I introduce a new explicit form of vertical seismic profile (VSP) traveltime approximation for a 3D model with non-horizontal boundaries and anisotropic layers. The goal of the new approximation is to dramatically decrease cost of time calculations by reducing the number of calculated rays in a complex multi-layered anisotropic model for VSP walkaway data with many sources. This traveltime approximation extends the generalized moveout approximation proposed by Fomel and Stovas. The new equation is designed for borehole seismic geometry where the receivers are placed in a well while the sources are on the surface. Testing this new approximation for the 3D anisotropic model with dipping boundaries shows its very high accuracy for offsets up to three depths. The new approximation can be used for 3D anisotropic models with tilted symmetry axes for practical VSP geometry calculations. The new explicit approximation eliminates the need of massive raytracing in a complicated velocity model for multi-source VSP surveys.


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