1. An elastic wave propagated in a layer where most of the energy is trapped. (a) The layer may have lower velocity than those on either side of it (so that total reflection can occur at the boundaries), or (b) a layer boundary may be a free surface (so that the reflectivity is nearly one). Instead of having a sharp interface as a boundary, a velocity gradient that bends rays that tend to escape back toward the channel may provide a channel. The SOFAR channel (q.v.) in the deep oceans is an example. However, some energy may escape (leaking mode), e.g., by converting to another wave mode. See Figure C-2. A channel is also called a wave guide and channel waves are also called guided waves and normal-mode propagation. Coal seams and a surface water layer often carry channel waves (also called seam waves). See Sheriff and Geldart (1995, 483-487).
FIG. C-2. Channel waves. (a) The Sofar channel is formed by velocity inversion. The velocity-versus-depth curve changes with latitude and other factors. (b) Energy from a source in the channel is repeatedly refracted or reflected back toward the velocity minimum and so undergoes less divergence than normal. (From Ewing et al., 1948.) (c) Phase and group velocity versus normalized frequency for a liquid layer on an elastic substratum. (From Ewing et al., 1957.) (d) First-mode wavetrain from a source 4 km distant where the ocean constitutes the channel. (e) The high-frequency portion of (d), the water wave; its arrival is used in refraction work to determine the range. (From Clay and Medwin, 1977.)