If the dispersion characteristics of coal-seam channel guides do not extend smoothly at frequencies below cut-off, the number of parameters required to process in-seam seismic data increases. Unwanted modes become more difficult to suppress, the arriving pulses from targets become more difficult both to identify and to pulse compress. The connection between leaking and normal channel mode dispersion is established here by first synthesizing and then analyzing theoretical in-seam seismograms. Calculation of synthetic seismograms is based on numerical evaluation of the spatial Fourier integral for elastic displacements in the complex wavenumber plane.Theoretical seismograms are presented for three-layered models. Phase velocity characteristics are recovered from these signals and compared with those obtained from the zero loci of the period equation in the complex wavenumber plane. Under cut-off the former method yields smooth extensions of the normal mode dispersion characteristics, in contrast to the velocity curves obtained from the period equation only. It is found that the dispersion characteristics obtained from analyzing the seismograms can be used to recompress the dispersed arrivals.