Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

3.4. SWS MEASUREMENTS AT WEYBURN
eigenvalue minimisation analysis. Cluster analysis is performed to chose the best cluster of results,
from which the best result is selected. To conduct the analysis I use the SHEar-wave Birefringence
Analysis (SHEBA) algorithm developed by Teanby et al. (2004b).
Figure 3.8 shows the results for an example event from the dataset. In panel (a), the seismograms
are rotated into the ray frame. There should be no P-wave energy on the Sv and Sh components.
Panel (b) shows the S-wave components rotated into radial and transverse components with respect
to the initial source polarisation, both before and after the splitting correction. After correction, the
transverse component should be minimised. Panel (c) shows the S-wave particle motion before and
after correction, and the fast and slow waveforms before and after correction. The particle motion
should be linearised after the splitting correction, and the fast and slow waveforms should be similar.
Panel (d) shows a contour plot of the energy on the transverse component after correction as a
function of ψ and δt stacked for each of the 100 picked splitting analysis windows. An F-test is used
to normalised this surface such that a value of 1 represents the 95% confidence interval. The cluster
analysis is also displayed to the right of this panel. The upper cluster analysis panel shows the results
(ψ and δV S ) of the splitting measurement for each of the splitting analysis windows. The lower cluster
analysis panel maps the results for each window in ψ-δV S space, highlighting the clusters identified
by SHEBA.
Teanby et al. (2004b) provide a number of requirements that must be fulfilled if a splitting result
is to be deemed reliable:
• Well defined S-wave and successful rotation, with all the P-wave energy on the ‘c’ component
((quasi-)parallel to the raypath), and S-wave energy on the ‘a’ and ‘b’ components (perpendicular
to ‘c’).
• Energy minimised on the transverse component after correction.
• Linear S-wave particle motion after correction.
• Good match between fast and slow waveforms.
• Unique solution, with a well defined and small error ellipse.
If a splitting measurement fulfils all of the above criteria then it is graded class A. Should it fulfil
all but one of the above requirements it is graded class B. If it fails to satisfy two or more of these
criteria, it is classed as C and considered to be unreliable.
3.4.2 Splitting results for Weyburn
Of the 688 possible splitting measurements (86 events × 8 receivers) only 72 provided class A and B
results. This is a very low success rate for SWS analysis (compare with, for e.g., Al-Harrasi et al.,
2010). There are two probable reasons for this. Firstly, the data generally has a very poor signalto-noise
ratio. Secondly, many of the events are located close to the array (