VSP multi-algorithm shear-wave anisotropy study
Zandong Sun, Michael J. Jones
A principal application of shear-wave birefringence is the determination of fracture orientation from shear-wave splitting in multicomponent VSPs. Three different analysis techniques for birefringence were implemented and their results are compared.
We have carried out nine-component VSP experiments in central Alberta. These full-wavefield datasets were analyzed by using three different algorithms to see the features of azimuthal anisotropy, the applicability of different algorithms, and how the results from converted shear waves compare to those from direct shear waves. In four-component rotation, shear-wave polarization directions were determined by minimizing energy on off-diagonal components of the 2-by-2 shear-wave data matrix, followed by layer stripping. Shear-wave polarization and time lag were also determined by applying a crosscorrelation modelling algorithm which is based on modelling crosscorrelation between rotated radial and transverse field components. This algorithm can be used for analysis on data acquired with a single source polarization, but it needs layer stripping for local anisotropy information.
Parametric inversion is the algorithm for shear-wave data with a single source polarization. Three unknown parameters (fast and slow shear-wave velocities and polarization direction) can be inverted by modelling a set of up- and down-going orthogonal plane shear waves, The advantage of this algorithm is that it provides the local anisotropy - it does not require shallower information as previous algorithms do, and fracture orientation is not constrained to remain constant with depth.
Even though the three different algorithms measure different aspects of anisotropy in different ways, our final results from the different algorithms corroborate each other. Analysis indicates shear-wave birefringence exists in some of the formations studied.