Thick Coal Seam Research Articles

Influencing Factors Analysis and Prediction of Gas Emission in Mining Face

Mine gas emission is one of the main causes of gas disasters. In order to achieve the accurate prediction of gas emission, a gas emission prediction model based on the random forest (RF) method was proposed in combination with the analysis of its influencing factors. The prediction results were compared with the support vector regression (SVR) and BP neural network (BPNN) methods, and then they were verified and analyzed through the Dongqu coal mine. The results show that the gas emission prediction model based on random forest has strong generalization and robustness, and RF has a wide range of parameter adaptation during the modeling process. When the number of trees (ntree) exceeds 100, its training error tends to stabilize, and changes in ntree have no substantial impact on the prediction performance. The SVR prediction model has significant bias in both the training and testing stages. Meanwhile, the BPNN model has excellent prediction results in the training phase, but there is a large error in the testing stage, which indicates that there is an “overfitting” phenomenon in the training stage, resulting in weak generalization. The evaluation of variable importance shows that the extraction rate, coal seam depth, daily production, gas content in adjacent layers, and coal seam thickness have a significant impact on gas emission. Meanwhile, through application analysis, it is further demonstrated that the random forest method has high accuracy, strong stability, and universality, and it can achieve good predictive performance without the need for complex parameter settings and optimization, making it is very suitable for predicting gas emission.

Research and applications of the cross-fusion mechanism of high gas thick coal seam influenced by dip angle

Research and applications of the cross-fusion mechanism of high gas thick coal seam influenced by dip angle

Geological Condition Characterization and Coal Resource Estimation at the “KJG” Pit of PT Bukit Asam Tbk, Muara Enim Regency, South Sumatra Province

Abstract Coal is a conventional energy source found in tropical countries (e.g., Indonesia) with varying distribution and thickness in the South Sumatra Basin. However, the thickness of the coal seams was not evenly distributed. This affected the estimation of coal resources in the mining area of PT Bukit Asam Tbk in Tanjung Enim, Muara Enim Regency. We used field observation methods and borehole data analysis to determine the geological conditions and estimate the coal resources. The geological condition seems to indicate a claystone unit with tuffaceous sandstone interbeds and a sandstone unit with claystone interbeds. These two units might host coal seams (i.e., A1, A2, B1, B2, and C), where the coal seam thickness may determine increased towards the south and west in the study area. The coal resource estimation based on the SNI 5015: 2019 classification, under moderate geological conditions, can result in a total of 26.034.991 tonnes of measured resources, 25.209.688 tonnes of indicated resources, and 11.189.554 tonnes of inferred resources. Therefore, to ensure efficient coal mining in accordance with the measured reserves, the stripmining method may be used by the company.

Air leakage mechanism and hole sealing technology in directional long-drilled perimeter rock-borehole composite fissures: for gas enrichment in varying coal-seam thickness change areas

The issue of gas enrichment in the tectonic zone of coal seams, which poses significant threats to coal mine safety and leads to gas disasters, is primarily addressed through borehole extraction. The quality of this extraction is notably influenced by air leakage and sealing technologies used during drilling. To tackle the problem of gas leakage in directional long boreholes within the floor, a composite fissure leakage model for the surrounding rock and borehole is developed. This model inverts the characteristics of coal seam thickness variations and abnormal gas enrichment through heterogeneous geostatistical modeling. It enables the quantitative characterization of the air leakage mechanism in directional long boreholes of the bottom plate and proposes an effective sealing process. Due to damage and disturbance, fissures and irregular plastic zones form around the excavated roadway, which are major contributors to air leakage. Over time, the air content in the coal and rock seams gradually increases while the gas content and concentration decrease continuously. Compared to the polyurethane "two plugs and one injection" sealing process, the multi-stage grouting and sealing process provides a more stable gas extraction concentration and flow rate in the sealed drill holes. This process effectively seals the fissure zones of the roadway, reduces air leakage from the boreholes, and improves gas extraction concentration.

