Monitoring deep geothermal reservoirs with ambient seismic noise: a case study at Rittershoffen, France

Abstract

In this work we analyze ambient seismic noise recorded by a permanent network of seismological stations near the two deep geothermal sites of Soultz-sous-Forêts (GEIE-EMC) and Rittershoffen (ECOGI project), France. The available data have been acquired between 2009 and 2014 and cover the drilling of two boreholes at Rittershoffen (GRT1 and GRT2) as well as several stimulation phases and production tests. These operations were associated with a moderate seismicity, not felt by the population, but suggesting that the medium may have been perturbed. Using the Passive Image Interferometry method (PII) that is based on the correlation of the noise records between pairs of receivers, we evaluate our ability to detect small variations of the medium induced by the geothermal activities. This method is theoretically sensitive to changes in the velocity of the seismic waves or in the scattering properties of the medium and does not rely on deterministic sources (artificial, natural or induced sources). We show that time-dependent spectral lines observed within the period band dominated by the anthropogenic activity (below 1s) have a significant impact on the method and can induce apparent velocity variations. At period 3s, we observe a significant change in the cross-correlation waveforms that follows the stimulation of the reservoir at Rittershoffen. It might indicate a perturbation of the geothermal reservoir induced by the stimulation.