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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