Dynamic Instability Mechanism and Stress Staged Evolution of Gob‐Side Entry During Cross‐Fault Mining for Thick Coal Seam

ABSTRACTDuring cross‐fault mining, the stress concentration in the surrounding rock of thick coal seam gob‐side entry was prone to dynamic disasters. Based on the in‐site geological conditions of the No. 6305 coal face of the Xinjulong coal mine, a FLAC3D numerical simulation model was established to research the failure and stress staged evolution of coal pillar in the gob‐side entry during cross‐fault mining. By analyzing the relation of surrounding rock structure, the mechanical models of different stages during cross‐fault mining were established. Furthermore, the mechanical mechanism of coal pillars' dynamic instability under the influence of fault activation was revealed, and the mechanical criterion n was given. The control technology of ‘asymmetric strengthening support + roof cutting and pressure relief’ was proposed and designed. Field practice showed that the maximum roof‐to‐floor and two‐side displacements of the gob‐side entry are 249.3 mm and 150.4 mm, and the force of anchor cable is 184.2 kN. This research provided theoretical guidance and reference for the stability control of roadways within the influence range of fault under deep mining conditions.

Monitoring of Overburden Failure with a Large Fractured-Height Working Face in a Deep Jurassic Coal Seam Based on the Electric Method

The development height of a water-flowing fractured zone is the key parameter to consider when carrying out mining under water pressure and coal mining with water conservation. In this paper, Jurassic coal seam 3-1 in the Menkeqing Coal Mine was taken as the research target, and a three-dimensional mining geological model was established by using FLAC3D to study the deformation and failure rules of overburden. Three roof boreholes were drilled in the auxiliary transportation roadway of adjacent working faces for dynamic monitoring by the resistivity method, which can better observe the whole process from failure to stability of the overburden. The results show that due to the complex sedimentary environment and large buried depth of coal seams in western China, there is a large deviation between the calculation results of the empirical formula of the fractured zone height under the “Regulations of buildings, water, railway and main well lane leaving coal pillar and press coal mining” (three regulations) and the simulation and on-site measurement. Based on the comprehensive analysis, the influence range of mining advance abutment pressure is approximately 60 m. The height of the water-flowing fractured zone is approximately 106 m, and it is located at the interface between sandy mudstone and mudstone. The height of the caving zone is approximately 22 m, and it is located at the interface between fine sandstone and medium sandstone. The ratio of the fractured height and coal seam thickness (Rf) reached 24.4, which was basically consistent with the test result of the adjacent Yushenfu mining area (which was 26 on average). There is no obvious change in the development height of the caving zone and water-flowing fracture zone from the working face to the drilling borehole position of more than 120 m, which reflects that the height of the overburden failure zone is related to the control of lithological combination.

Energy release and disaster-causing mechanism of ore-pillar combination

Coal and other mineral resources are also commonly found in bauxite mining. The process of bauxite mining is usually affected by retained ore pillars in other strata, which leads to the formation of combinations composed of different strata. Once these combinations become unstable, they can cause serious disasters that threaten production safety. Aiming at the safe mining of co-associated resources in the overlying coal seams of bauxite mines, in this paper, the strength, fracture development, and energy evolution of the coal-rock-aluminum (C-R-A) combination under varying thickness proportions of coal, rock, and aluminum were studied by means of particle flow code (PFC) numerical simulation, SPSS statistical analysis, and other methods. The results indicate that the strength of the combination is significantly negatively correlated with the thickness of the soft coal seam and remarkably positively correlated with the thickness of the hard aluminum layer. Under the same stress conditions, fractures in the combination mainly occur in the coal seam. As the thickness proportion of the coal seam in the overall structure increases, the number of fractures there grows correspondingly. Under a larger thickness proportion of the rock stratum, the combination releases its elastic energy faster after instability, and the fractures develop more intensely. As the thickness proportion of the rock stratum decreases, the elastic energy index (WET) in the C-R-A combination rises, and the burst proneness strengthens. Areas where the thickness proportions of coal, rock, and aluminum lie in the ranges of 30%–60%, 10%–20%, and 30%–60% respectively are considered high-risk zones, and rock burst accidents are most likely to occur when the thickness ratio of coal, rock, and aluminum is 4: 1: 5. These research findings can provide guidance for the safe mining of similar coal and aluminum associated resources.

