Shear wave critical angle

Alternative data collection possibilities to the immersion technique are shown in Figure 19. Squirter systems, couplant flow systems, and various contact methods may also be used to employ the normal beam shear wave, critical angle, plate wave, surface wave, back-scattering, or frequency perturbation techniques. 

In materials of lower velocity the situation becomes simpler. If the shear critical angle is outside the lens angle, then there will be no Rayleigh phenomena. Such materials would include many polymers and composites. There may still be longitudinal head waves, and these can reveal some of the features associated with Rayleigh wave imaging, such as surface cracks. But two further factors enhance subsurface imaging. First, because the velocity is lower, the... 

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