Comparison of two P-S conversion-point mapping approaches for Vertical Transversely Isotropic (VTI) media

Jianli Yang and Don C. Lawton

ABSTRACT

Determination of the conversion point is an important step in P-SV converted-wave survey design and in data processing. Unlike the midpoint determination in P-P wave exploration that is determined geometrically, the conversion point in P-S exploration is determined by physical properties of the formations. In practical processing, it is obtained by calculation followed by depth-variant sorting. The depth-dependent conversion-point position is often approximated by asymptotic conversion point, which is at a constant offset to the source. The influence of anisotropy on the P-S conversion point has to be considered together with the effect of reflector depth, offset, as well as layering. We developed a general raytracing algorithm for multi-layered VTI modelling using exact velocity equations for weak anisotropy to map the raypath and the conversion point location. The conversion point can also be determined by using the γ eff method, where γ eff refers to effective velocity ratio in anisotropic media. Both methods were applied for a one-layer model and a multi-layered model. In a single-layer model, the relative error is shown to be less than 8% for short-to-intermediate offsets. The γ eff method can be applied to obtain the conversion point for offset to depth ratio less than 1.5. In the multi-layered model, the relative error increases with the increasing offset and can reach 14% at offsets of twice the depth.

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