ALBERT

Description: Passive seismic interferometry is a new promising methodology for seismic exploration. Interferometry allows information about the subsurface structure to be extracted from ambient seismic noise. In this study, we apply the cross-correlation technique to approximately 25 hr of recordings of ambient seismic noise at the Ketzin experimental CO2 storage site, Germany. Common source gathers were generated from the ambient noise for all available receivers along two seismic lines by cross-correlation of noise records. This methodology isolates the interstation Green's functions that can be directly compared to active source gathers. We show that the retrieved response includes surface waves, refracted waves and reflected waves. We use the dispersive behaviour of the retrieved surface waves to infer geological properties in the shallow subsurface and perform passive seismic imaging of the subsurface structure by processing the retrieved reflected waves.

Description: The GLATIS project (Greenland Lithosphere Analysed Teleseismically on the Ice Sheet) with collaborators has operated a total of 16 temporary broadband seismographs for periods from 3 months to 2 years distributed over much of Greenland from late 1999 to the present. The very first results are presented in this paper, where receiver-function analysis has been used to map the depth to Moho in a large region where crustal thicknesses were previously completely unknown. The results suggest that the Proterozoic part of central Greenland consists of two distinct blocks with different depths to Moho. North of the Archean core in southern Greenland is a zone of very thick Proterozoic crust with an average depth to Moho close to 48 km. Further to the north the Proterozoic crust thins to 37–42 km. We suggest that the boundary between thick and thin crust forms the boundary between the geologically defined Nagssugtoqidian and Rinkian mobile belts, which thus can be viewed as two blocks, based on the large difference in depth to Moho (over 6 km). Depth to Moho on the Archean crust is around 40 km. Four of the stations are placed in the interior of Greenland on the ice sheet, where we find the data quality excellent, but receiver-function analyses are complicated by strong converted phases generated at the base of the ice sheet, which in some places is more than 3 km thick.

Description: Teleseismic Love and Rayleigh waves recorded by the GLATIS project (Greenland Lithosphere Analysed Teleseismically on the ice sheet) have been analysed for phase velocities using a two-station method. The data set consists of broad band recordings from 19 seismograph stations in Greenland, as well as data from IRIS station ALE in northern Canada. Four of the BB seismographs in Greenland are permanent, the rest are temporary with deployment periods ranging from three months to four years. We have obtained good quality phase velocities for Love and Rayleigh waves in the period range from 25 to 150 seconds for 45 different two-station paths across Greenland. This provides us with information about velocity heterogeneities through the entire lithosphere and covering a large part of central to southern Greenland. An isotropic tomographic inversion was used to combine the phase velocity information from the dispersion curves in order to produce phase velocity maps for Greenland at several different periods. The most significant lateral variation in phase velocity is found at intermediate periods, where a high-velocity anomaly is resolved beneath central-southwestern Greenland, and a low-velocity anomaly is found beneath southeastern Greenland. Results from Love and Rayleigh waves will be compared.

Description: We present the results of a surface wave study carried out across Greenland as part of the 'GLATIS' (Greenland Lithosphere Analysed Teleseismically on the Ice Sheet) project. Rayleigh wave phase velocity dispersion curves were estimated for 45 two-station paths across Greenland, using data from large teleseismic earthquakes. The individual dispersion curves show characteristics broadly consistent with those of continental shields worldwide, but with significant differences across the Greenland landmass. Reliable phase velocity measurements were made over a period range of 25-160 s, providing constraint on mantle structure to a depth of 300 km. An isotropic tomographic inversion was used to combine the phase velocity information from the dispersion curves, in order to calculate phase velocity maps for Greenland at several different periods. The greatest lateral variation in phase velocity is observed at intermediate periods (50-80 s), where a high-velocity anomaly is resolved beneath central-southwestern Greenland, and a low-velocity anomaly is resolved beneath southeastern Greenland. The results of the phase velocity inversion were used to construct localized dispersion curves for node points along two parallel north-south profiles in southern Greenland. These curves were inverted to obtain models of shear wave velocity structure as a function of depth, again with the assumption of isotropic structure. A similar inversion was carried out for two two-station dispersion curves in northern Greenland, where the resolution of the phase velocity maps is relatively low. The models show a high-velocity 'lid' structure overlying a zone of lower velocity, beneath which the velocity gradually increases with depth. The 'lid' structure is interpreted as the continental lithosphere. Within the lithosphere, the shear wave velocity is 4-12 per cent above global reference models, with the highest velocities beneath central-southwestern Greenland. However, the assumption of isotropic structure means that the maximum velocity perturbation may be overestimated by a few per cent. The lithospheric thickness varies from 100 km close to the southeast coast of Greenland to 180 km beneath central-southern Greenland.