Abstract
The energy distributions of guided waves in multilayered elastic solid media are investigated in three dimensions. A guided wave is the result of the interaction of the acoustic source and the interfaces in the material structure, and does not lose energy in the course of propagation along the horizontal direction. It should be pointed out that the guided wave cannot be excited alone by a practical acoustic source in this paper. The mean energy flux density of the guided waves (excited by a nonaxisymmetric acoustic source) has the tangential component except the radial component, but the effective part of the mean energy flux density has only the radial component. Only in the case that the propagation distance is greater than the wavelength, is the propagation velocity of the mean value of the total energy equal to the group velocity of the guided wave. It is found that the propagation velocity of the mean energy density is equal to the phase velocity of the guided wave in the lowest layer medium in the multilayered media, but in other layers, the propagation velocity of the mean energy density is related to the distance from the free surface to the receiving point. Two categories of guided waves, Rayleigh and trapped waves, are also numerically investigated in this paper in the multilayered media in which a low-velocity area is comprised. It is also found that one category of the guided waves decays rapidly with the distance from the free surface while the another category of guided waves concentrates its energy within the low-velocity area and decays with the distance from the low-velocity area. These two categories of guided waves have different energy distributions and propagation characteristics. However, since they are closely related, it is not always easy to distinguish them from each other. The excitation and propagation mechanism of the guided waves are useful for exploring the structures of the interfaces and the low-velocity area under the free surface.
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