Numerical and physical modeling of P-S converted waves in VTI media

Jianli Yang

ABSTRACT

The earth's crust is not isotropic as is traditionally assumed, but is anisotropic and inhomogeneous. Vertical Transverse Isotropy, also called polar anisotropy with vertical axis, is the most commonly considered type of velocity anisotropy. Converted wave (P-S) analysis in VTI media was conducted, focusing on conversion point mapping. It was found that the location of P-S conversion point in VTI media depends on the anisotropic parameters ε and δ , Vp/Vs and offset/depth. When ε is larger than δ , for the same offset, the conversion point moves towards the source in VTI media relative to that in the isotropic case. The displacement of the conversion point from that in the isotropic case increases with the increasing offset-to-depth ratio and the value of ε . The velocity ratio doesn't have a significant influence on the displacement of the conversion point for small offset/depth, but has an influence on for large offset/depth. The P-S conversion point was also calculated by using the γ eff method and compared with the results obtained from raytracing method. It is found that for long offsets, i.e. offset-to-depth ratio greater than 2, the γ eff method is an insufficient approximation for mapping the P-S conversion-point.

A 2C (P-P and P-S) seismic survey was designed and conducted on a scaled VTI physical model and data were acquired and processed. It is shown that the overburden VTI layer does not have significant influence on P-P imaging. When isotropy is assumed in P-S data processing, the slab edge position obtained has a lateral shift from its true position.

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