This paper discusses reducing computation costs for 3D VSP P-wave traveltime calculations in orthorhombic multilayered anisotropic models with curvilinear layers. An efficient implementation of a two-point P-wave ray tracing approach is described. The method is based on Fermat’s principle with respect to the ray intersection points. To calculate the initial ray approximation, it uses intersection point derivatives with respect to the source coordinates. Newton method is applied to determine the ray that minimizes traveltime as a non-linear function of intersection points. To find the ray derivatives with respect to the source and receiver coordinates, I utilize the same matrix that is used in Newton method to solve the system of non-linear equations. Instead of solving system of two non-linear equations to determine group velocity in each layer, I use an analytical approximation of interval group velocities. Model tests demonstrate high speed performance and small amount of Newton iterations. Because the ray prediction is accurate, for normal 3D VSP data with many source points, the average number of Newton iterations is less than two.


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