The propagation speed of the in-seam wave has a functional relationship with the frequency, and the Airy phase with high frequency and large amplitude existing in the in-seam wave group velocity dispersion curve is the most important feature of the presence and strength of the in-seam wave signal (Yang et al. 2020). The main factors influencing the in-seam wave dispersion characteristics are the thickness of the coal seam, the degree of gangue, the degree of difference between the physical parameters of the coal seam and the surrounding rock, and the lithological characteristics of the roof and floor (Zhao et al. 2019; Gao et al. 2020).
The No.3 Coal Seam coal seam and roof and floor rock samples of Shuangliu Coal Mine were selected for related parameter testing to provide basic information for detection, data inversion, and result interpretation. The statistical results of the rock wave velocity test under no-load conditions are shown in Table 1. By comparison, it is found that the sequential changes of the ratio of P-wave and P-wave velocities with different lithologies are basically consistent with the sequential changes of lithological P-wave velocities. Since the seismic wave velocity of the rock formation is directly proportional to the density of the rock formation, the following table shows that the density of the coal seam in this area is compared with the density of the roof and floor rocks, that is, VP coal: VP mud ≈ 1:1.36, so this area has the conditions for in-seam wave formation.
Table 1
Test results of rock wave velocity under unloaded condition
Lithology
|
P-wave velocity /m·s− 1
|
S-wave velocity /m·s− 1
|
vp/vs
|
Coarse sandstone
|
3562.64 ~ 4269.13
3897.08
|
2701.67 ~ 3007.90
2891.81
|
1.28 ~ 1.44
1.35
|
Medium sandstone
|
3971.20 ~ 4194.94
4053.25
|
2784.44 ~ 3059.08
2896.07
|
1.37 ~ 1.43
1.40
|
Fine sandstone
|
3366.10 ~ 4021.14
3675.45
|
1858.91 ~ 2788.02
2276.84
|
1.40 ~ 2.14
1.77
|
Argillaceous sandstone
|
2777.16 ~ 3080.25
2928.70
|
1751.52 ~ 1770.87
1761.19
|
1.57 ~ 1.76
1.66
|
Too limestone
|
5346.49 ~ 6943.80
6355.61
|
2672.65 ~ 3445.10
3059.97
|
1.56 ~ 2.35
2.13
|
Austrian limestone
|
3977.73 ~ 5201.57
4771.74
|
1893.06 ~ 2593.99
2312.98
|
2.01 ~ 2.12
2.07
|
No.3 Coal Seam
|
2144.40
|
1340.97
|
1.6
|
The No.3 Coal Seam coal seam and roof and floor rock samples of Shuangliu Coal Mine were selected for related parameter testing to provide basic information for detection, data inversion, and result interpretation. The statistical results of the rock wave velocity test under no-load conditions are shown in Table 1. By comparison, it is found that the sequential changes of the ratio of P-wave and P-wave velocities with different lithologies are basically consistent with the sequential changes of lithological P-wave velocities. Since the seismic wave velocity of the rock formation is directly proportional to the density of the rock formation, the following table shows that the density of the coal seam in this area is compared with the density of the roof and floor rocks, that is, VP coal: VP mud≈1:1.36, so this area has the conditions for in-seam wave formation.
Using the obtained coal and rock sample parameters, using Norsar and Tesseral 3D numerical calculation software, using coal seam thickness and roof and floor lithology as variables, a three-dimensional numerical analysis model is established and forward simulation is performed. The model is set to a homogeneous and isotropic medium, and the finite difference method is used to perform a full-wave field forward simulation of the elastic wave equation. The size of the model is 100 0 m × 500 m (length × width), the length, width and height of the model grid are all 1 m, and the height of the model is determined according to the actual plan. There are 20 seismic sources and geophones, the shot spacing and the track spacing are both 20 m, the sampling length is 1000 ms, and the sampling rate is 0.1 ms. The excitation source is a Rake wavelet spherical source with a dominant frequency of 200 Hz. Regarding the influence of changes in coal seam thickness and roof and floor lithology characteristics on the in-seam wave seismic response characteristics, the transmission method is used to arrange the observation system in the numerical simulation, which is more conducive to the study of the regularity of its response characteristics.
3.1 Response characteristics of in-seam wave to coal seam thickness
According to the geological conditions of the mine, select the physical parameters of coal and rock as shown in Table 2,with the thickness of the coal seam as a variable (1 m, 3 m, 5 m, 8 m), the top and floor thickness of the model were selected as 50 m respectively, and a three-dimensional "roof-coal-floor" 3-layer symmetrical level was established. The three-dimensional numerical analysis model of the medium is shown in Fig. 1. The physical parameters of the model are shown in Table 2.
