Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

4.5. INITIAL S-WAVE POLARISATION
of δt as a function of the difference between ψ and θ S , the signal/noise ratio and δt. From this figure
I note that the measurement of δt is not always accurate. When δt N =0.075 the boundary between
accurate and inaccurate δt measurement appears to follow a linear relationship between (ψ − θ S ) and
(signal-to-noise) −1 . When δt N is increased to 0.15 or above, the limit for accurate measurement of δt
appears to be where (ψ − θ S ) ≥ 10 − 15 ◦ , excepting where there is a very low signal-to-noise ratio.
This result is in agreement with the more in depth analysis on this issue conducted by Wüstefeld and
Bokelmann (2007).
This analysis used a Gaussian white noise distribution, which may not be the most appropriate
way to add noise. A method to analyse this issue in greater depth would be to use the noise as
it is found in the pre-first-arrival traces, which will be of a more correlated nature. Such a noise
distribution might start to interfere with the signal at higher signal:noise ratios. However, using such
a distribution, while it might affect the signal:noise ratios at which small ψ − θ S values become an
issue, it would not affect the limit at which small ψ − θ S becomes an issue for accurate measurement
(i.e. the ±10 ◦ limit identified here).
4.5.2 θ S and ψ in the data
The average splitting magnitude in the real data is approximately 2ms, and the dominant frequencies
are ≈ 150Hz. Hence, δt N is approximately 0.3. At this limit (Figure 4.18c), the splitting measurements
will be reliable so long as the difference between θ S and ψ is greater than 10 ◦ (the same applies when
θ S is close to the slow direction, 90 ◦ away from ψ).
The difference between ψ and θ S for the data is plotted in Figure 4.19. I note at this point that
the selection of class A events was made before the modelling work was undertaken, and so did not
explicitly take into account the difference between θ S and ψ in the selection of reliable splitting results.
The sole selection criteria were those discussed in Teanby et al. (2004b). The overall pattern is for θ S
and ψ to be similar (i.e., as expected, both are sub-horizontal). I would expect that, in reality, the
overall majority of recorded events would have θ S and ψ very similar. However, I note that no reliable
events are found when θ S and ψ are within 10 ◦ of each other. These events have been rejected not
explicitly because θ S and ψ are within 10 ◦ , but because they do not fulfil the criteria of Teanby et al.
(2004b). However, it is evident that, as anticipated from the synthetic analysis, almost no reliable
results are found when θ S and ψ are within 10 ◦ (or 80-100 ◦ ). None are found in the water injection
data, 2 are found in the CO 2 injection data, so 2 out of 92 measurements fall within this 10 ◦ limit.
This serves as a verification of the conclusions presented above regarding unreliable results when θ S
and ψ are similar, and demonstrates that the selection criteria outlined by Teanby et al. (2004b) are
effective in removing unreliable splitting measurements. An example of a typical null event rejected
during the analysis is shown in Figure 4.20. I conclude that, although the polarisation of the initial
S-waves and the fast splitting direction do appear to be similar, the splitting measurements that we
use are still reliable.
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