One aspect of the modification of seismic waves on passage through the Earth is the partitioning of energy at subsurface interfaces as described by the Zoeppritz equations. These equations have been applied to simple two- and three-layer models to determine the variations in the amplitude and phase of a reflection signal at nonnormal angles of incidence. Synthetic seismograms have been produced to illustrate the effect of these variations on a seismic wavelet. It is found that the phase variations can lead to substantial changes in the character of the reflected wavelet as the source-geophone distance increases. These changes are dependent on the angle of incidence and on the elastic properties of the subsurface layers. In particular, for models approximating an overburden over bedrock situation, the reflected pulse is predicted to 'change phase' when the velocity contrast between the two layers is relatively small. This effect has been observed on field records. Geophysicists should be aware of this phenomenon, both in terms of the problems it may cause in observing a bedrock reflection and in terms of the potential it has for indicating subsurface elastic properties.

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