Abstract：On the basis of calculating effective normal moveout (NMO) velocity of layered media with dipping interfaces using the derived analytic solution of NMO-velocity surface of the reflected wave in single-layer TI media with a dipping interface, rapidly calculating the travel time of reflected waves from common middle point (CMP) and common shot point (CSP) gathers is realized with the travel time-distance relation of CMP under conventional spread (the ratio of the maximum spread length to the reflection depth is close to 1). This method only needs to trace one zero-offset ray in line with the normal incidence theory and achieve an effective NMO velocity ellipse (encompassing all-around NMO velocities) with the Dix average algorithm proposed by Grechka and the analytic solution of NMO-velocity surface. Thus, it has a calculation efficiency of several orders of magnitude higher than multi-azimuth, multi-offset ray tracing, suitable for travel time inversion in TI media. Numerical tests show that the relative error between the effective NMO velocity and the NMO velocity when the travel time is calculated by ray tracing of optimal fitting under the conventional spread is in the magnitude of 10-3, while the relative error between the travel time calculated from the travel time-distance relation of CMP with effective NMO velocity and the travel time obtained by ray tracing is in the magnitude of 10-4~10-3. In short, the method has high calculation accuracy and is suitable for the media without too complex structures and conventional spread. It can be extended to layered anisotropic models with smooth curved interfaces.
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