## 4 General Discussion
### Comparison with Observations from the Literature  
This study presented a comprehensive experimental investigation into the effects of both \(\sigma_{1}\) and \(\sigma_{2}\) on the P- and S-wave velocities for a wide range of sedimentary and metamorphic rocks present at the Kizuldere geothermal site. In this subsection, laboratory results obtained for limestone sample TS2-SZL4, in experimental series A (UCS) and B (CCS with \(\sigma_{1}=\sigma_{2}\)), and acoustic results related to calcschist sample TD12-CS5 (where \(\sigma_{1}=\sigma_{2}\); experimental series B) are compared with data from the literature. Figure 13 shows the P-wave velocity as a function of increasing \(\sigma_{1}=\sigma_{2}\) for the calcschist and limestone samples assessed in this study (Table 3), and for two limestone [46, 47] and one calcschist [29] samples investigated by other researchers. The calcschist sample studied in [29] is part of the Mandanici Unit from the Peloritani mountain belt in Northern Sicily, Italy. Its mineralogy is heavily dominated by coarse-grained calcite minerals (\(\sim\)71%) and to a lesser extent by quartz (\(\sim\)16%). The limestone investigated in [46] is characterised by a porosity of 13% (mainly consisting of voids or large pores: a vuggy limestone) and by a mineralogy that is greatly dominated by calcite and dolomite minerals. The limestone sample assessed in [47] was sourced from the Molasse Basin, located at the northern front of the Alps, and is part of the Kimmeridgian Reef Complex (Upper Jurassic). It contains a porosity of 13% and a mineralogy that solely consist of calcite minerals, yielding a matrix density of 2.72 g/cm\({}^{3}\). The data presented in Figure 13 show that the acoustic-related results for limestone and calcschist, obtained in this study as part of experimental series B, correspond fairly well with the velocity profiles related to one of the two limestones [46] and the calcschist [29] taken from the literature at similar experimental conditions. For the calcschist, the velocities obtained in this study are somewhat higher than the ones reported in [29], nonetheless they follow the same trend. The reason for the difference in absolute velocity might be partly related to its mineralogy. The calcschist samples utilised in this study contain  
Figure 13: P-wave velocity as a function of axial stress for limestone (TS2-SZL4) and calcschist (TD12-CS5) samples studied in experimental series B (Figure 6). For comparison purposes, acoustic data related to two limestone samples and one calcschist sample were taken from [46, 47] and [29], respectively. Experimental conditions involve no differential stress, i.e., axial stress equals radial stress at all times. approximately 98% recrystallized calcite and dolomite minerals (Table 7), whereas the one described in [29] contains also a fair amount of quartz, as mentioned above. Since the P-wave velocity in calcite minerals [48; 49; 50; 51; 52] is of a higher magnitude compared to quartz [52; 53; 54], this might contribute to the slightly lower calcschist velocities reported in [29]. The presented P-wave velocities corresponding to limestone sample TS2-SZL4 are similar to the ones stated in [46] for \(\sigma_{1}=\sigma_{2}\leq 20\) MPa. For \(\sigma_{1}=\sigma_{2}>20\) MPa, the limestone sample investigated in this work reveals lower velocities compared to the one studied in [46]. The latter might be related to the amount of compaction, i.e., closure of open pore space, as a function of increasing stress. Since the limestone studied in [46] is characterised by a vuggy porosity, it contains relatively large pores and/or voids. Most probably, these large voids were still not entirely closed at \(\sigma_{1}=\sigma_{2}=20\) MPa and compaction could continue afterwards, hence its P-wave velocity was able to increase further at \(\sigma_{1}=\sigma_{2}>20\) MPa.