STUDY OF ENERGY-DISTRIBUTION OF GUIDED-WAVES IN MULTILAYERED MEDIA

The energy distributions of guided waves in multilayered elastic solid media are investigated in three dimensions. A guided wave is the resu lt of the interaction of the acoustic source and the interfaces in the material structure, and does not lose energy in the course of propaga tion 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 ex cept the radial component, but the effective part of the mean energy f lux density has only the radial component. Only in the case that the p ropagation distance is greater than the wavelength, is the propagation velocity of the mean value of the total energy equal to the group vel ocity 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 othe r layers, the propagation velocity of the mean energy density is relat ed to the distance from the free surface to the receiving point. Two c ategories of guided waves, Rayleigh and trapped waves, are also numeri cally 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 surfac e while the another category of guided waves concentrates its energy w ithin the low-velocity area and decays with the distance from the low- velocity area. These two categories of guided waves have different ene rgy distributions and propagation characteristics. However, since they are closely related, it is not always easy to distinguish them from e ach other. The excitation and propagation mechanism of the guided wave s are useful for exploring the structures of the interfaces and the lo w-velocity area under the free surface. (C) 1998 Acoustical Society of America.