Study on the accurate detection method of full‐polarimetric ground‐penetrating radar faults in mines based on modified Yamaguchi decomposition

AbstractObtaining information on tectonic tendencies is a prerequisite for intelligent and accurate mining in mines. In the special mine environment, the co‐polarized ground‐penetrating radar can only identify the spatial location of faults, and it is difficult to analyse the inclination information of fault structures. This paper proposes a mine full‐polarimetric ground‐penetrating radar fault tendency detection method based on this. First, based on the stacking characteristics of the coal depositional, this paper analyses the propagation law of the pulse electromagnetic wave in the coal seam and puts forward the assumption of the overlapping echo reflection of the fault structure. The reasonableness of the fault reflection assumption is verified through a numerical simulation study. Second, based on the cutting relationship of the fault to the coal seam, we divided the reflection structure of the fault structure into plane scattering and dihedral angle scattering. We realized the mingled echoes’ decomposition using the improved Yamaguchi decomposition technique. To analyse the applicability of the modified Yamaguchi and Freeman decomposition methods in the identification of fault inclination, we use the upright fault simulation data for the discussion, and we find that the improved Yamaguchi decomposition method is more advantageous in the identification of fault inclination in the mine. The decomposition results based on the simulation data of fault models with different dip angles found that when the dip angle of the fault is less than 90°, the scattering of the fault structure is dominated by planar scattering and dihedral angle scattering; when the dip angle of the fault is greater than or equal to 90°, the scattering of the fault structure is dominated by planar scattering, and the scattering power of the dihedral angle model is zero. By analysing the effect of fault strike on the decomposition results, it is found that the fault strike angle has little effect on the identification of fault tendency. Finally, the application potential of this paper's method is tested by constructing complex numerical models and probing experiments. Therefore, the method proposed in this paper can solve the fault tendency identification under a thick coal seam.

Advanced modeling of seepage dynamics and control strategies in thick coal seams under high-confined aquifer conditions: A case study

The hydraulic behavior of the connection between the floor failure area and the aquifer water-conductive zone is considered to be the root cause of mine water inrush disasters. Therefore, unraveling the floor failure mechanism is particularly important for safe coal mining above the high-confined aquifer. This paper estimates the depth of the baseplate failure to be 18.4–27.3 m by combining network parallel electrical methods with drilling visualization technology. The FLAC3D-based numerical model considering the strain hardening of caved rock was established with rigorous calibration and verification. The results showed that the depth of damage to the floor is 23.1 m, and the dominating floor failure mechanism is shear failure caused by the vertical stress exceeding the rock bearing capacity. Moreover, the stress recovery process of the baseplate does not alter the failure morphology of the baseplate. Based on the above research findings, the in-situ floor control technique of the working face No. 4305 is proposed and practiced in the field. Field measurements show that floor control performance is satisfactory with water inflow in the goaf being roughly stable at 50 m3/h. Our results can provide useful reference for safe mining above confined aquifer and prevention and mitigation of water-related hazards.

Static expansion fracturing mechanism for enhancing gas permeability in low permeability coal seams

Extraction of gas from low-permeability thick coal seams poses challenges globally, attributed to low extraction efficiency, limited enrichment content, and extraction complexities. Investigating static fracturing in low-permeability thick coal seams holds substantial engineering significance and practical utility. This research delves into the current conditions in the Western region, characterized by low gas permeability and challenging extraction in thick coal seams. Through the utilization of FLAC3D and COMSOL, simulation schemes are devised to analyze the influence of borehole parameters on fracture efficacy, elucidating the mechanisms by which external loads and internal gas pressure impact coal seam permeability. Field monitoring tests are employed to evaluate and model gas extraction enhancement via borehole positioning. The results suggest that the application of a static fracturing agent inducing 40 MPa expansion stress, along with a 75 mm borehole diameter and 0.5 m spacing, effectively fractures low-permeability thick coal seams. The spacing between extraction and fracturing holes adversely affects gas extraction efficiency due to the limited range of static fracturing. Field experiments demonstrate that static fracturing significantly improves gas extraction from low-permeability thick coal seams, resulting in a twofold increase in average gas extraction purity post-fracturing. This study establishes a robust theoretical foundation for optimizing gas extraction and mining activities in challenging low-permeability thick coal seam environments.

