2D and 3D Seismic Surveying at the CO2SINK Project Site, Ketzin, Germany: The Potential for Imaging the Shallow Subsurface

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.