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Shallow velocity–depth model using first arrival traveltime inversion at the CO2SINK site, Ketzin, Germany
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
2007 (English)In: Journal of Applied Geophysics, ISSN 0926-9851, Vol. 63, no 2, 68-79 p.Article in journal (Refereed) Published
Abstract [en]

The CO2SINK project, a carbon dioxide storage and monitoring project, has been in operation in the Ketzin area, Germany since 2004. Goals are to investigate the techniques and understand the processes related to geological storage of CO2 in a saline aquifer. A key element of the project is comprehensive monitoring and the development of verification methods to track the long term spread of the injected CO2. Time lapse seismic surveys in 3D and 2D are important monitoring components for tracking the movement of CO2. Prior to the 3D baseline seismic survey in 2005, a pilot seismic survey was performed in 2004 in order to provide input parameters for the 3D survey and information on target depth. This pilot study, consisting of two perpendicular 2.4 km long profiles confirmed the geological structure based on 1960s vintage seismic data. However, the image is rather poor in the uppermost parts of the seismic sections. In order to obtain additional structural information and to improve the velocity function estimates, first arrival traveltimes were used to image the near-surface structure and to provide an improved velocity function for interpretation of reflection data. We used a generalized linear inversion (GLI) method, based on iterative least-squares inversion, to reconstruct the velocity–depth model. A simple and smooth starting velocity model was derived from traveltime plots and the stacked reflection seismic sections. Reliability of the resulting velocity models were estimated from analysis of the traveltime residuals (RMS error) and qualitative analysis of ray coverage. There appears to be a good correlation of layer boundaries in our model and higher amplitude reflections observed on the stacked sections. Comparison of the seismic depth sections computed using stacking velocities with the GLI inverted velocity shows that the depth sections obtained from the latter agrees better with the velocity models. The combination of borehole data and the depth converted stacked sections provides key constraints for interpretation of the velocity models. The estimated investigation depth of the seismic profiles is on the order of 400 m with rays penetrating the surface cover and sedimentary sequences. These sedimentary rocks are characterized by a gradual increase in the velocity field with depth without strong contrasts and insignificant lateral velocity variations.

Place, publisher, year, edition, pages
2007. Vol. 63, no 2, 68-79 p.
Keyword [en]
Traveltime inversion, Generalized linear inversion, Velocity–depth model, CO2SINK project
National Category
Earth and Related Environmental Sciences
URN: urn:nbn:se:uu:diva-97519DOI: 10.1016/j.jappgeo.2007.05.001ISI: 000250767700002OAI: oai:DiVA.org:uu-97519DiVA: diva2:172499
Available from: 2008-09-05 Created: 2008-09-05 Last updated: 2013-11-01Bibliographically approved
In thesis
1. 2D and 3D Seismic Surveying at the CO2SINK Project Site, Ketzin, Germany: The Potential for Imaging the Shallow Subsurface
Open this publication in new window or tab >>2D and 3D Seismic Surveying at the CO2SINK Project Site, Ketzin, Germany: The Potential for Imaging the Shallow Subsurface
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2008. 80 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 550
Inversion, Traveltime tomography, 3D seismic surveys, Seismic velocity, Seismic source, CO2SINK project
National Category
urn:nbn:se:uu:diva-9273 (URN)978-91-554-7276-4 (ISBN)
Public defence
2008-10-07, Axel Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00
Available from: 2008-09-05 Created: 2008-09-05 Last updated: 2016-05-13Bibliographically approved

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