Paleoenvironmental reconstruction of coal deposition during the Cretaceous-Paleogene transition in the Eastern Cordillera Basin, Colombian Andes

Stratal stacking patterns and factors influencing peat accumulation in coastal and continental settings represent a significant problem in studying coal-bearing sequences. To address this issue, this work focused on the Cretaceous-Paleogene Guaduas Formation on the Checua-Lenguazaque Syncline (CLS) coalfield in the Eastern Cordillera Basin (Colombian Andes). This study relies on geological survey, facies analysis, sequence stratigraphy, organic geochemistry, and coal petrography. Through these methods, depositional systems and sequences were characterized, and their relationship with coal composition was established. Sedimentary facies were categorized into four Facies Associations (FAs): lagoon, tidal flat, delta plain, and mixed fluvial system. Five T-R sequences (S1 to S5, in ascending order) were identified. S1 consists of lagoon and tidal sandstone, mudstone, and coal. S2-S4 comprise tidal, deltaic, and fluvial deposits. S5 is composed mainly of deltaic and fluvial facies. Thick coal seams (> 0.7 m) were concentrated in the regressive system tracts of S1 and S3, while the transgressive coals were deposited in S2-S3 and are associated with tidal environments. The organic petrography showed enrichment in vitrinite (30.00–85.20 %), while liptinite (0.00–16.60 %) and inertinite (4.60–34.40 %) varied according to depth and paleoenvironments. CLS coalfield displays an environmental evolution from shallow marine and lagoon deposits to deltaic and fluvial environments. Minor sea-level fluctuations, changes in accommodation, siliciclastic influx, and plant community distinguish this sedimentary succession. The deposition of the Guaduas Formation is characterized by a prograding pattern with dominant shallowing-upward cycles in a high accommodation setting. The organic matter accumulated under limno-telmatic to telmatic conditions in mesotrophic to ombrotrophic environments with nutrients derived mainly from rainfall. The paleoclimate for the Guaduas Formation indicates wet and hot conditions for flora expansion. This investigation determined paleoenvironments of the Maastrichtian-Paleocene coastal to fluvial successions within the tropical latitudes, indicating a strong relationship between depositional systems, sequence stratigraphy, paleoclimate, and coal composition.

Optimization of drawing sequence in longwall top coal caving mining through an FDM‐DEM model

AbstractThe Longwall top coal caving (LTCC) technology is regarded as one of the most crucial approaches for exploiting thick coal seams. A crucial and effective approach for improving the recovery rate of top coal and reducing coal resource losses in LTCC faces is to reasonably select process parameters based on actual mining and geological conditions of different mines. The main focus of this paper is the engineering background of the 12,309 LTCC face in Wangjialing Coal Mine. A numerical model is developed using FALC3D and PFC3D software, employing a finite difference method and discrete element method. This model takes into account predetermined cutting and caving ratios, as well as drawing intervals. To examine the caving process and roof particles, three different drawing sequences were examined: sequential drawing, segmented sequential drawing, and intermittent drawing. The findings suggest that, in terms of the reset shape of the drawing body before individual and entire caving, the segmented sequential drawing method exhibits noticeable drawbacks compared to the other two methods. From the perspective of the drawing weight, following “closing drawing opening when seeing gangue”, the sequential drawing, segmented sequential drawing, and intermittent drawing methods can yield 32.42 t, 26.87 t, and 35.78 t of top coal, with corresponding recovery rates of 73.39%, 60.81%, and 82.97%. Therefore, it can be concluded that intermittent drawing is suitable for implementation on LTCC working face 12,309.

Study on the rockburst risk of alternate exterior entry under island coal pillar

The extraction of extremely thick coal seams through slicing mining requires leaving behind several island coal pillars in the top slicing, which could lead to rockburst in the bottom slicing. This article proposed a method of alternate exterior entry (AEE) for rockburst prevention based on the stress distribution of bottom slicing. The influences of coal pillar width [Formula: see text] and stress concentration factor [Formula: see text] on stress distribution were explored using two-dimensional stress imaging. The results indicated that K value had a greater effect on stress distribution comparing with B value. Both stress relaxed and stress rate angles increased linearly with an increase in K value. In order to get the optimized location of AEE in the coal mine stress rate angle was chosen as the stress extension angle, which was further verified by field data from 1210 coal faces. Finally, numerical simulation was used to explore the optimized location of special tail entry under dip island coal pillars for rockburst prevention. It was revealed that plan first was found to be most effective due to its presence in a stress-relaxed area, thus verifying the effectiveness of the AEE method for rockburst prevention.

