Download Modelling of Rayleigh-type seam waves in disturbed coal seams

Modelling of Rayleigh-type seam waves in disturbed coal seams
and around a coal mine roadway
K. Essen1 ∗ , T. Bohlen2 , W. Friederich1 and T. Meier1
1
Ruhr-University Bochum, Institute of Geology, Mineralogy and Geophysics, D-44780 Bochum, Germany
2
TU Bergakademie Freiberg, Institute of Geophysics, Gustav-Zeuner-Str. 12, 09596 Freiberg, Germany
For a site investigation of the rock structure beyond the heading face during the
drivage of new tunnels, several methods have been developed in the past few years in civil
engineering (e.g. Sattel et al., 1992; Borm et al., 2003; Otto et al., 2002; Kneib et al.
2000; Inazaki et al. 1999). The existing systems are adapted to seismic reconnaissance
for tunnelling in hard rock or soft ground and make use of reflected body waves. Up to
now no system based on seismic methods exists that provides such information in coal
mining. Such a technique can help to reduce the financial risk of shut downs of the mining
operation and the hazards for the miners.
The coal layer is a channel of lower seismic velocities and density compared to the
country rock in the neighbourhood of the coal, if this rock consists e.g. of shale or
sandstone as typically in the Ruhr area (Germany). In such coal seams dispersive channel
waves, so called seam waves, can be generated. Because of their cylindrical geometrical
spreading seam waves are suitable for the investigation of the structure and layering of coal
seams. Since in coal mining, roadways are usually driven parallel to the seam, dispersive
seam waves can be generated by locating for example explosive sources within the seam.
Special methods have been developed for the processing and analysis of dispersive channel
waves and used for the localisation and mapping of seam disturbances within a seam panel.
Detailed treatments of in-seam seismics are given e.g. by Booer (1982), Buchanan (1983)
and Dresen & Rüter (1994). For a site investigation of the rock structure beyond the
heading face during the drivage of a new coal mine roadway, no special purpose seismic
system exists that makes use of seismic methods. To interpret measured data acquired
in such field experiments synthetic data are needed, as waves are propagating in complex
structures including several disturbances of the coal seam. Moreover, wave propagation
effects due to the presence of the tunnel or roadway have to be considered and can be
investigated with synthetic wavefields.
Wave propagation in coal seams is numerically modelled in order to identify approaches
towards the reconnaissance beyond the heading face of an advancing coal mine roadway
(Essen et al., 2007). Complete synthetic wavefields including P-SV body waves and
Rayleigh-type seam waves are calculated using a Green’s function approach (Friederich &
phone:
bochum.de
∗
+49-234/32-23275; fax:
+49-234/32-14181, E-mail address:
1
katja.dietrich@ruhr-uni-
S−waves
RS−S
RS−RS
10
Depth [m]
Depth [m]
S−waves
5
RS
15
20
T=0.030 s
10
20
30
40
50
60
Distance [m]
70
80
90
5
RS
10
15 RS−S
20
T=0.030 s
10
100
20
30
40
50
60
70
Distance [m]
80
90
100
110
120
80
90
100
110
120
P−waves
RS−RS
10
Depth [m]
Depth [m]
P−waves
5
RS
15
20
10
20
30
40
50
60
Distance [m]
70
80
90
5
RS
10
15
20
10
100
20
30
40
50
60
70
Distance [m]
Figure 1: Snapshots of the divergence and curl of the seismic wavefield in the X-Y-plane
for a time step of 30 ms. The coal layers are framed with white lines. The source was
located in the centre of the seam at distances of 39 m and 44 m respectively. Left hand
side: Washout or pinch completely interrupting the seam. When the direct Rayleigh-type
seam wave (RS) reaches the disturbance, one part is reflected as seam wave (RS-RS). The
main part of the wave is converted into S-waves (RS-S) and radiated into the roof of the
neighbouring rock. Right hand side: Seam splitting. Amplitudes of the reflected seam
waves are very low, so they are likely to be hidden by noise in measured data. The main
part of the wave is again converted into S-waves (RS-S).
