Available online at ScienceDirect. Energy Procedia 76 (2015 ) European Geosciences Union General Assembly 2015, EGU

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1 Available online at ScienceDirect Energy Procedia 76 (015 ) European Geosciences Union General Assembly 015, EGU Division Energy, Resources & Environment, ERE Investigation of the presence of transverse isotropy in the 3D baseline seismic data from Ketzin, Germany Mingkhwan Kruachanta a 0F0F0F0F0F0F*, Monika Ivandic b and Christopher Juhlin b a Chiang Mai University, 39 Huay Kaew Road, Chiang Mai 5000, Thailand b Uppsala University, Villavagen 16, Uppsala 75 36, Sweden Abstract In this study, we investigated the presence of transverse isotropy at the Ketzin CO pilot storage site, Germany. The anisotropy parameter, eta (ŋ), is used as an indicator of anisotropy. Preliminary results show that ŋ ranges from to , which indicates weak anisotropy. Introducing ŋ into the velocity estimation and applying the nonhyperbolic moveout correction resulted in an improvement in the continuity of reflections in the shallow part of the final migrated seismic data. 015 The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the GFZ German Research Centre for Geosciences. Peer-review under responsibility of the GFZ German Research Centre for Geosciences Keywords: anisotropy parameters, eta, transverse isotrppy, velocity analysis, 3D seismic. 1. Introduction A 3D seismic baseline survey was acquired in the EU funded CO SINK project at Ketzin, Germany in 005. The survey results from Juhlin et al. [1] reveal anticlinal and faulting structures at the Ketzin site, which indicate directional variations of the compressional stress. Moreover, well data show the presence of several intervals of claystone interbedded with siltstone, sandstone and anhydrite throughout the successions above the injection interval * Corresponding author. Tel.: ; fax: address: mingkhwan_k@cmu.ac.th The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the GFZ German Research Centre for Geosciences doi: /j.egypro

2 550 Mingkhwan Kruachanta et al. / Energy Procedia 76 ( 015 ) [1, ]. The structural and lithological layering implies that seismic anisotropy may be present, in particular transverse isotropy. Alkhalifah [3] proposed the application of nonhyperbolic moveout to evaluate the ŋ values by the following equations [4]. t x X to Vnm o t o 4 X 4 Vnmo X (1 ) V nmo (1) where 1 V V h nmo 1 () and t x is the traveltime, t o is the traveltime at zero offset, X is the offset and V h is the horizontal velocity. The obtained ŋ values can be used as an indicator for the relative contributions of anisotropy. Therefore, reprocessing of the baseline seismic data has been performed (1) to check for the presence of transverse isotropy, and () to evaluate whether an improved image can be obtained by using an approximation of the velocity field which takes into account transverse isotropy.. Geology of Ketzin, CO SINK site The Ketzin pilot CO storage site is located in the North-East German Basin (NEGB) which was initiated in the early Permian [1]. At Ketzin, the thickness of the sedimentary fill is approximately 4000 m with age ranging from Permian to Quaternary. Permian salt diaperism (Zechstein) generated ENE-WSW striking double anticlines with gently dipping Triassic and lower Jurassic formations [5]. There were two uplift events that resulted in erosional troughs that cut the Tertiary clay and the Jurassic succession. The Tertiary trough was filled with Quaternary sediments [] (Fig. 1). Fig. 1. Simplified geology at the Ketzin pilot site, modified after Juhlin et al. and Förster et al. [1, ]. The study area shows the anticlinal structure with aquifer (yellow) and aquitard (gray) units. The central graben faults are marked by red lines. U1 and U indicate two uplift events occurred in the periods of the Jurassic and Tertiary, respectively.

3 Mingkhwan Kruachanta et al. / Energy Procedia 76 ( 015 ) Data acquisition The acquisition was carried out using a template system with the same acquisition parameters for each template. Table 1 shows the template acquisition parameters. Fig. (a) shows the location of the Ketzin pilot site, Germany. Fig. (b) illustrates the theoretical source and receiver locations for a template. Table 1 Acquisition parameters [1]. Parameter Value Receiver line spacing / number 96 m / 5 Receiver station spacing / channels 4 m / 48 Source line spacing / number 48 m / 1 Source point spacing CDP bin size Nominal fold 5 Geophone Sampling rate Record length Source Acquisition unit 4 m or 7 m 1 m x 1 m 8 Hz single 1 ms 3 s 40 kg accelerated weight drop, 8 hits per source point Sercel 408 UL Fig.. (a) Location of the Ketzin pilot CO storage site in Ketzin, Germany; (b) Theoretical source (blue) and receiver (red) locations for each template.