Table 2
Table 2 Physical parameters of coal and rock
Lithology
|
P-wave velocity /m·s− 1
|
S-wave velocity /m·s− 1
|
Density /kg·m− 3
|
Coal seam
|
2000
|
1200
|
1400
|
Mudstone
|
3000
|
1800
|
2100
|
The format conversion and Gabor change processing of the seismic wave information received in different coal thicknesses are performed, and the characteristics of the in-seam wave dispersion curve under different coal thickness conditions are obtained as shown in Fig. 2. It can be seen that the thickness of the coal seam has an obvious influence on the development of the in-seam wave: it is difficult to form a in-seam wave on the working face with a coal thickness of 1 m, and the reflected wave reception time and frequency are basically linear. After analysis, it is a S-wave of the coal seam; the coal thickness is 3 m When the coal thickness is 5 m, the in-seam wave has obvious characteristics; when the coal thickness is 8 m, the in-seam wave develops poorly again.
3.2 Response characteristics of in-seam wave to roof and floor lithology
A three-dimensional numerical analysis model of a homogeneous medium with different roof and floor lithology levels is established (Fig. 3). The forward numerical model scheme is shown in Table 3, and the physical parameters of the model are shown in Table 4. When the coal seam thickness is 5 m, the in-seam wave dispersion characteristics under different roof and floor conditions are shown in Fig. 4. It can be seen that when the direct roof and floor are mudstone with the same lithology, the in-seam wave is not affected by the change of the double-layer roof and floor. The development characteristics of the in-seam wave are the same as the frequency dispersion characteristics of the single roof and floor of mudstone in 3.1. It shows that the number of roof and floor rock layers has no effect on the formation of in-seam wave, and is only related to the lithology of the direct roof and the direct bottom.
Table 3
Forward numerical model scheme
Plan
|
Lithology of the roof
|
Top plate thickness/m
|
Coal seam thickness/m
|
Floor lithology
|
Thickness of bottom plate/m
|
Plan1
|
Sandstone/mudstone
|
25/25
|
5
|
Mudstone/Sandstone
|
25/25
|
Plan2
|
Sandstone/mudstone
|
25/25
|
5
|
Sandstone/sandstone
|
25/25
|
Plan3
|
Sandstone/limestone
|
25/25
|
5
|
Mudstone/Sandstone
|
25/25
|
Table 4
Physical parameters of coal and rock mass
Lithology
|
P-wave velocity /m·s− 1
|
S-wave velocity /m·s− 1
|
Density /kg·m− 3
|
coal seam
|
2000
|
1200
|
1400
|
sandstone
|
3500
|
2200
|
2650
|
Mudstone
|
3000
|
1800
|
2100
|
Limestone
|
6500
|
2700
|
2800
|
3.3 Analysis of simulation results
Through the analysis of the response characteristics of in-seam wave to coal thickness and roof and floor lithology in Shuangliu Coal Mine, the following results are obtained:
(1) The thickness of the coal has obvious influence on the characteristics of the in-seam wave dispersion curve. The in-seam wave has the characteristics of dispersion. The frequency of the in-seam wave decreases with the increase of the thickness of the coal seam, while the coal thickness basically has no effect on the Airy phase velocity of the in-seam wave. In the numerical simulation, the frequency of the Airy phase is higher than that of the measured results. The analysis is mainly due to the fact that the frequency of the seismic source selected by the simulation is inconsistent with the frequency of the actual explosive source. According to the working face length of 200 m, the time for the geophone to receive the in-seam wave is about 200 ms, and the calculation shows that the in-seam wave velocity is about 1000 m/s.
(2) When the top and bottom lithological wave speeds are close, the influence on the frequency of the in-seam wave Airy phase is small. When the top and bottom wave speeds differ greatly, the dispersion response of the Airy phase increases, but the amplitude is small. Different roof and floor lithology combinations have no obvious influence on the in-seam wave velocity, and the in-seam wave velocity is still about 100 0 m/s.
Through the relevant parameter test of No. 3 coal seam and roof and floor rock samples of Shuangliu Coal Mine, it provides basic information for detection, data inversion and result interpretation. According to the simulation results, the response characteristics of in-seam wave under different geological conditions in Shuangliu Coal Mine are mastered, which provides a theoretical basis for further research on the reflection of in-seam wave in fault-bearing coal seams in Shuangliu Coal Mine.