Smiles and frowns in migration/velocity analysis

Jinming Zhu, Laurence R. Lines and Samuel H. Gray.

ABSTRACT

Depth migration can be defined as the process of positioning reflected seismic arrivals at their proper subsurface locations. The accurate depth migration of seismic reflectors requires accurate velocity estimations. Inaccurate velocity estimates will cause moveout artifacts such as "smiles" and "frowns" to appear on depth migrated images. The elimination of these moveout features by the adjustment of seismic velocities allows depth migration to be used as a powerful velocity analysis tool. It can be argued on the basis of model studies (Lines et al., 1993) and with real data examples (Whitmore and Garing, 1993) that iterative prestack depth migration provides a very general velocity analysis method for structurally complex media. In this paper, we examine migration moveout for both poststack and prestack depth migration of point diffractors both mathematically and geometrically. Point diffractors are used due to their simplicity and the fact that reflected wavefields can be considered as a superposition of point diffractor arrivals according to Huygens'principle. We show the effects of velocity on the depth migration of point diffraction arrivals so that one can establish criteria for velocity analysis. We do this for bothzero offset (poststack) and non-zero offset (prestack) recording configurations.

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