Velocity Modeling in a Vertical Transversely Isotropic Medium Using Zelt Method

In the present paper, the Zelt algorithm has been extended for ray tracing through an anisotropic model. In anisotropic media, the direction of the propagated energy generally differs from that of the plane-wave propagation. This makes velocity values to be varied in different directions. Therefore, velocity modeling in such media is completely different from that in an isotropic media. The velocity model for ray tracing is parameterized in terms of blocky trapezoid cells where the velocity changes inside the cells linearly. Thomsen’s approximations in weakly anisotropic media were used to estimate anisotropic velocity vectors. Rays were traced in direction of group vector in the vertical transversely isotropic (VTI) media, whereas, the anisotropic Snell’s law must be satisfied by the phase angle and phase velocities across the interface. The synthetic examples are given to demonstrate and verify the ray tracing algorithm. Reflected and turning waves were traced through the isotropic and anisotropic velocity models. Lateral and vertical velocity variation caused deviation on trajectory of the traveltime curve. The results show that the difference between isotropic and anisotropic traveltimes increases with offset, especially when the ratio offset/depth exceeds 1.5.