# Comparison of DFN Modelled Microfracture Systems with Petrophysical Data in Excavation Damaged Zone
## 4 Discussion  
Technical limitations introduce some bias into the study, e.g., the most damaged sections physically cannot be included in the specimens, meaning that no macro-scale  
Figure 11: Fracture length index \(E^{*}\) and constant \(F\) plotted against physical properties. Red is PGR and light blue is VGN.  
fractures are present in the specimens. The specimens are typically required to be relatively intact, and as a result may not be representative of macro-scale EDZ.  
Analysis of the geometric parameters from the prepared specimens should be a straightforward process with not much room for error, and the process remained systematic [13]. This means that fluorescent images were produced in fixed conditions (light, camera settings, possible post processing) before the segmentation to avoid biases. Considering the focus of the study is on the microfractures, it is possible that the preparation of the specimens has a disturbing effect.  
The effect of drilling was studied by looking at a section that transects a specimen perpendicular to the drillhole axis. No difference in the microfracture network properties was found with respect to the distance from the specimen axis. This means that the effect of drilling was irrelevant to the studied problem. However, it must be noted that this test was limited to a single specimen, and for future studies more comprehensive testing might be in order.  
Cutting and grinding the specimen ends might have an effect, but as cut and ground surfaces are always present their effect is impossible to distinguish. It is, however, likely that for similar specimens, the effect is similar and would therefore not affect the results within a specimen set to any significant degree.  
The geometric parameters of the fracture network were only studied in sections taken in the x-y plane. Gneiss has an anisotropic structure, which may affect the results. However, the Olkiluoto gneiss has centimetre-scale variation in the foliation direction, which means the sections represent multiple orientations with respect to foliation.  
The resistivity values varied approximately four to five times between the smallest and largest value for both the granite and gneiss, respectively. The resistivity value is sensitive to single \"channels\" of microcracks; thus, the orientation of microcracks in relation to the measurement direction is of importance. The microcracks and their orientations also affect the wave velocities but with a smaller impact. The fact that the specimens are fluid saturated could decrease the effect of microcracks for the P-wave velocity [33]. The porosity of the specimens is very small and was found to be practically isotropic in this context.  
The modelling was limited to 5 models of varying compositions and 10 runs per model. For future work, the effect of the stress field on the orientation of formed fractures should be studied. Additional rock types would increase the understanding on the subject.  
More fractures in total and more connected fracture clusters with more fractures per cluster in the simulations were observed in granite compared to gneiss. Based on the results, the number of larger micro fracture clusters and maximum number of fractures per cluster increases linearly with increasing volumetric granite content. It must be noted, however, that the fractal-based approach of the modelling assumes that the statistical distribution of the parameters remains the same regardless of scale.  
Differences in the microfracture properties may either be natural or caused by drill and blast excavation. Differentiating the effect of fracturing from the effect of rock type, when the type of fracturing seems to depend on the rock type is tricky. The observed differences in the fracturing and electrical properties between the pegmatoid and gneiss specimens may suggest that pegmatoids are more prone to fracturing. Alternatively, it may also be that the natural heterogeneity of the gneissic specimens has an effect that exceeds the effect of excavation damage, disguising it.