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When seismic survey energy is initiated within a coal seam, internally reflected S and P waves constructively interfere to form a seam wave. The seam wave is a high-frequency, dispersive wave which may exhibit both Rayleigh-type and Love-type modes. Since the seam wave is confined to the coal seam, its frequency content and dispersion characteristics are a function of the elastic properties and thickness of the coal seam. Large variations in the elastic properties or thickness of the coal seam, such as faults, sand bodies and pinch-outs, will affect the frequency and dispersion of the seam wave.
Seam-wave studies applied to coals in the eastern United States are few. The paper gives an acount of a transmission survey shot across a block of coal in the Oak Park Mine located near Cadiz, Ohio. The coal seam, under investigation is the lower Freeport (6A) which is 54 in. (14 cm) thick at the Oak Park Mine. Conducting an underground seismic survey presents many difficulties including: (1) mine accessibility; (2) underground-safety regulations; (3) source and receiver coupling; and (4) maneuverability. Comparison of the observed dispersion and predicted dispersion of the seam wave reveals its dominant mode and frequency content.
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