RTM and Kirchhoff Angle Domain Common-image Gathers for Migration Velocity Analysis

Abstract

Angle domain common image gathers are recommended in Kirchhoff and reverse time migration for velocity model building in complex area. For these both approaches there is a general agreement that the tomographic ray pairs are fully defined by the reflection and azimuth angle information and the reflection dip and that if the velocity model is correctly updated down to a given horizon, it is not necessary to shoot the tomographic ray pairs upwards through this horizon. We show here through examples and a theoretical analysis that these both statements have to be mitigated when the common image gathers exhibit a significant residual move out on. We also show how to accurately compute the tomographic ray pairs allowing then for an accurate angle domain migration velocity analysis.

Surface-offset RTM Gathers - What Happens When the Velocity is Wrong?

“…Figure 1 shows on a very simple experiment that the RMO curvatures of the two migrations differ when a wrong velocity model is used. This behaviour was observed earlier when dealing with angle domain common image gathers (Bartana 2006;Montel et al 2011). It means that there are kinematic differences between the two migrations when the velocity is incorrect.…”

“…The main challenge is that for each picks we have two equations and 5 unknowns (in 2D, three map derivative terms, one surface slope misfit and one initial ray direction to source since the other one to receiver will be constrained by the offset at the surface).This can be done for example by setting a system of equation using consecutive neighbouring picks in the x,z and direction on which we will had some additional constraints to link them such as a local constant stationary map derivative. We have indeed a similar situation with respect to angle domain migration (Montel and Lambaré, 2011). The computation of the tomographic rays will require a nonlinear optimization and they do not systematically satisfy Snell-Descartes considering the dip observed in the common specular offset migrated section.…”