Estimation of shear wave velocities from P-P and P-S seismic data using Equivalent Offset Migration

Thais A. Guirigay

Prestack migration of converted wave data requires accurate acoustic and shear wave velocities Vp and Vs. This thesis presents new methods to estimate the shear velocities using a single converted wave velocity Vc, that combines Vp and Vs.

The acoustic velocity Vp, and a constant value for the Vp/Vs ratio are used to make an initial estimate of the converted wave velocity, Vc. Narrow range gathers are formed using the initial estimate of Vc that are then processed to obtained a refined value of Vc. This refined value is then used to estimate Vs. The estimated Vs is used with Vp in a full prestack migration using the equivalent offset method (EOM) to form complete prestack migration gathers. Velocity analysis of these gathers produces a more accurate Vc which is used to complete the prestack migration.

The quality of the method is demonstrated for the cases of one synthetic dataset and two real datasets. The results show superior imaging when compared with alternative migration algorithms.