear problems. However, SVM’s classification accuracy is closely related to its built-in penalty parameter C and kernel function parameter g. The traditional SVM must find and set the best parameters and select the appropriate kernel function by experience. The parameter settings significantly influence the generalization ability and accuracy, and it takes considerable time to adjust the parameters. As an optimization algorithm, particle swarm optimization (PSO) is more suitable for global optimization. The particle swarm optimization algorithm embedded in SVM can directly operate the structural objects and optimize the traditional support vector machine parameters to the maximum extent. This algorithm improves the risk prevention and control of coal and gas outbursts in deep coal mines, changes the safety risk from passive risk control to pre-control source prevention, and can effectively improve the safety control of coal mines.
In conclusion, the risk assessment of coal and gas outbursts in deep coal mines has a wide range of practical needs. A coal and gas outburst in a deep coal mine is an extraordinarily complicated and changeable system. The system’s complex and inherent changeable properties greatly aggravate the risk degree of coal and gas outbursts in a deep mine. It also means that even minor vulnerabilities can cause significant coal and gas outbursts. Therefore, when evaluating the risk of coal and gas outbursts in deep coal mines, the possibility of safety accidents should be considered from all angles. Traditional risk assessment methods are difficult to apply to the risk assessment of coal and gas outbursts in deep coal mines due to the lack of information processing ability for risk factors and the complex and changeable factors affecting the risk of coal and gas outbursts in deep coal mines.
5. Discussion
According to the objective reality of deep mining in Chinese coal mines, this study adopted an appropriate mathematical transformation and reasonable modeling to judge the risk degree of deep coal and gas outbursts. A deep coal and gas outburst is a typical small-sample, high-dimensional, and complex nonlinear problem. The premise of the accurate evaluation of deep coal and gas outbursts is to capture the nonlinear variation law of deep coal and gas outbursts by proper methods. Based on balancing the calculation time and detection accuracy, this paper proposes a risk assessment method for deep coal and gas outbursts based on IQPSO-SVM.
First, in the evaluation accuracy, an essential basis that affects the model training accuracy is the characteristics of the sample data. In machine learning, neural networks have advantages in dealing with significant sample size problems. However, a deep coal and gas outburst is a small-probability event and naturally has the characteristics of a small sample. The total number of samples collected in this paper is 124 groups, a small sample of data. SVM has particular advantages in solving small-sample, high-dimension, local-minimum, and nonlinear problems. Moreover, the SVM algorithm is simple and robust and has good generalization ability. However, SVM’s classification accuracy is closely related to its built-in penalty parameter C and kernel function parameter g. Therefore, based on the correct selection of the kernel function, this study used IQPSO to optimize the penalty parameter C and kernel parameter g of SVM to balance the global search and local search problems in the algorithm design. The data fitting shows that IQPSO can combine the unique advantages of SVM in solving small-sample, high-dimension, local-minimum, and nonlinear problems. The penalty parameter C and kernel parameter G of SVM are optimized, and the parallelism, stability, robustness, global optimality, and model generalization ability of data fitting are improved. Compared with the test results of the standard SVM, PSO-SVM, and QPSO-SVM models, IQPSO-SVM constructs a more reasonable adaptive recognition model that can be used in the field of gas outburst recognition in deep coal mines. This model can significantly improve the detection accuracy of deep coal and gas outbursts, and the accuracy of the target identification of coal mine disasters is increased to 94%.
Secondly, in terms of training time, the traditional SVM takes considerable time to adjust the parameters, while PSO, as an optimization algorithm, has better global optimization ability. By embedding the PSO algorithm into SVM, the object structure can be operated directly, and the parameters of the traditional support vector machine can be optimized to the maximum extent. However, the reliability of the PSO algorithm is closely related to the restriction degree of the relationship between particles. In solving many high-dimensional and nonlinear problems, the PSO algorithm is limited to a specific search range due to the substantial homogeneity of particles and the insufficient difference between particles. It cannot find the ideal value in the whole range. This situation is called convergence stop or premature convergence. Based on the above understanding, this study constructed the IQPSO-SVM evaluation method for deep coal and gas outbursts through an appropriate transformation. The problems of optimal local risk and premature convergence in the training process of PSO and QPSO are effectively solved, and the learning ability of SVM and the global search function of the IQPSO algorithm are used. The data fitting shows that PSO-SVM > QPSO-SVM > IQPSO-SVM in terms of overall time consumption. Therefore, IQPSO-SVM maintains a high detection accuracy, saves time, and is more efficient in evaluating deep coal and gas outbursts.
