## 2 Materials and Methods
### Experimental Procedure: UCS and CCS Tests  
In experimental series A, i.e., the UCS experiments combined with acoustic measurements (Table 2), the effect of increasing \(\sigma_{1}\) on the seismic velocities as well as the mechanical behaviour (e.g., static elastic properties) of the various rock samples were studied. The samples were placed inside a uniaxial loading apparatus with a 500 kN loading frame. A hydraulic ram was used to provide and control \(\sigma_{1}\). Axial and radial strain were recorded by two linear variable displacement transformers (LVDTs) and an extensometer chain, respectively. All rock samples were deformed until they reached their respective ultimate strengths (i.e., the maximum \(\sigma_{1}\) that the sample can withstand before it breaks or weakens). Active-source acoustic measurements were taken during the course of the experiments. The variable in all UCS tests conducted is the axial stress, which was increased using a constant axial displacement rate of 0.0005 mm/s until rock failure occurred. During the course of experimental series B, C, and D, i.e., the CCS tests combined with active-source acoustic measurements, the effect of both \(\sigma_{1}\) and \(\sigma_{2}\) on the seismic velocities was investigated. Here, the rock samples were positioned in a Hoek cell [39], which allows one to apply a certain confining pressure (i.e., \(\sigma_{2}\)) on the sample, before being placed on the abovementioned 500 kN loading frame. The hydraulic ram controlled \(\sigma_{1}\) whereas an ISCO pump was used for regulating the confining pressure. Axial strain is again recorded by the two LVDTs. During the experiments active-source acoustic measurements were taken, where attention was paid to how the seismic signal varies as a function of \(\sigma_{1}\) and \(\sigma_{2}\); reflecting various locations within the subsurface at the Kizildere reservoir. In experimental series B (Table 3), \(\sigma_{1}\) was set equal to \(\sigma_{2}\) at all times. Active-source acoustic measurements were taken at intervals of 5 MPa, until a maximum of 60 MPa was reached. In series C (Table 4), \(\sigma_{\rm diff}\) (i.e., \(\sigma_{1}\)- \(\sigma_{2}\)) was kept constant at either 45 or 10 MPa. Due to the relatively weak nature of the silt- and mudstone samples, \(\sigma_{\rm diff}\) was set equal to 10 MPa, instead of 45 MPa, for these specific experiments. Once again, acoustic measurements were performed at 5 MPa intervals, until \(\sigma_{1}\) and \(\sigma_{2}\) equalled 95 MPa (or 60 MPa for silt- and mudstone samples) and 50 MPa, respectively. Experimental series D (Table 5) reflect the seismic response characterisation at field-representative stress conditions. Here, \(\sigma_{2}\) was fixed whereas \(\sigma_{1}\) was varied, the latter reflecting different depths within the Kizildere reservoir. Once more, the active-source acoustic measurements were taken as a function of varying \(\sigma_{1}\). Appropriate magnitudes for \(\sigma_{1}\) (ranging from 9-70 MPa) and \(\sigma_{2}\) (ranging from 9-33 MPa), that prevail at the Kizildere geothermal field (Table 5), were taken from the literature [35]. For all abovementioned CCS experiments, the axial load was increased at a rate of 0.05 kN/s between two consecutive acoustic measurements.