Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007geoji.170..511e&link_type=abstract
Geophysical Journal International, Volume 170, Issue 4, pp. 511-526.
Astronomy and Astrophysics
Astronomy
1
Finite Difference Methods, Green'S Function, Mode Conversion, Scattering, Seam Waves, Synthetic Wavefields
Scientific paper
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. Complete synthetic wavefields including P-SV body waves and Rayleigh-type seam waves are calculated using a Green's function approach for simple, laterally homogeneous models and a parallel elastic 2-D/3-D finite difference modelling code for more realistic geometries.
For a simple three-layer model the wavefield within the seam is dominated by a fundamental Rayleigh seam mode symmetrical with respect to the centre of the seam on the vertical component and antisymmetrical on the horizontal component. If the seam contains an interleaved dirt band with higher velocities and density, higher modes dominate the wave propagation, depending on the thickness of the dirt band. 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. Amplitudes of seam waves reflected from these 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. However, seam waves are not reflected from a seam splitting disturbance. Thus a detection of seam splitting with reflected seam waves appears to be impossible.
FD computations for 3-D 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. This effect has to be taken into account for subsequent treatment of experimental data, where the locations of sources and receivers are restricted to a small seismic layout in the vicinity of the tunnel. As tunnel surface waves have slightly lower frequencies than seam waves, it may be feasible to separate tunnel waves from the seam wave reflections, particularly because higher frequency phases of the seam wave are preferably reflected at seam disturbances. Polarization analysis showed, that the elliptically polarized 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 polarization in the radial-transversal plane.
Bohlen Thomas
Essen Katja
Friederich Wolfgang
Meier Thomas
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