Converted-wave Prestack Time Migration Research Articles

Anisotropic converted wave amplitude-preserving prestack time migration by the pseudo-offset method

In this paper, we use the method of pseudo-offset migration (POM) to complete converted wave pre-stack time migration with amplitude-preservation in an anisotropic medium. The method maps the original traces into common conversion scatter point (CCSP) gathers directly by POM, which simplifies the conventional processing procedure for converted waves. The POM gather fold and SNR are high, which is favorable for velocity analysis and especially suitable for seismic data with low SNR. We used equivalent anisotropic theory to compute anisotropic parameters. Based on the scattering wave travel-time equation in a VTI medium, the POM pseudo-offset migration in anisotropic media was deduced. By amplitude-preserving POM gather mapping, velocity analysis, stack processing, and so on, the anisotropic migration results were acquired. The forward modeling computation and actual data processing demonstrate the validity of converted wave pre-stack time migration with amplitude-preservation using the anisotropic POM method.

Anisotropic velocity updating for converted-wave prestack time migration

The conventional method used to estimate velocities for converted-wave (C-wave) prestack time migration is awkward because the P-wave velocity [Formula: see text] comes from P-wave processing, the velocity ratio gamma [Formula: see text] is estimated from C-wave data, and the S-wave velocity [Formula: see text] is then derived from [Formula: see text] and gamma. Instead, by using the C-wave velocity [Formula: see text], effective gamma [Formula: see text], and anisotropy parameter [Formula: see text], velocity updating becomes straightforward and more reliable. To update [Formula: see text] for converted-wave time migration, one can carry out hyperbolic moveout analysis on the hyperbolic-moveout-migrated-common-midpoint (HMO-MCMP) gathers. However, the errors in initial [Formula: see text] and anisotropy parameter [Formula: see text] can only be corrected by trial and error. In this article, we propose to remove the effects of initial [Formula: see text] and [Formula: see text] in the HMO-MCMP gathers by inverting the moveout related to the initial [Formula: see text] and [Formula: see text]. This enables a full nonhyperbolic velocity analysis to update not only [Formula: see text] but also [Formula: see text] and [Formula: see text]. To obtain reliable [Formula: see text], we also develop a simultaneous PP/PS anisotropic-parameter estimation method so the [Formula: see text] estimated from P-wave data is compared immediately with the [Formula: see text] derived from [Formula: see text] by using C-wave data. This provides a better constraint for estimating anisotropy parameters. The method has been tested and shows consistent improvement in converted-wave prestack time-migration velocity estimations.

Analysis and application of pseudo-offset method in the converted-wave prestack time migration

Pseudo-offset migration (POM) is a new method for prestack time migration of converted waves that improves on equivalent-offset migration (EOM). The mapping of POM is different than EOM but the purpose of the two methods is to map the input samples to the common conversion scatter point (CCSP) gathers. This paper introduces the principles of the two migration methods and model parameter sensitivity tests for both POM and EOM. At large offset-to-depth ratios the hyperbolic approximation, the three-term approximation and the double square root (DSR) equation are used to NMO-correct the mapped POM gathers to obtain more accurate migration velocities. These equations were derived and calculated by small pseudo offset. POM is then used to image complex structure and prestack time migration.