4 55 Mingkhwan Kruachanta et al. / Energy Procedia 76 ( 015 ) Data Processing The processing steps applied to the 3D baseline seismic data were performed in two streams following the former processing as in Juhlin et al. [1]. Both processing sequences are summarized in Fig. 3. The first processing sequence (solid arrows) used an isotropic assumption in the velocity analysis. Reprocessing was performed in order to (1) prepare the data for the fourth order velocity analysis and estimation of ŋ, as well as () using the final migrated image as a comparison tool to detect any improvement of this image after introducing ŋ into the nonhyperbolic moveout correction. The second processing sequence (dashed arrows) performed identical processing steps with the same processing parameters as the former one until reaching the second pass of second order velocity analysis (V nmo ). After running the second pass of velocity analysis, the data were used for the fourth order velocity analysis to estimate ŋ values followed by a nonhyperbolic moveout correction. Fig. 3. The original processing workflow applied to the 3D baseline seismic data by Juhlin et al. [1] (solid arrows) and the reprocessed workflow with the fourth order velocity approximation (dashed arrows).

5 Mingkhwan Kruachanta et al. / Energy Procedia 76 ( 015 ) Velocity analysis In order to estimate ŋ, it is required to pick the horizontal velocities (V h ) by applying the fourth order velocity estimation. After acquiring V nmo, V h were picked by using semblance velocity analysis. The picked V nmo and V h profiles are illustrated in Fig. 4. The difference between V nmo and V h indicates a non-zero value of ŋ. Equation () gives ŋ values that range from to and show a variation in both vertical and horizontal directions. By introducing the obtained ŋ values into the nonhyperbolic moveout correction, the far offset reflections become flatter. Fig. 5(a) illustrates one CDP gather after applying a normal moveout correction. It shows the crooked shape of the reflections at the far offset, indicating a velocity misfit for the far offset of the seismic data. Fig. 5 (b) shows the same CDP gather after applying a nonhyperbolic moveout correction, which has flatter reflections at the far offsets. Fig. 4. (a) V nmo profile at inline 1160; (b) V h profile at inline Fig 5. (a) Illustrates one CDP gather after applying normal moveout correction; (b) After applying nonhyperbolic moveout correction.

6 554 Mingkhwan Kruachanta et al. / Energy Procedia 76 ( 015 ) Fig. 6. The final migrated sections using (a) second order velocity estimation; (b) fourth order velocity estimation. The fourth order velocity estimation shows a stronger amplitude reflection than that of the second order velocity estimation (red arrows). 4.. Final migrated images After applying moveout corrections to both of the processed data volumes, the data were stacked, FXYdeconvolved and migrated to obtain the final migrated sections. The final migrated section of the fourth order velocity estimation shows some stronger reflection amplitudes than that of the second order velocity estimation (Fig. 6). 5. Conclusion The difference between V nmo and V h shows that transverse isotropy is present in the study area. ŋ ranges from to , indicating weak anisotropy. Additionally, the inclusion of ŋ into the seismic data processing improves the final migrated data. In our study, it results in stronger amplitudes in some areas of the seismic cube, but only in the shallow part of the processed volume. The depth of the improvement depends on the offset, i.e. the longer the offset, the deeper is the expected depth of improvement. Acknowledgements GLOBE ClaritasTM under licence from the Institute of Geological and Nuclear Sciences Limited, Lower Hutt, New Zealand was used to process the seismic data. The European Commission is gratefully acknowledged for funding CO Storage by Injection into a Natural Storage site (CO SINK), Project no DPST project (Royal Thai Government) provides scholarship and support research funding professional training. References [1] Juhlin C., Giese R., Zinck-Jørgensen K., Cosma C., Kazemeini H., Juhojuntti N., Lüth S., Norden B., Förster A. 3D baseline seismics at Ketzin, Germany: The CO SINK project. Geophysics 007; B11-B13. [] Förster A., Giese R., Juhlin C., Norden B., Springer N., CO SINK Group. The geology of the CO SINK site: From regional scale to laboratory scale. Energy Procedia 009; [3] Alkhalifah T. Velocity analysis using nonhyperbolic moveout in transversely isotropic media. Geophysics 1997; [4] Alkhalifah T., Tsvankin I. Velocity analysis for transversely isotropic media. Geophysics 1995; [5] Norden B., Frykman P. Geological modelling of the Triassic Stuttgart formation at the Ketzin CO storage site, Germany. Int. J. of Greenhourse Gas Control 013;