Finally, the IQPSO-SVM method proposed in this paper comprehensively uses the knowledge and research tools of management science, security science, computer science, security engineering, and other fields. Real-time data are integrated with the intelligent algorithm for machine learning classification processing, and the risk level is determined. It provides a method for improving the early warning system of deep coal and gas outbursts and carrying out the hierarchical prevention and control management of deep coal and gas outbursts. For the scientific evaluation of deep coal and gas outburst risk, this method promotes deep coal mine safety management and provides a new train of thought on deep coal mine safety, but it can also be extended to other areas, such as machine-learning-integrated intelligent algorithms of data processing methods to solve other complex small-sample, high-dimensional, nonlinear problems, and provides an essential reference. However, the difference between theory and practice is a specific problem situation. The problem situation determines the input and output settings, the problem’s index and dimension, the sample size requirement, and the training model, directly affecting the evaluation results. Therefore, the evaluation method of machine learning classification processing based on data fusion by the intelligent algorithm proposed in this paper enriches the theoretical method of coal mine safety evaluation and shifts the safety risk evaluation of deep coal mines from theory to the broader application level.
6. Conclusions
6.1. A Set of Crucial Index Evaluation Systems of Coal and Gas Outbursts in the Deep Coal Mines Is Established
This paper’s key contribution is establishing an index of deep coal and gas outbursts. Deep coal and gas outbursts are closely related to the mining depth. Significant changes occur in the deep mining environment, typically characterized by three high parameters and one low one, namely, high ground temperature, high ground pressure, high gas content, and the low permeability of coal and rock. In this environment, the gas permeability of the coal body is insufficient, and the influence of mining disturbance enhances the brittleness of coal and rock. The internal force of coal rock is much higher than usual, and it is more likely to become unstable under the action of an external force. More importantly, in the deep environment, the nonlinear changes in single vital indexes, such as the coal seam gas pressure, gas content, initial gas release velocity of coal, and coal firmness coefficient, are aggravated, and the degree of interaction between each index is increased, significantly aggravating the risk of deep coal and gas outbursts. Based on the above understanding, in this paper, based on a literature review, combined with guiding documents such as Rules for Prevention and Control of Coal and Gas Outburst and Coal Mine Safety Regulations, the type of coal damage, mining depth, original gas pressure, gas content (including original gas content and resolvable gas content), the initial gas release velocity of coal, and the ruggedness coefficient of coal are established as a critical index system of deep coal and gas outbursts.
6.2. IQPSO-SVM Model Based on Small-Sample Features Is Constructed According to the Data Characteristics of Deep Coal and Gas Outbursts
The premise of the accurate evaluation of deep coal and gas outbursts is to capture the nonlinear variation law of deep coal and gas outbursts by proper methods. At the same time, a deep coal and gas outburst is a small-probability event, which naturally has the characteristics of a small sample. SVM has unique advantages in solving small-sample, high-dimension, local-minimum, and nonlinear problems. Based on the comprehensive consideration of the calculation time and detection accuracy, in this paper, using the IQPSO-SVM risk assessment method for deep coal and gas outbursts, combined with the appropriate transformation, the gas outburst index of a deep coal mine is transformed into a specific problem suitable for the IQPSO-SVM algorithm model. Based on the correct selection of the kernel function, improved quantum particle swarm optimization (IQPSO) is used to optimize the penalty parameter (C) and kernel parameter (g) of SVM to balance the global search and local search problems in the algorithm design. The research shows that the detection accuracy of the methods is: standard SVM < PSO-SVM < QPSO-SVM < IQPSO-SVM; in terms of overall time consumption, PSO-SVM > QPSO-SVM > IQPSO-SVM. Therefore, IQPSO-SVM has an excellent performance in saving time, increasing accuracy, and improving efficiency in evaluating the risk of deep coal and gas outbursts.
