The pure-mode Rayleigh wave is a two-dimensional
wave which propagates along the surface of a solid
substrate. The particle displacements are in the plane
normal to the surface, the sagittal plane. The wave
consists of a longitudinal component and a shear compo-
nent normal to the surface (Fig. 4). Its velocity is close
to the shear velocity. The penetration depth of the
Rayleigh waves is just over one wavelength, as can be
seen in Fig. 5.
For use as a gas sensor, the surface of the substrate