 |
| Titel |
Ground Water and Frost Induced Seismic Velocity Changes in Ketzin (Germany) |
| VerfasserIn |
Martina Gassenmeier, Christoph Sens-Schönfelder, Michael Korn |
| Konferenz |
EGU General Assembly 2014
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250095673
|
| Publikation (Nr.) |
 EGU/EGU2014-11142.pdf |
|
|
|
|
|
| Zusammenfassung |
| The principle of Seismic Interferometry (SI) is that the correlation of a random wave field
like seismic noise recorded by distant receivers can be used to infer the Green function (or at
least part of it) of the medium between the receivers. Beside tomographic inversion for the
subsurface velocity it can also be used to detect small temporal changes in the propagation of
the seismic wave field.
As these changes can be related to changes of elastic properties in the propagation
medium, SI can characterize dynamic processes in the earth’s crust. This technique was
successfully applied, inter alia, to monitor seasonal variations in response to environmental
influences, shaking caused by earthquakes or material changes due to the eruption of
volcanoes.
We work with data acquired with a seismic network in Ketzin (Brandenburg, Germany),
where CO2 is injected into a saline aquifer at a depth of about 650m. We calculated daily
cross-correlation functions (CCFs) of the ambient noise field for a time period of about 4
years from the beginning of the injection. Spectra showed that the frequency band between 1
and 3.5Hz does neither show an annual periodicity (like for microseism) nor temporal shifts
of peak frequencies.
For this frequency band we estimated the noise propagation direction over two years and
found a predominant direction from north-east. This direction matches with the location of a
large wind park a few km away from the array. The direction of the noise wave field shows a
good stability, which excludes variations of the noise source distribution as a cause of
spurious velocity variations.
To analyze possible velocity changes for each day, we computed stretched versions of a
reference CCF and calculated correlation values between different time windows in the coda
part of the stretched traces and the reference trace. We can observe velocity variations with a
period of approx. one year that are not caused by the CO2 injection. Due to the almost
continuous injection of CO2 we would expect a monotonic decrease of the seismic velocities
if caused by the CO2.
Based on an amplitude decrease when using time windows in the later part 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 the depth of the ground water level and
the seismic velocity. The influence of ground frost on the seismic velocities is documented in
a sharp increase of velocity when the maximum daily temperature does not exceed 0°C. |
|
|
|
|
|
|