A seismic ray is a trajectory describing high-frequency seismic energy propagation in a medium whose properties vary slowly compared with the wavelength. In an isotropic medium it is normal to local travel-time wavefronts, and its travel time is along the path.
Changes in wave speed bend rays, while interfaces can reflect or refract them. The approximation is useful for interpreting arrival times, but diffraction, interference and caustics require more than a single-ray description. In anisotropic media, energy propagation and the wavefront-normal direction need not coincide.
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