Optimization of small coal pillar width and control measures in gob-side entry excavation of thick coal seams

This study introduces an innovative optimized bolting support system specifically tailored for gob-side entry excavation in thick coal seams at a coal mine in southwestern Shandong, China. Employing theoretical analysis, numerical simulation, and field measurements, the research focuses on examining the failure characteristics of surrounding rock during gob-side entry excavation. The key innovation lies in the development of a 5-meter optimal coal pillar width, ensuring balanced stress distribution and structural integrity. Additionally, a lag time of at least 46 days between gob-side entry excavation and the upper working face retreat is recommended to mitigate roof subsidence and surrounding rock deformation. The optimized bolting support system, featuring increased bolt pretension, utilization of high-strength steel strips, and reinforcement of weak points, effectively reduces deformation of the roadway surrounding rock, meeting support requirements for normal production. This novel approach successfully addresses the support challenges in thick coal seam gob-side entry excavation, enhancing mining safety and resource recovery rates.

Numerical analysis of water injection in coal seams under mining influence

This study investigates the diffusion characteristics and optimization of borehole water injection in the 3# coal seam of Licun Coal Mine under mining influence. The distribution of stress and permeability ahead of working face using FLAC3D combined COMSOL Multiphysics. The influence of distances between the borehole along with the diameters of the boreholes on the stress distribution around the borehole is studied. Additionally, the diffusion characteristics of coal seam water injection are analyzed under various parameters. The study found that borehole proximity to the working face significantly enhances stress relief and permeability, while borehole diameter has a minor impact. Simulation results from COMSOL Multiphysics indicated that increasing injection pressure significantly improves seepage velocity and wetting range, though borehole diameter effects remain negligible. Optimal water injection parameters were identified, suggesting that the most effective water injection occurs at a borehole 3 m from the working face, with a diameter of 0.11 m and an injection pressure of 10 MPa, achieving saturation within 48 h. This research contributes to the theoretical and practical understanding of optimizing borehole water injection in complex geological settings and thick coal seams.

A NIRS-based recognition of coal and rock using convolution-multiview broad learning system

Achieving high production in the top coal caving process from thick coal seams is crucial. Thus, the timely decision of when to stop caving poses an urgent challenge to impact the mining loss rate and cost recovery. To address this issue, an innovative recognition system has been developed using Near-Infrared Spectroscopy (NIRS) technology. It stands out for its on-site usability, it enables rapid data collection and local recognition at the longwall face. Furthermore, to overcome the limitations of existing methods in adapting to variations in spectral data quality during on-site collection and the lack of integration of spectral data across different feature processing stages, a coal-rock recognition method has been developed which can ignore the influence of acquisition factors(granularity, light source angle, and detection sensor angle). This method incorporates the features of convolution and multi-view into the BLS model, the designed model structure exhibits a remarkable recognition accuracy of 99.78 %. The model was deployed into the recognition system, and experimental tests were conducted on the working face. The results showed that the recognition system can effectively identify the entire coal-caving process and achieve a recognition accuracy of 92.3 %. This capability is crucial for determining the optimal point to stop roof caving.

Based on Flac3D Research and Analysis of the Mining Pressure Development Law in the Top Coal Section of the Roadway Support of the Working Face Transport

Top coal supporting roadway is widely used in the transportation roadway of thick coal seam face, which can not only increase the coal extraction rate but also reduce the driving cost. Based on the engineering geological conditions of the transportation lane of the working face of Zhongma Cun mine, the law of mine pressure development in the roadway of the top coal section is studied and analyzed based on Flac3D and field monitoring methods. The numerical simulation analysis shows that when the working face is pushed to 55m for the first time, the displacement of the area affected by the leading abutment pressure and the surrounding rock on the mining side of the roadway reaches the peak value, and the step distance of the first time is 55m. According to the analysis of stress monitoring data of single pillar and roof anchor cable on site, the first pressure occurs when the working face advances to 51m, which is consistent with the numerical simulation conclusion.

