Freeman Gilbert, Stanley J. Laster, Milo M. Backus, Richard Schell; Observation of pulses on an interface. Bulletin of the Seismological Society of America ; 52 (4): 847–868. doi:
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Recent theoretical results show that, in addition to P and S pulses, two pulse-like phenomena, denoted by P and S, can be defined near the boundary of an elastic solid. The S pulse is the classic Rayleigh pulse in Lamb's problem and is the classic Stoneley pulse in Cagniard's problem. When the Stoneley existence conditions are violated, the S pulse still exists for many solid pairs and can be regarded as a radiating interface pulse. The P pulse is distinct only when σ (Poisson's ratio) is greater than about 0.4. It seems to be the dual to the S pulse. It is the classic Stoneley pulse in Strick's problem. The P pulse has been observed by Kisslinger in loess and clay near Florrisant, Missouri. The radiating S pulse has been observed by Pod''yapol'ski and Vassil'ev on a clay/granite interface, and by Roever and Vining on a fluid/solid interface. Roever and Vining may also have observed the P pulse on pitch. Two dimensional seismic model studies of the S pulse suggest that it is most easily recognized from its particle orbit. Model results for both the trapped and the radiating S pulse agree well with theoretical calculations of orbital motion and orbital tilt. The tilt of the P orbit in Lamb's problem is virtually independent of the elastic parameters and the P velocity is very nearly twice the S velocity. In dispersion problems both radiating P and S pulses appear as kinks in the dispersion curves.
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