ESTIMATING ANISOTROPIC PARAMETERS FROM PS CONVERTED-WAVE DATA: A CASE STUDY

摘要

Anisotropic parameters are estimated from PS converted-wave (C-wave) data. The data were acquired by digital MEMS (micro-electro-mechanical system) sensors over a volcanic gas reservoir in Northeast China with a mixed sand and shale sequence in the overburden. This gives rise to both shear-wave splitting and anisotropy moveout effects due to presence of vertical transverse isotropy (VTI). We use a four-parameter theory to evaluate the effects of non-hyperbolic moveout due to an asymmetric raypath and VTI. These four parameters include the PS converted wave stacking velocity (V_(c2)), the vertical velocity ratio (γ_0), the effective velocity ratio (γ_(eff)), and the anisotropy parameter (x_(eff))- This four-parameter theory leads to an improvement in imaging quality and correlation between the P-waves and converted-waves. After compensating for the VTI effects, we analyze the shear-wave splitting in the post stacked data. We find that the amount of splitting determined from the data can be correlated to the known gas reservoirs, and is also consistent with other amplitude and spectral anomalies associated with the gas reservoirs, revealing a potential for using shear-wave splitting to delineate gas reservoirs in volcanic rocks.