Four years of experience with a permanent seismic monitoring array at the Ketzin CO2 storage pilot site

CO2 was injected into a saline aquifer near the town of Ketzin in Germany from July 2008 to August 2013. To monitor CO2-migration close to the injection well, TNO installed a fixed 2D seismic array of 120 meters length in 2009, with 3-component (3-C) geophones at the surface, 4-component receivers at 50 meters depth and a central vertical array of 4-component receivers. The test-bed is used both for the recording of high-quality active time-lapse seismic data as well as for continuous passive seismic data recording. Here we focus on the latest results obtained from active source experiments, micro-seismic data analysis and ambient noise seismic interferometry (ANSI) of passive seismic data. Data from two experiments with a source driven by linear motors (permanent) and an impact hammer source (semi-permanent) were evaluated in order to assess the repeatability of the two source types. The repeatability of the first source was relatively high compared to the second source. After the stop of injection in August 2013, active seismic shots were acquired repeatedly on a daily basis. These data did not show significant impedance changes at reservoir level. Microseismic event analysis of passive seismic data detected a few local events originating from reservoir depth. In order to quantify these events the seismic array was successfully calibrated by using independent earthquake recording. This yielded estimates of the local magnitude for local events within the range [-2.5,0.5]. Recent work on ANSI focused on the application of this method on both synthetic and field data. The resulting reflection response shows a strong resemblance with outcomes from active shot reflection profiles, and the key reflectors at reservoir level have comparable characteristics. (C) 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of GHGT-12