SECT. 1] SUB -OCEANIC STRUCTURAL EXPLORATION BY SEISMIC SURFACE WAVES 



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but differ in that a minimum of the group velocity exists at some intermediate 

 period. Love waves are unaffected by a hquid layer, and so the above remarks 

 apply equally to the case of a water-covered or non-water-covered region if the 

 solid layers are the same in each case. 



In the case of Rayleigh waves, as illustrated in Fig. 3, the fundamental or 

 Rayleigh mode differs from all the higher modes in that it not only includes the 

 entire spectrum of periods, but also in that the phase velocity approaches 

 the Rayleigh-wave velocity in the upper medium as the wave periods decrease, 

 and the Rayleigh velocity in the lower medium as the periods increase. Higher 



3 4 5 



10 



20 30 40 50 

 Period (sec) 



500 1000 



Fig. 4. Summary of Rayleigh-wave group-velocity observations. Effective depth of 

 penetration of the surface waves increases with period, leading to the terms crustal 

 and mantle Rayleigh waves. These curves are intended to represent average con- 

 tinental and oceanic conditions. However, for periods less than about 50 or 75 sec, 

 regional differences within continents and ocean basins correspond to observed data, 

 which depart significantly from these average cvirves. The configuration of the 

 mantle-oceanic Rayleigh-wave curve and of the oceanic first-shear-mode curve are 

 discussed in detail in this paper. 



modes of the Rayleigh type, here termed shear modes, have the same limits as 

 the Love modes, and also have similar long-period cut-offs. When the upper 

 layer is liquid, waves of the Rayleigh type are strongly affected and the very 

 short waves have velocities approximately, but not exactly, equal to the 

 velocity of sound in water (Biot, 1952). As in the Love-wave case, the group- 

 velocity curves all have a minimum at an intermediate value of the period. 



In practice, it is not possible to approximate the earth adequately through 

 the use of such simple models. The observed curves, as well as the theoretical 