Dalkolmo, 1995) for simple, laterally homogeneous models and a parallel elastic 2-D/3D finite-difference modelling code (Bohlen, 2002) for more realistic geometries. For a
simple three-layer model the wavefield within the seam is dominated by a fundamental
Rayleigh seam mode. If the seam contains an interleaved dirt band with higher velocities
and density, higher modes dominate the wave propagation. Wave propagation in laterally inhomogeneous coal seam models with disturbances like seam ends, faults, thinning,
washouts and seam splitting is strongly influenced by the type of disturbance.
Pinches, washouts or the very common seam splitting (Fig. 1) are examples of such
seam interruptions. It is important to detect these types of disturbances prior to the
drivage of a new roadway. In particular when seam splitting occurs, it might in some
cases no longer be profitable to mine the seam if the seam thickness gets too small. In
the model with a pinch completely interrupting the seam only a small part of the incident
fundamental mode Rayleigh-type seam wave is reflected as a fundamental mode seam
wave (left hand side of Fig. 1). Mode conversion is observed generated by reflection
at the fault zone. The main part of the wave is converted into S-waves and radiated
into the roof of the neighbouring rock. On the contrary, in the seam splitting model no
seam wave is reflected at the disturbance (right hand side of Fig. 1) or the amplitudes of
the reflected seam wave are very small. Waves propagate into the surrounding rock and
into the thinner split seam layers. According to these results a reconnaissance of seam
splitting disturbances beyond a single advancing coal mine roadway is not possible solely
by searching for reflected seam waves.
2
Amplitudes of seam waves reflected from disturbances strongly depend on the fault
throw and the degree of thinning or washout. In some cases, conversion to higher modes
can occur. In all investigated models, those Rayleigh seam wave phases are preferably reflected, which have frequencies above the fundamental mode Airy phase. Lower frequency
phases are preferably transmitted.
FD computations for 3D models containing an ending tunnel parallel to the seam
and a source beyond the heading face of the tunnel show, that seam waves are converted
into Rayleigh waves at the tunnel face. They propagate along the surface of the tunnel
and interfere with the seam waves propagating beside the tunnel. Polarisation analysis
showed, that the elliptically polarised Rayleigh-type seam waves in the vertical-radial
plane can be distinguished from Rayleigh tunnel waves propagating on the sidewall of the
tunnel adjoining the coal layer with elliptical polarisation in the radial-transversal plane.
References
Bohlen, T., 2002. Parallel 3-D viscoelastic finite difference seismic modelling, Computers &
Geoscience, 28, 887–899.
Booer, A. K., 1982. Developments in geophysical exploration methods, vol. 3, chap. Underground geophysics of coal seams, Applied Science Publischers, London and New York.
Borm, G., Giese, R., Klose, C., Mielitz, S., Otto, P., & Bohlen, T., 2003. ISIS - integrated
seismic imaging system for the geological prediction ahead of underground construction,
65th Conference & Exhibition, EAGE, Stavanger, Norway.
Buchanan, D., 1983. In-seam seismology: a method for detecting faults in coal seams, vol. 5,
chap. Underground geophysics of coal seams, Applied Science Publischers, London and
New York.
Dresen, L. & Rüter, H., 1994. Seismic Coal Exploration, Part B: In-Seam Seismics, vol. 16B,
Pergamon.
Essen, K., Bohlen, T., Friederich, W. & Meier, T., 2007. Modelling of Rayleigh-type seam waves
in disturbed coal seams and around a coal mine roadway, Geophys. J. Int. (accepted).
Friederich, W. & Dalkolmo, J., 1995. Complete synthetic seismograms for a spherically symmetric earth by a numerical computation of the Green’s function in the frequency domain,
Geophys. J. Int., 122, 537–550.
Inazaki, T., Isahai, H., Kawamura, S., Kurahashi, T., & Hayashi, H., 1999. Stepwise application
of horizontal seismic profiling for tunnel prediction ahead of the face, The Leading Edge,
18, 1429–1431.
Kneib, G., Kassel, A., & Lorenz, K., 2000. Automatic seismic prediction ahead of the tunnel
bore machine, First Break , 18, 295–302.
Otto, R., Button, E., Bretterebner, H., & Schwab, P., 2002. The Application of TRT - True
Reflection Tomography - at the Unterwald Tunnel, Felsbau, 20(2), 51–56.
Sattel, G., Frey, P., & Amberg, R., 1992. Prediction ahead of the tunnel face by seismic methods
- pilot project in Centovalli Tunnel, Locarno, Switzerland, First Break , 10, 19–25.
3