## 4 General Discussion
### Velocity Profile of Rocks at the Kizaldere Geothermal Field  
The experimental study presented in this manuscript covers an investigation on the stress- and temperature-dependency of seismic velocities for multiple rock types found at  
Figure 15: Acoustic signal obtained during the Silstone TK-B1-3 UCS experiment at \(\sigma_{1}\) = 5.08 MPa (Figure 5). Note that the error, related to S-wave arrival time picking, can be interpreted as being equal to 12 wave period. the Kizildere geothermal field. Considering the results shown in Figure 8 (i.e., velocities at field-representative stress conditions), one may apply linear regression to each individual dataset/rock type. The latter will result in velocity-depth relationships, one per rock type, that are solely stress-dependent and reflect dry conditions. Aforementioned velocity-depth trends can subsequently be applied to any given stratigraphic section, in or around the Kizildere geothermal site, in order to construct a velocity profile. This was done for a specific stratigraphic section of interest. Corresponding results are presented in Figure 16.  
The stratigraphic section shown in Figure 16 represents the planned CO2 injection location at the Kizildere geothermal site. The red intervals characterise'schists' formations. Since it remains questionable to what extend these intervals contain quartz-, calc-, and/or micaschists, a 50% quartz-schist and 50% calcschist distribution was assumed, because they yield similar velocities (Figure 8). As it is known that layers of micaschists do occur within the red intervals in Figure 16, potentially serving as local seals within the second reservoir (Table 1), the effect of the addition of micaschists on the resulting velocities was investigated for one single interval. Resulting velocities for the addition of 25, 50, 75, and 100% micaschists (remaining percentage is equally divided between calc- and quartz-schist), are shown in blue, green, purple, and red, respectively. Due to its relatively low velocities (Figure 8 and Figure S6), the presence of micaschists will lower the velocity substantially. As mentioned earlier, the velocity profiles presented only reflect the stress-dependency of the seismic velocities for dry conditions. One might wonder to what extend dry conditions are representative of the reservoir. As previously mentioned in Section 2.1, the  
Figure 16: Seismic velocity profiles (**A**), P-wave/S-wave velocity ratio, i.e., \(\mathrm{Vp}/\mathrm{Vs}\) (**B**), Poisson ratio (**C**), and the stratigraphic section of interest (**D**). The stratigraphic section represents the planned CO2 injection site at the Kizildere geothermal reservoir. For the intervals that contain multiple rock types (mud- and siltome, marble and calcschist, and calc- and quartz-schist), a 50/50 distribution was assumed. Velocity–depth relationships, i.e., linear regressions, from Figure 8 (P-wave) and Figure S6 (S-wave) were used to construct the profile shown in (**A**). Since the claystone could not be assessed in this study, a literature value for its P-wave velocity, and for its \(\mathrm{Vp}/\mathrm{Vs}\) ratio, was assumed [67]. In one calc- and quartz-schist layer, micaschists were added in varying degrees (25, 50, 75, and 100%) to show its effect on the resulting velocities. Note that the velocity profiles presented (black and red profiles in (**A**)) reflect only the stress-dependency of the seismic velocities for dry conditions. In order to give an impression of the effect of temperature, on related velocities, the maximum temperature effect (dashed red and black profiles in (**A**)) was added for the limestone interval. This represents the reduction in velocity with increasing temperature at atmospheric pressure (as presented in Figure 12 and Figure S8). It is expected that the real dry velocity of the limestone interval in (**A**) lies somewhere in between the dashed and continuous lines, as it is assumed that the temperature-dependency is highest at atmospheric conditions, hence the ‘maximum temperature effect’.