|
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 Ketz in(Germany) where 67271 tonsof 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 4yr of passive seismic data in a frequency range of 0.05–4.5Hz we found periodic velocity variations with a period of approximately 1yr 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 spacialstability excludes variations of the noises our cedistribution 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 ground water level data reveals a direct correlation between depth of the ground water level and these is micvelocity. The influence of ground frost on these is mic velocities is documented by a sharp increase of velocity when the maximum daily temperature stays below0◦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.
|
Kembali