RAY PERTURBATION-THEORY, DYNAMIC RAY-TRACING AND THE DETERMINATION OFFRESNEL ZONES

Ray perturbation theory and dynamic ray tracing both describe the beha viour of seismic rays near a reference ray. The equations of dynamic r ay tracing relate changes in position and direction of a ray in the vi cinity of a reference ray (paraxial ray) to changes in the reference r ay's initial conditions. Ray perturbation theory relates changes in pa raxial rays to changes in the slowness field or the reference ray's en dpoints. Dynamic ray tracing is performed in ray-centred coordinates, whereas ray perturbation theory may be performed in either ray-centred coordinates or in an external reference frame. Both methods derive fr om asymptotic ray theory, but their developments have proceeded along largely independent paths. In this paper we explore the relationship b etween dynamic ray tracing and ray perturbation theory and outline an efficient scheme, based on ray perturbation theory, to compute approxi mate Fresnel zones in inhomogeneous media.