In the case of long-range propagation of forward scattering, due to the accumulation of phase change caused by the velocity perturbations, the validity of the Born approximation is easily to be violated. In contrast, the phase-change accumulation can be easily handled by the Rytov transformation. In this study, we present a generalized Rytov approximation which has an improved phase accuracy compared to the first-order Rytov approximation. We first analyze the integral kernel of Rytov transform using the WKBJ approximation and we demonstrate that integral kernel is a function of velocity perturbation and scattering-angle. By applying a small scattering-angle approximation to the integral kernel, a new integral equation is derived where no assumption of velocity perturbation is made. Therefore it is referred to as the generalized Rytov approximation (GRA) because it is valid for arbitrary velocity perturbation. Numerical examples show that the GRA method can provide an improved phase accuracy no matter how strong the velocity perturbation is, as long as the scattering-angle is small. The proposed GRA method has the potential to be used for traveltime modeling and inversion for large-scale and strong perturbation media.


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