6.3. A Research Idea for Solving Complex Nonlinear Problems Is Provided
According to the data characteristics of the safety evaluation system of gas outburst risk in the deep coal mine, this paper explores and proposes a set of intelligent evaluation methods based on improving the traditional standardized data processing methods. This method can not only deal with uncertain information but also provide the quantitative risk probability value, effectively solving the safety risk assessment of coal and gas outbursts in deep coal mines. However, a deep coal and gas outburst is one of many disaster risks, such as heat damage, water damage, and rock bursts, in the deep coal mine. Complex accident causation theory shows that many aspects increase the safety risk of the deep coal mine. Its causes are complex and interrelated, each kind of disaster affects the other, and the evolution process of the accident is dynamic and changeable. Therefore, a deep coal mine is a complex giant system involving secondary characteristics, derivation, sudden changes, crossover, variability, nonlinearity, fuzziness, strong coupling, and so on. It is a complex system under the joint action of multiple subjects, factors, scales, attributes, levels, and departments. It entails the integration and penetration of cross-level, cross-disciplinary edge-crossing comprehensive problems. The data for this problem are typically complex, highly dimensional, and nonlinear. Machine learning and intelligent algorithms can serve as essential resources for solving this problem.
6.4. It Opens up a New Possibility for Promoting the Classification Management of Coal Mine Safety and Improving the Early Warning System of Coal Mine Safety
At present, shallowly buried coal seams are being gradually mined. Deep mining is urgently needed in China’s energy structure and coal mine safety management practice. The deep coal seam faces the complicated objective reality of high ground temperature, high ground pressure, high gas content, and low coal permeability in the mining process, which leads to the significantly increased risk of deep coal and gas outbursts. In this paper, intelligent algorithms and deep learning are effectively integrated. The evaluation of coal and gas outbursts in the deep coal mine is transformed into a model suitable for a specific intelligent optimization algorithm. In the training of the deep learning model, the dialectical relationship between the global search and local search is fully considered to comprehensively improve the intelligence level of data processing and the evaluation accuracy and generalization ability of the evaluation model. A safety assessment method for gas outbursts, closely combined with the situation of the deep coal mine, is proposed. This method opens up a new possibility for promoting deep coal mines’ hierarchical prevention and control management and improving the safety early warning system.
7. Contributions and Limitations
According to the current situation of China’s energy structure and coal and mine safety management practice, this paper effectively combines sample data processing, data normalization method improvement, complex evaluation algorithm optimization, and other aspects with scientific and reasonable system design. It not only promotes a new way of thinking for the scientific evaluation of deep coal and gas outburst risk and deep coal and mine safety management but also provides an essential reference for the scientific evaluation of high-dimension, nonlinear problems in other fields. However, between theory and practice is a specific problem situation. The problem situation determines the input and output settings, the problem’s index and dimension, sample size requirements, and training models. As such, the choice of deep coal and gas outbursts involves the thermal damage in the deep coal mine, the damage and impact ground pressure, and so on. The safety risk of a deep coal mine is a disaster risk with many types of complex accident causes. Influenced by complex, interrelated, and mutual influences in the mined-out area and the changeable dynamic evolution process of the accident, the cause is a complex giant system involving secondary characteristics, derivation, sudden changes, crossover, variability, nonlinearity, fuzziness, strong coupling, and so on. Furthermore, it is under the joint action of multiple disciplines, factors, scales, attributes, levels, and departments, entailing the integration and penetration of cross-level, interdisciplinary edge-crossing comprehensive problems. In the future, the safety risk assessment of multiple disasters in deep coal mines should be studied in combination with the specific situation of deep coal mines, especially the coupling evaluation of numerous disasters in deep coal mines in complex environments. Based on a thorough grasp of the problem situation, it can gradually be extended to other high-dimensional and nonlinear problems to expand its application value.