Comparison of two P-S conversion-point mapping approaches for Vertical Transversely Isotropic (VTI) media
Jianli Yang, Donald C. Lawton
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 eff method, where 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 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.