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4. Results
A Wave 3D model was created for the geometry of the polyaxial test on a sandstone cube (Haycox
and Pettitt, 2004)[1], modelling fluid filling by allowing the cracks to have a normal stiffness but no
shear stiffness. Models consisted of a 50mm x 50mm x 50mm cube and used approximately 8x10 6
finite difference zones. The cube was modelled as an isotropic, linear elastic material with P- and Swave
speeds of 4800 m/s and 3300 m/s respectively and a density of 2431 kg/m 3 .
Best fit models were obtained for the 6 measurements of amplitude ratio and phase difference on
the P and two orthogonal S-waves. An example of the correlation between measured and modelled
data is shown in Fig. 1. The model that was found to best represent all the data was a set of aligned
cracks with crack length = 4mm, normal stiffness = 1x10 13 Pa/m, and crack density = 0.2.
A similar strategy was followed in the case of the in-situ experiment at SKB’s HRL, for the period
following the excavation of the prototype borehole (Haycox and Pettitt, 2004) [1]. A number of ray
paths were chosen which pass through specific regions around the void, ranging from ray paths
which pass within centimetres of the deposition hole’s edge and exhibit the greatest changes in velocity
(Pettitt and Haycox, 2004)[3] to ray paths passing away from the deposition hole wall where
no significant changes in transmitted P-waves are observed before and after the excavation. The
comparison of the amplitude and phase spectra from the ray paths crossing through a tensile (or
low-compressive-stress) zone induced in the vicinity of the hole’s wall before and after the excavation
show a decrease in amplitude over a wide range of frequencies.
Results from the best-fit analysis confirm interpretations based on the ultrasonic data and acoustic
emissions alone. The ray path that passes through the tensile zone (which experienced acoustic
emissions and large velocity drops over the excavation period) has the largest crack sizes and density.
For simplicity, we restricted the modelling of elastic wave velocities using the EMT in both the
laboratory and the field case studies to non-interacting dry microcracks, a valid approximation for
the case where no unstable crack propagation is observed. For the laboratory case study, only measurements
of wave velocity carried out along the three principal stress axes have been considered.
For the inversion, matrix elastic parameters were used, with Eo= 65 GPa and � = 0.24. Figure 3a
shows the changes in propagation velocity along the three principal axes measured during the different
episodes of hydrostatic (up to 100 MPa) and deviatoric loading. The results of the modelled
velocities and the associated changes in crack density are shown in Fig. 2. A velocity increase is
observed during hydrostatic loading associated with crack closure. During the deviatoric loading
there is a significant decrease in the velocity of waves propagating in the �3 direction. The opening
of macroscopic fractures in the plane normal to �3 was confirmed by the source location of Acoustic
Emission events during the deviatoric loading (Haycox and Pettitt, 2004) [1].
Figure 4 presents the results of the modelled velocity changes using the EMT for a ray path skimming
the surface of the deposition hole at SKB’s HRL. The changes in transmission velocity were
measured during its excavation, showing a sharp decrease of velocities after the start of the opening
of the void followed by an asymptotic decrease to a value ~30 m/s below the original transmission
velocity in the case of vP. The magnitude of the stress changes associated with the excavation and
the nature of the host rock in this case results in significantly smaller induced changes in transmission
velocity and crack density compared with the laboratory case. A reasonably good fit is observed
between the observed and modelled velocity changes. The results for the heating phase, pre-
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