Monitoring of environmental influences on seismic velocity at the geological storage site for CO2 in Ketzin (Germany) with ambient seismic noise

Regarding the exploitation of natural resources, storage of waste or subsurface construction, is an increasing need to obtain comprehensive knowledge about the subsurface and temporal changes. We investigate the possibility of a passive monitoring using ambient noise, which is cheap and continuous compared to active seismics.We work with data with a seismic network in Ketzin (Germany) where 67 271 tons of CO2 were injected 2008 June until 2013 August into a saline aquifer at a depth of about 650 m. Monitoring expansion of the CO2 plume is essential for the characterization of the reservoir as well the detection of potential leakage. By cross-correlating about 4 yr of passive seismic data a frequency range of 0.05–4.5 Hz we found periodic velocity variations with a period of 1 yr that cannot be caused by the CO2 injection. The prominent direction of the wavefield indicates a wind farm as the dominant source providing the temporally stable field. This spacial stability excludes variations of the noise source distribution as a cause spurious velocity variations. Based on an amplitude decrease associated with time windows later parts of the coda, we show that the variations must be generated in the shallow. A comparison to groundwater level data reveals a direct correlation between depth the groundwater level and the seismic velocity. The influence of ground frost on the seismic is documented by a sharp increase of velocity when the maximum daily temperature below 0 ◦C. Although the observed periodic changes and the changes due to ground frost only the shallow subsurface, they mask potential signals of material changes from the depths.