ANISOTROPIC VELOCITY UPDATING FOR CONVERTED-WAVE PRESTACK TIME MIGRATION

摘要

The conventional method to estimate velocities for converted-wave (C-wave) prestack time migration is an awkward procedure, because the P-wave velocity (V_P) comes from P-wave processing, the velocity ratio gamma (V_P/V_S) is estimated from C-wave data, and the S-wave velocity (V_S) is then derived from V_P and gamma. Instead, by using the C-wave velocity (V_C), effective gamma (γ_(eff)), and anisotropic parameter x_(eff), velocity updating becomes straightforward and more reliable. To update Vc for converted-wave time migration, one can carry out hyperbolic moveout analysis on the hyperbolic moveout migrated common mid-point (HMO-MCMP) gathers (Dai and Li (2004). However, the errors in initial γ_(eff) and anisotropic parameter x_(eff) can only be corrected by a trial-and-error method. In this paper we propose to remove the effects of initial γ_(eff) and x_(eff) in the HMO-MCMP gathers by inverting the moveout related to the initial γ_(eff) and x_(eff). This enables a full non-hyperbolic velocity analysis to be conducted in order to update not only V_c but also γ_(eff) and x_(eff). To obtain reliable x_(eff), we also developed a simultaneous PP/PS anisotropic parameter estimation method so that the η_(eff) estimated from P-wave is immediately compared with the η_(eff) derived from x_(eff) by using C-wave data. This provides a better constraint for estimating anisotropic parameters. The method has been tested and has shown consistent improvement in converted wave prestack time migration velocity estimations.