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