How acoustic waves are guided in buried subsurface channels in unconsolidated granular media

We show the existence of natural subsurface channels that guide acoustic waves for periodic arrays of spherical grains under the influence of gravity. Downward propagation of acoustic rays can be stopped altogether as a result of a caustic whose origin lies in the continuous bending of rays as a result of a gravitationally induced increase of rigidity with depth. Upward propagation of acoustic rays, on the other hand, is attenuated by Bragg reflection arising from the continuous diminishing of rigidity in the direction of the surface, resulting in a vertical propagation constant that approaches a limited allowed value. Waves with different frequencies are shown to travel at different depths below the surface. The key conditions for the manifestation of the predicted wave modes are microscopic periodicity and macroscopic inhomogeneity of the medium.