ALBERT

Description: The capability of seismic interferometry to create virtual sources atreceiver sites from records of ambient seismic noise is used for seismic monitoringand tomography of different targets.We present hardware developed specificallyfor the needs of seismic data acquisition in the context of monitoring and ambientnoise tomography. Digitizers are capable of continuous recording and real timewireless data transmission in self organizing meshes to allow for robust telemetry indifficult circumstances such as cities or landslides that may cause the loss of stations.A software tool is described that implements required processing and analysis proceduresfor the interferometric processing.We have applied the novel 3Dambient noisesurface wave tomography approach to the Issyk-Ata fault in Kyrgyzstan. It showsthat seismic interferometry can successfully be used for structural investigations onlength scales of only 100 m. The method uses 3D sensitivity kernels for a singlestepinversion of phase velocity dispersion curves for subsurface S-wave velocitystructure and incorporates topography. We recover lateral differences in sedimentvelocities and an offset of the bedrock depth across the fault. Applications of interferometricmonitoring to the geological CO2 storage test site in Ketzin (Germany)and to the Piton de la Fournaise volcano (La Reunion island) emphasize the valueof this approach. At Ketzin site we identify variations of the subsurface velocitiesthat are correlated with changes in the ground water level and mask potential signalsfrom the reservoir depth. At Piton de la Fournaise volcano, seismic velocity changesare linked to volcanic processes as shown by comparison with surface displacementand seismicity that are typically used to characterize volcanic activity.We observe aclear distinction between phases of inflation prior to eruptions and deflation duringperiods of quiescence.

Description: Im Rahmen der DFG-Forschergruppe "Projekt Großhang: Koppelung von Strömungs- und Deformationsprozessen zur Modellierung von Großhangbewegungen" wird eine Hangrutschung in Heumös, bei Ebnit in Österreich untersucht. Für die passive Seismik zeichnen drei Miniarrays (Apertur: 40m) mit jeweils vier Seismometern die Bodenbewegungen im Frequenzband 1-200Hz seit September 2009 kontinuierlich auf (Abstand zwischen Miniarrays: 200m-500m). Für einen Messzeitraum von vier Tagen wurde das Netzwerk mit 12 zusätzlichen Seismometern verdichtet. Es hat sich gezeigt, dass sich die Kreuzkorrelationen des seismischen Hintergrundrauschens im Frequenzband von 1-20Hz schon nach weniger als 24h stabilisieren und langzeit-stabile Strukturen bis zu Lagzeiten von 5s auftreten. Werden die Kreuzkorrelationen in Abhängigkeit von der Stationsentfernung und der Lagzeit dargestellt, so ist kein kohärentes Muster propagierender Wellen erkennbar, was wir auf die komplexe Struktur unter dem Netzwerk zurückführen. Es ist daher unklar, welche Wellentypen zu den Signalen in den Kreuzkorrelationen beitragen. Außerdem wurde untersucht, ob und wie Veränderungen in den Kreuzkorrelationen mit Wettereinflüssen zusammenhängen. Dabei zeigte sich, dass Veränderungen in den Kreuzkorrelationen mit einem erhöhten Wassergehalt im Boden sowie mit dem Einfrieren und Auftauen des Bodens in engen Zusammenhang gebracht werden können.

Description: In seismology, Passive Image Interferometry (PII), based on ambient seismic noise, can be used to detect small temporal changes in the propagation of the seismic wavefield. As these changes can be related to changes of elastic properties in the propagation medium, PII can be used to observe dynamic processes in the earth’s crust. This technique was successfully applied, inter alia, to monitor seasonal variations in response to environmental changes or stress changes caused by earthquakes or material changes due to the eruption of volcanoes. PII is based on the possibility of reconstructing the Green’s function between a pair of receivers from continuous records of seismic noise. With two seismometers as receivers, the Green’s Function describes the propagation of a seismic wave between the two receivers. It can be reconstructed by cross-correlating the ambient seismic noise, recorded at the receivers. A change in the medium between the seismometers directly affects the shape of the cross-correlation functions (CCFs), from which a change in the seismic velocities can be derived. Our idea is to investigate the potential application of this technique to monitor the emplacement of CO2 at the test site for CO2 sequestration in Ketzin (Brandenburg, Germany).We calculated CCFs of the ambient noise field for a time period of about 4 years from the beginning of the injection. The analysis of the cross-correlations showed that they are asymmetric and dominated by a phase traveling with about 300 m/s, which is consistent with Rayleigh waves traveling in the shallow sediments. The noise direction was analysed with an optimal rotation algorithm over 1 month of data and showed a prominant direction incoming from north-east in a frequency range between 0.5 and 4 Hz. This direction matches with the location of a large windpark a few km away from the array. For lower frequencies, the noise is dominantly incident from north-west. To analyse possible velocity changes for each day, we computed stretched versions of a reference CCF in different frequency bands and calculated correlation values between time windows in the coda part of the stretched traces and the reference trace. Due to the almost continuous injection of CO2 we expect a monotonic decrease of the seismic velocities. So far, we can observe velocity variations with a period of approx. one year that indicates a seasonal influence, most probably due to environmental influences, which overlay the effect of the CO2 injection.