| 1. |
- Kalscheuer, Thomas, 1975-
(författare)
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Improvement and Assessment of Two-Dimensional Resistivity Models Derived from Radiomagnetotelluric and Direct-Current Resistivity Data
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Two-dimensional (2-D) models of electrical resistivity are improved by jointly inverting radiomagnetotelluric (RMT) and direct-current resistivity (DCR) data or by allowing for displacement currents in the inversion of RMT data collected on highly resistive bedrock. Uniqueness and stability of the 2-D models are assessed with a model variance and resolution analysis that allows for the non-linearity of the inverse problem.Model variance and resolution are estimated with a truncated singular value decomposition (TSVD) of the sensitivity matrix. In the computation of model errors, inverse singular values are replaced by non-linear semi-axes and the number of included eigenvectors is increased until a given error threshold is reached. Non-linear error estimates are verified with most-squares inversions. For the obtained truncation levels, model resolution matrices are computed. For RMT data, non-linear error appraisals are smaller than linearized ones. Hence, the consideration of the non-linearity in RMT data leads to reduced model errors or enhanced model resolution.The dielectric effect on RMT data is investigated with a new 2-D forward and inverse code that allows for displacement currents. As compared to the quasi-static approximation, apparent resistivities and phases of the impedance tensor elements are found to be significantly smaller and the vertical magnetic transfer function exhibits more distinct sign reversals. More reliable models of electrical resistivity are obtained from areas with highly resistive bedrock, if displacement currents are allowed for. In contrast, inversions with a quasi-static scheme introduce artefactual structures with extremely low or high resistivities.A smoothness-constrained 2-D joint inversion of RMT and DCR data is presented. The non-linear model variance and resolution analysis is applied to single and joint inverse models. For DCR data, the errors estimated by most-squares inversions are consistently larger than those estimated by the non-linear semi-axes, indicating that DCR models are poorly resolved. Certain areas of the joint inverse models are better resolved than in the single inverse models.
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| 2. |
- Lebedeva-Ivanova, Nina
(författare)
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Geophysical Studies Bearing on the Origin of the Arctic Basin
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Deep troughs and ridges of the Arctic Basin are some of the least known features of the Earth's crust. Some of the ridges, eg. Chukchi and Nordwind, are connected directly to the continental shelves and are certainly submarine promontories of the latter. The character of the Lomonosov Ridge as a narrow slice of continental crust that separated from the Eurasian margin in the early Cenozoic (by opening of the Eurasian Basin), is not in doubt. Recent drilling (ACEX) and piston coring have confirmed this interpretation. However there are many other ridges and some of the troughs that are of uncertain origin. Seismic research in combination with potential field data over the East-Siberian margin, Podvodnikov and Makarov basins and the Mendeleev Ridge, presented here, provides a framework for understanding this enigmatic part of the Earth. The constrained models of the crust illustrate their structure. The crust beneath the East Siberian margin is up to 40 km thick; it thins to about 20 km towards to the Podvodnikov Basin. The models over the Arlis Gap, in the middle of the Podvodnikov Basin, and the Mendeleev Ridge have shown that the crust beneath both these features is anomalously thick (up to 28–32 km) and has a velocity structure that suggests the presence of highly attenuated continental crust. The crustal thickness over the Makarov Basin varies from 8 km to 15 km. Reflection profiles provide evidence of the character and thickness of the sedimentary cover (mostly Cenozoic and late Mesozoic), both on the ridges and beneath the troughs. Presented here is evidence that some of the ridges (eg. Marvin Spur) appear to be fragments of continental crust rifted off the Lomonosov Ridge (with a similar, unconformable Cenozoic cover); however, they gently plunge into and beneath troughs (eg. Makarov Basin). Reflection seismic data collected by the HOTRAX expedition in 2005 over the central part of the Lomonosov Ridge illustrate the sedimentary structure on the top of the Ridge and in an internal basin. The main sedimentary units can be interpreted by correlation with the ACEX results. The major fault separating the surrounding ridges from the internal basin appears to have a roll over anticline in the hanging wall, suggesting that the basin was created by a growth fault. The seismic lines provide evidence of gently folded basement beneath the Lomonosov Ridge with intra basement reflections are usually parallel to the upper surfaces; in combination with velocities (c. 4–5 km/s), these suggest the presence of old well-consolidated sediments.
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| 3. |
- Yordkayhun, Sawasdee, 1975-
(författare)
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2D and 3D Seismic Surveying at the CO2SINK Project Site, Ketzin, Germany: The Potential for Imaging the Shallow Subsurface
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Seismic traveltime inversion, traveltime tomography and seismic reflection techniques have been applied for two dimensional (2D) and three dimensional (3D) data acquired in conjunction with site characterization and monitoring aspects at a carbon dioxide (CO2) geological storage site at Ketzin, Germany (the CO2SINK project). Conventional seismic methods that focused on investigating the CO2 storage and caprock formations showed a poor or no image of the upper 150 m. In order to fill this information gap, an effort on imaging the shallow subsurface at a potentially risky area at the site is the principal goal of this thesis.Beside this objective, a seismic source comparison from a 2D pilot study for acquisition parameter testing at the site found a weight drop source suitable with respect to the signal penetration, frequency content of the data and minimizing time and cost for 3D data acquisition.For the Ketzin seismic data, the ability to obtain high-quality images is limited by the acquisition geometry, source-generated noise and time shifts due to near-surface effects producing severe distortions in the data. Moreover, these time shifts are comparable to the dominant periods of the reflections and to the size of structures to be imaged. Therefore, a combination of seismic refraction and state-of-the-art processing techniques, including careful static corrections and more accurate velocity analysis, resulted in key improvements of the images and allowed new information to be extracted. The results from these studies together with borehole information, hydrogeologic models and seismic modeling have been combined into an integrated interpretation. The boundary between the Quaternary and Tertiary unit has been mapped. The internal structure of the Quaternary sediments is likely to be complicated due to the shallow aquifer/aquitard complex, whereas the heterogeneity in the Tertiary unit is due to rock alteration associated with fault zones. Some of the major faults appear to project into the Tertiary unit. These findings are important for understanding the potentially risky anticline crest and can be used as a database for the future monitoring program at the site.
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