Scattering of guided SH and PSV modes by lateral disruptions in coal seams: A finite-element model study

In-seam seismic surveys are conducted to image coal seam structures both in reflection and transmission. It is important to understand the frequency-domain reflection and transmission characteristics of commonly occurring seam disruptions. Coal seams act as seismic waveguides carrying up to 6 modes of SH and PSV polarizations in the frequency range normally used. Fundamental symmetric modes of either polarization are predominantly excited by sources in midseam. We present the results of two-dimensional model calculations of mode scattering matrix elements observationally significant in in-seam seismic. The calculations are performed using a hybrid method: 2-D finite elements to model the central scattering region and 1-D finite elements (Rayleigh-Ritz method) to calculate the waveguide modes in regions of uninterrupted seam. Matching of solutions at the vertical boundaries of the scattering region is performed using the consistent boundary conditions of Lysmer and Wass (1972). We calculated the frequency-domain S matrix elements for incidence normal to strike in 124 models representing faults of varying throw and dip, dikes of varying thickness, hardness and dip, sills of varying hardness, stonerolls of varying height, and washouts terminating the seam with varying degrees of irregularity.