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| | Abstract | | When the land surface is paved or the soil is subject to compaction, the seismic velocities in near-surface layers are higher than the underlying layers. In this case, dispersion curves contain invers dispersion parts in normal modes, where phase velocity increases with increasing of frequency. In normal mode calculations, the phase velocity should be smaller than the S-wave velocity of the lowermost layer. This restriction comes from the physical requirement that the amplitude of surface wave must converge to zero at the infinite depth. Velocities of the normal modes do not exceed the velocity larger than the S-wave velocity of the lowermost layer. In the velocity range larger than the S-wave velocity of the lowermost layer, there appear leakage modes which dissipate wave energy during propagation in horizontal direction. Leakage modes appear associated with the fast S wave at the near-surface layers, and dissipate wave energy downwards. We calculated the normal and leakage modes for layered models that have layers of pavement in the frequency range 10-100 Hz. This frequency range has not been well studied, but important for most of active geophysical prospecting in which surface waves are excited by near-surface vibration sources. It is important to compare the inverse dispersion and observed surface-wave dispersion curves for estimating subsurface velocity structures. |
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