Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

3.4. SWS MEASUREMENTS AT WEYBURN
the different parameters are. In the case with subhorizontal arrivals (Figure 3.5). Fracture strike and
γ are well constrained. However, from the elongation of the misfit contours along the δ axis, I infer
that this parameter is not as well constrained. This is because splitting of subhorizontal shear waves is
not significantly affected by the size of δ, and therefore it does not have an influence on the inversion.
For the case with subvertical arrivals (Figure 3.6), there is a trade-off in the inversion between δ
and γ, meaning that neither is well constrained. Essentially, for a set of splitting measurements on
subvertical S-waves, any modelled value of γ, with the appropriate value of δ, can produce splitting
patterns that match well with the actual splitting.
For the case with obliquely arriving waves (Figure 3.7) there is still some trade-off between γ and δ,
though both are better constrained than with the subvertical arrivals. In all the examples the fracture
strike and density are both well imaged. This is because I use a full range of arrival azimuths from
0 - 180 ◦ . Much as horizontal fabrics are most sensitive to the range of arrival inclinations, vertical
fabrics (such as fractures) will be most sensitive to the range of azimuths available. As can be seen
in Chapter 4, constraints on fracture strike and density will be dependent on the azimuthal range of
S-wave arrivals.
This section does not intend to cover every possible source-receiver geometry, these will obviously
be specific to the problem being investigated. However, I have outlined how synthetic modelling
can guide the interpretation of SWS results, and highlight what real data is likely to identify, and
what it cannot. This capacity may well be of use to field engineers when selecting sites to place
geophones because geophones sites can be selected to maximise the structures that SWS can constrain.
In subsequent sections I will construct further synthetic models that are appropriate to the actual
problems being investigated. However, before I do so I will first discuss the SWS measurements made
at Weyburn.
3.4 SWS measurements at Weyburn
3.4.1 Method
In order to analyse SWS, the seismograms must first be rotated into the ray frame coordinates. I do
this using the P-wave particle motion orientation to indicate the direction of ray propagation, using
the protate algorithm described by Al-Anboori (2006). Where the P-wave has not been picked, or
where this does not produce a satisfactory rotation, the events are rotated using the azimuth of the
located event from the receivers and the inclination assuming a straight source-receiver path.
If SWS has occurred, the S-wave particle motion will be elliptical. However, a rotation of the
components by ψ and a time-shift of δt will remove the effects of splitting and leave the particle
motion linearised. I use the methodology of Silver and Chan (1991), performing a grid search over
ψ and δt to find values that best linearise S-wave particle motion (indicated by a minimised second
eigenvalue of the covariance matrix). To ensure a stable and reliable result, the analysis is conducted
over a range of windows centred on the S-wave arrivals. This ensures that the result is not dependent
on S-wave picking accuracy. 100 picked windows are automatically generated on which to perform the
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