ABSTRACT
The ray method allows a high-frequency asymptotic description of a wavefield, provided that the diffraction coefficients that define the amplitude distribution along the wavefront are known. The problem of finding diffraction coefficients is beyond the scope of the ray method. Ray expressions are perturbed plane waves. They were derived under the assumption that the wavefronts can be locally approximated by planes. Approaching the source point, the curvatures of the fronts unrestrictedly grow, and the ray theory fails. The condition formally coincides with that of the far-zone for a homogeneous medium. In the case of a homogeneous medium, the radiation pattern of a spherical wave can thus be identified with the corresponding leading-order diffraction coefficient. The choice of a characteristic scale is prompted by the size of the domain where the ray formulas fail.