Research on Velocity Feedforward Control and Precise Damping Technology of a Hydraulic Support Face Guard System Based on Displacement Feedback

The hydraulic support face guard system is essential for supporting the exposed coal wall at the working face. However, the hydraulic support face guard system approaching the coal wall may cause impact disturbances, reducing the load-bearing capacity of coal walls. Particularly, the hydraulic support face guard system is characterized by a large turning radius when mining thick coal seams. A strong disturbance and impact on the coal wall may occur if the approaching speed is too fast, leading to issues such as rib spalling. In this paper, a feedforward fuzzy PID displacement velocity compound controller (FFD displacement speed compound controller) is designed. The PID controller, fuzzy PID controller, feedforward PID controller, and FFD displacement speed compound controller are compared in terms of the tracking characteristics of the support system and the impact response of the coal wall, validating the controller’s rationality. The results indicate that the designed FFD displacement speed compound controller has significant advantages. This controller maintains a tracking error range of less than 1% for target displacement with random disturbances in the system, with a response adjustment time that is 34% faster than the PID controller. Furthermore, the tracking error range for target velocity is reduced by 8.4% compared to the feedforward PID controller, reaching 13.8%. Additionally, the impact disturbance of the support system on the coal wall is suppressed by the FFD displacement speed compound controller, reducing the instantaneous contact impact between the support plate and the coal wall by 350 kN. In summary, the FFD compound controller demonstrates excellence in tracking responsiveness and disturbance rejection, enhancing the efficacy of hydraulic supports, and achieving precise control over the impact on the coal wall.

The Gas Production Characteristics of No. 3 Coal Seam Coalbed Methane Well in the Zhengbei Block and the Optimization of Favorable Development Areas

The characteristics and influencing factors of gas production in CBM wells are analyzed based on the field geological data and the productivity data of coalbed methane (CBM) wells in the Zhengbei block, and then the favorable areas are divided. The results show that the average gas production of No. 3 coal seam CBM wells in the study area is in the range of 0~1793 m3/d, with an average of 250.97 m3/d; 80% of the wells are less than 500 m3/d, and there are fewer wells above 1000 m3/d. The average gas production is positively correlated with gas content, critical desorption pressure, permeability, Young’s modulus, and Schlumberger ratio, and negatively correlated with fracture index, fault fractal dimension, Poisson’s ratio, and horizontal stress difference coefficient. The relationship between coal seam thickness and the minimum horizontal principal stress is not strong. The low-yield wells have the characteristics of multiple pump-stopping disturbances, unstable casing pressure control, overly rapid pressure reduction in the single-phase flow stage, sand and pulverized coal production, and high-yield water in the later stage during the drainage process. It may be caused by the small difference in compressive strength between the roof and floor and the coal seam, and the small difference in the Young’s modulus of the floor. The difference between the two high-yield wells is large, and the fracturing cracks are easily controlled in the coal seam and extend along the level. The production control factors from strong to weak are as follows: critical desorption pressure, permeability, Schlumberger ratio, fault fractal dimension, Young’s modulus, horizontal stress difference coefficient, minimum horizontal principal stress, gas content, Poisson’s ratio, fracture index, coal seam thickness. The type I development unit (development of favorable areas) of the Zhengbei block is interspersed with the north and south of the block on the plane, and the III development unit is mainly located in the east of the block and near the Z-56 well. The comprehensive index has a significant positive correlation with the gas production, and the prediction results are accurate.

Identification and Application of Wave Field Characteristics of Channel Waves in Extra-Thick Coal Seams

Channel wave seismic activity often occurs with thin and medium-thick coal seams being the main target layer. To address the problem of channel wave applicability detection in extremely thick coal seams, the propagation and identification characteristics of channel waves remain the focus of research. Therefore, this paper takes the in-seam wave exploration of a 27 m extremely thick coal seam as an example and uses the staggered mesh finite difference method to construct a three-dimensional medium model for numerical simulation. An analysis of the physical parameters of coal and rock, along with the dispersion characteristics of channel waves in extra-thick coal seams, is utilized, through the Zoeppritz equation and the total reflection propagation method, to calculate the imaging. We found the following: (1) The dispersion areas and weak dispersion areas along the detection direction are extremely thick coal seams. (2) There are apparent channel waves in extra-thick coal seams, with a waveform similar to body waves; the length of the wave train is shorter than that of the conventional channel wave, and the arrival time can be estimated accurately. The amplitude of the apparent channel wave is affected by the degree of dispersion, with lower attenuation and higher resolution. The characteristic of total reflection in extremely thick coal seams is that the incident angle is equal to the critical angle, and the dispersion characteristics are weak. (3) The channel waves with weak dispersion characteristics in extra-thick coal seams are mainly Love-type waves.