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SEISMIC MODELLING OF SEAM WAVES EXCITED BY ENERGY TRANSMISSION INTO A SEAM 1
Author(s) -
GRITTO R.,
DRESEN L.
Publication year - 1992
Publication title -
geophysical prospecting
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.735
H-Index - 79
eISSN - 1365-2478
pISSN - 0016-8025
DOI - 10.1111/j.1365-2478.1992.tb00547.x
Subject(s) - rayleigh wave , excited state , geology , microseism , amplitude , seismic wave , surface wave , geophysics , acoustics , seismology , physics , optics , atomic physics
A bstract The traditional method of exciting channel waves in coal deposits underground consists of firing explosive sources in a mid‐seam position generating seam waves of the Rayleigh and Love type. We investigate various source positions and excitation mechanisms within the bedrock structure surrounding the seam to evaluate the effects of source positions adjacent to the seam. The investigation is based on analogue and numerical modelling of half‐ and full‐space cases, for which the excitation and the nature of Rayleigh channel waves are examined. In the analogue modelling, sources, located from mid‐seam out into the bedrock, along the edge of a 2D plate model, excited channel waves through a conversion of the free surface Rayleigh wave at the edge of the plate. The excited channel wave belongs to the normal mode range. Frequency‐wavenumber analysis shows that the symmetric 2nd mode of the channel wave is excited with frequencies comparable to the forcing frequency of the source signal. The polarization changes from retrograde to prograde, as the wave develops from the front to the rear of the seam, respectively. The amplitude‐depth distribution resembles that of an ordinarily excited seam wave, for the symmetric component. However, the antisymmetric component does not show the characteristic change of sign in amplitudes across the mid‐seam axis. Numerical modelling with sources located in the bedrock (full‐space case) shows that relocating the source away from the seam lowers the frequency content of the excited channel wave. Based on these investigations, the influence of a lower‐frequency source signal on the excitation of the channel wave is examined in an analogue experiment. Sources are sited in the bedrock adjacent to the seam at three locations. A lower‐frequency wavelet is calculated for each source location from the results obtained in the numerical analysis. For comparison, a higher‐frequency wavelet is also used which is known to be optimal for this model geometry when excited by a mid‐seam source location. It is found that in two cases the use of the lower‐frequency wavelet improves the channel wave excitation, while no amplification is achieved in one case.