  Caustic formation occurs within a ray skeleton as optical or acoustic fields
propagate in a medium with variable refractive properties and are unphysical,
their presence being an artifact of the ray approximation of the field, and
methods of correcting the field near a caustic are well known. Differential
geometry provides a novel approach to calculating acoustic intensity, assessing
ray stability and locating caustics in acoustic ray traces when the properties
of medium are completely arbitrary by identifying points on the caustic with
conjugate points along various rays. The method of geodesic deviation is
applied to the problem of determining ray stability and locating caustics in
2-dimensional acoustic ray traces in a layered moving medium. Specifically, a
general treatment of caustic formation in sound ducts and in piecewise
continuous media is presented and applied to various idealized and realistic
scenarios.
