## 4 General Discussion
### Experimental Implications and Limitations  
As mentioned in Section 2.4, for several rock formations, exactly the same core sample was used for experimental series A-D (Figure 9). This was solely done for comparison purposes, in order to avoid the heterogeneity effect amongst different cores from the same formation. In these cases, first experiment B (\(\sigma_{1}=\sigma_{2}\)) was conducted followed by C (fixed \(\sigma_{\rm diff}\)), D (varying \(\sigma_{1}\)), and finally A (UCS). Acoustic-assisted UCS experiments were executed last as these tests resulted in rock failure, making the core sample useless for further testing. One may wonder whether the CCS experiments (series B-D), conducted prior to the UCS test, have altered the internal pore structure of a specific core sample. If the latter is true, the UCS experiments (series A) that followed may not be representative of the initial core sample anymore, thus yielding comparison implications. However, the data points symbolised by circles in Figures 6-8 (i.e., active-source acoustic measurements during the unloading stages) follow quite accurately the observed velocity profiles during loading. This may suggest that the tests were executed in the elastic regime, implying that any deformation, imposed by the elevated stresses, was temporarily and would disappear during the unloading stage. Even if the internal pore structure of the sample slightly changed during the first experiments, comparison among tests conducted on that specific sample might still be preferable over comparing different cores with one another. Another implication, e.g., source of error, is related to the arrival time picking of the P- and S-waves. Figure 15 presents the active-source acoustic signal obtained during the siltstone TK-B1-3 UCS experiment (series A) at \(\sigma_{1}\) = 5.08 MPa. In order to accurately pick the P-wave arrival time, a horizontal line (black dashed line in Figure 15), representing the maximum noise level in the signal prior to the P-wave arrival, was drawn. The P-wave arrival time was defined as the time at which the signal exceeds this level of maximum noise. In this case this equals 3.433 \(\times\) 10\({}^{-5}\) s, yielding a P-wave velocity of 2333 m/s. Using this methodology, it becomes challenging to quantify the error related to picking P-wave arrival times. In a qualitative way, one can state that the error in picking the P-wave arrival  
Figure 14: P-wave velocity as a function of axial stress for the limestone sample (TS2-SZL4) used in experimental series A, i.e., the UCS experiments (Figure 5). For comparison purposes, UCS-related acoustic data corresponding to an Indiana limestone sample were taken from [57]. Experimental procedure involves an increasing axial stress until rock failure occurred. times is directly proportional to the noise level of the signal. For determining the S-wave arrival times, a somewhat different approach was adopted. Firstly, looking at the complete amplitude spectrum in the time domain, a time frame was identified that corresponds to a significant increase of the signal's amplitude. In the case of Figure 15, this corresponds to 6-8 \(\times\) 10\({}^{-5}\) s. Subsequently, one has to identify the first peak (1 in Figure 15) or trough (2 in Figure 15) that initiates the aforementioned high amplitude time period. Finally, the S-wave arrival time is defined as the time where the signal crosses the background line (i.e., zero amplitude line) just before the significant increase, or decrease, in amplitude. For the case in Figure 15, this resulted in two potential S-wave arrival times of 6.214 and 6.380 \(\times\) 10\({}^{-5}\) s, yielding velocities of 1337 and 1290 m/s, respectively. Thus, the maximum error in picking the S-wave arrival times equalled approximately half a wave period. Throughout this entire study the abovementioned picking procedure was applied in a consistent way, to all rock formations assessed, allowing for mutual comparison.