Frequency-dependent quality factors of S-coda waves from active ultrasonic transmission measurements in the STIMTEC project, URL Reiche Zeche, Germany

Mean S-wave coda quality factors (mean-Qc) were estimated from active ultrasonic transmission (UT) measurements acquired during the STIMTEC project in the URL Reiche Zeche (Saxony, Germany). We used S-coda waves of 88 selected UT measurements carried out in 3 differently oriented boreholes (BH10, BH12, BH16) to estimate the spatial change of the coda quality factor in the targeted rock volume, an anisotropic metamorphic gneiss. We also analysed temporal variation in attenuation before and after hydraulic stimulations performed in two boreholes (BH10, BH17). We formed in total 8 UT groups (see data table "2022-004_Blanke-and-Boese_mean_UT_event_locations") from neighbouring UT measurements within different depths and from separated time intervals (see also Tab. 1 in Blanke et al. 2023), and compare mean-Qc estimates of centre frequencies ranging 3-21 kHz of octave-width frequency bands. Our results show a characteristic frequency-dependence and we find that mean-Qc estimates reveal temporal-variations of attenuation more significantly than those obtained from velocity measurements. The temporal variations are strongly connected to hydraulic stimulation activities resulting in a reduction of the coda quality factor where AE events occurred. Analysis of mean-Qc estimates after a temporal resting phase (with no activity in the rock volume) suggests that frequencies > 15 kHz indicate healing of small-scale fractures induced by injections. The study shows that coda analysis is a powerful tool for the detection of damage zones and for monitoring changes of the local fracture network within reservoirs important for exploitation or underground storage of gases and liquids.