| Title | Monitoring of EGS Reservoir with Ambient Seismic Noise Interferometry: Case Study of Rittershoffen Site (France) |
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| Authors | D. KULA, J. VERGNE, J. SCHMITTBUHL, J. AZZOLA, D. ZIGONE |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | Seismic noise correlation, reservoir monitoring, artifacts, stimulation |
| Abstract | The monitoring of EGS reservoirs using ambient seismic noise is emerging as a potentially interesting tool for a low cost and continuous survey of deep geothermal reservoirs. We apply here ambient noise coda wave interferometry to almost 6 years of continuous data at two short period seismic stations located in the vicinity of a deep geothermal EGS project located in Rittershoffen (northern Alsace, France), seven kilometers to the southeast of the historical EGS pilot project of Soultz-sous-Forêts. The reservoir is located 2.5 kilometer deep and the exploitation is continuous since second half of 2016. Using a multiple reference approach, we compute the matrices of the optimal stretching coefficient and related maximum coherence for every pair of days and for the nine pairs of components and apply a weighted scheme to recover seismic velocity variations over time. For the 3 analyzed frequency bands (2-5Hz, 1-3Hz, 0.1-0.5Hz) we observe stable cross correlation functions and coherent velocity variations. Times series are compared to the various stages of the stimulation and exploitation of the Rittershoffen geothermal reservoir. We can associate most of them either to artefacts due to temporal changes in the frequency content of the ambient noise or to natural phenomena such as changes in the water table elevation or thermo-elastic effects due to annual atmospheric temperature changes. Accounting for such bias appears to be a mandatory step to allow monitoring such deep reservoir with ambient noise interferometry. However, a sudden decrease of coherence, not related to changes in the noise sources, is observed following the hydraulic stimulation that occurred in one of the wells. |