THE INFLUENCE OF OBLIQUELY DIPPING DISCONTINUITIES ON THE USE OF RAYLEIGH CHANNEL WAVES FOR THE IN-SEAM SEISMIC REFLECTION METHOD * |
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| Authors: | S FREYSTTTER L DRESEN |
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| Institution: | S. FREYSTÄTTER,L. DRESEN |
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| Abstract: | The geological sequence rock-coal-rock represents a seismic low-velocity channel. Channel waves generated in a coal seam and their reflections from discontinuities can be used for proving the minability of the seam. To investigate the process of reflection, two-dimensional models of the sequence rock-coal-rock have been investigated by means of the ultrasonic transducer technique. Two-dimensional models have the advantage that the wave field can be observed at an arbitrarily chosen point of the model plate. Thus, by means of these models the direct and the reflected Rayleigh channel wave can be observed along their path of propagation. From the geophysical point of view the various types of discontinuities of a coal seam can be divided into two basic types: one is restricted to the seam, the other includes in addition a fractured zone in the adjoining rock. The investigation of the symmetrical Rayleigh channel wave reflected by a discontinuity yields the following results: For dip angles γ between approximately 90° and approximately 60° the reflectivity is virtually independent of the type of discontinuity. This does not hold for the limiting case of γ= 90° (vertical dip) for which the reflectivity increases with increasing influence of the fractured zone. For dip angles γ between approximately 60° and approximately 40° the reflectivity is still independent of the fractured zone but the shape of the reflected wave deteriorates with increasing influence of the fractured zone. For dip angles γ below approximately 40° the reflected wave deteriorates such that the application of the in-seam seismic reflection method will be difficult or even impossible. The conversion of the direct wave of the symmetrical fundamental mode into a reflected wave of the antisymmetrical fundamental mode has been observed. |
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