Blackwell Publishing Journal Backfiles 1879-2005
The frequency equation for Stoneley-type waves with symmetric vibrations propagated along the interfaces between an internal stratum and two adjacent halfspaces, all perfectly elastic, homogeneous and isotropic, is obtained as the vanishing of a determinant of the fourth order. For large values of the frequency, the equation reduces to that of Stoneley waves at the interface between two halfspaces. Discussion of the sign of this determinant for suitable values of the unknown leads to the condition for the existence of such waves both for a low-velocity and a high-velocity internal stratum. The ranges of values which the ratios of the elastic constants of the stratum and either halfspace must have are obtained by numerical computation and the results are presented both in tabular form and graphically.Tt is found that: (i) Stoneley waves with symmetric vibrations, when they exist, have their phase velocity lying between the distortional and Rayleigh wave velocities of the lower velocity medium, (ii) If, as is usual, a velocity ratio less than unity is associated with a density ratio less than unity, then such waves cannot exist unless the smaller of the two distortional wave velocities is greater than the higher Rayleigh wave velocity, (iii) As the frequency of these waves or the thickness of the stratum is decreased, there is a cut-off value of either below which Stoneley waves cannot be propagated.
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