Passive monitoring of a deep geothermal reservoir in the Strasbourg area by interferometric approaches using ambient seismic noise.

We present preliminary results of ambient seismic noise monitoring near the deep geothermal reservoir at the Vendenheim site north of Strasbourg in France. From November 2019 to mid 2021, various operations led to an intense induced seismic swarm with several events of magnitudes above 3.0Mlv. This crisis is also characterized by the presence of an isolated swarm ~5km south of the geothermal site as well as the occurrence of the maximum magnitude event (3.9Mlv) 6 months after the cease of injection tests. Understanding these remote and delayed triggering mechanisms is essential for the successful development of future deep geothermal projects. We use ambient seismic noise correlations betweens pairs of sensors from a composite network of 137 permanent and temporary stations in the area. In particular, we intend to monitor the evolution of the upper crust around the reservoir by studying velocity variations and coda waveforms decorrelation in different frequency bands.

At high frequencies (1-3Hz), velocity variations appear to be correlated with fluctuations of the water table elevation. Strong decorrelations in waveform coda are also observed during holidays, suggesting changes in anthropogenic noise sources illumination. At low frequencies (3-6s), apparent variations of velocity and decorrelation with mainly an annual periodicity are observed, but could be associated with seasonal variations in the position of the sources of the second microseismic peak. This study shows that in order to observe temporal variation in the properties of deep geothermal reservoirs with ambient noise coda wave interferometry, it is necessary to understand and model variations in the subsurface layers and in the sources of ambient seismic noise.