MODELLING CHANNEL WAVES WITH SYNTHETIC SEISMOGRAMS IN AN ANISOTROPIC IN‐SEAM SEISMIC SURVEY 1

A bstract In‐seam seismic surveys with channel waves have been widely used in the United Kingdom and elsewhere to map coal‐seams and to detect anomalous features such as dirt bands, seam thinning and thickening, and particularly in‐seam faulting. Although the presence of cleat‐induced anisotropy has been recognized in the past, almost all previous analyses have assumed homogeneous isotropic or transversely isotropic coal‐seams. Channel waves, however, exhibit properties which cannot be fully explained without introducing anisotropy into the coal‐seam. In particular, Love‐type channel waves are observed for recording geometries where, in a homogeneous isotropic or transversely isotropic structure, the source would not be expected to excite transverse motion. Similarly, modes of channel‐wave propagation display the coupled three‐component motion of generalized modes in anisotropic substrates, which would not be expected for Rayleigh and Love wave motion in isotropy or in transversely isotropic media with azimuthal isotropy. We model the observed in‐seam seismic channel waves with synthetic seismograms to gain an understanding of the effects of cleat‐induced anisotropy on the behaviour of channel waves. The results show a reasonable good match with the observations in traveltime, relative amplitudes, dispersion characteristics and particle motions. We demonstrate that anisotropy in the surrounding country rocks contributes significantly to the coupling of channel wave particle motion, although its effect is not as strong as the anisotropy in the coal‐seam. We conclude that the effects of cleat‐ and stress‐induced anisotropy are observed and can be modelled with synthetic seismograms, and that anisotropy must be taken into account for the detailed interpretation of channel waves.