Summary The modular seismic acquisition system at the GFZ is basically suitable for underground exploration in crystalline, salt and clay rocks. Special features of the application result from the different properties of the media such as degree of heterogeneity, damping, anisotropy and their interaction with the excavation zone (EDZ) induced by the cavity (drifts, tunnels). This manifests itself in different frequency damping behaviour for compressional, shear and surface waves, which require a specific use of impulse and vibration sources at the joint depending on the task. Seismic borehole measurements can contribute to a significant improvement of the resolution.

The choice of an appropriate seismic source for a specific application is crucial for the success of seismic surveys. In this study, the utilization of two novel seismic sources is discussed for geoengineering applications in the bandwidth of 80-3000 Hz. These are a pneumatic impact and a magnetostrictive vibrator source mainly designed for seismic transmission and reflection applications. Seismic test surveys were carried out on Paleozoic sediment deposits to study a near surface fault zone at the southern rim of the Taunus mountain range in Germany and in crystalline rocks to investigate fracture zones along the Piora Adit of the Gotthard Base Tunnel in Switzerland, respectively. Both sources are evaluated with respect to their seismic wave energy, the sum of the squared amplitudes of first P- and S-arrivals and frequency content, signal to random noise ratio, as well as their suitability with regard to the specific site conditions and applications. Common Midpoint-stacked data of the Taunus survey and pre-stack depth migrated data of the Piora survey were analysed to assess the ability of the seismic sources to image geological structures and to give support for geotechnical aspects. For both test sites, the vibrator source proved to be more suitable to generate body waves than the impact source, which generates strong ground roll noise signals due to its non-controlled signal amplitude spectrum. In fact the control system of the vibrator source enabled improved adaption to varying ground conditions due to a controlled adjustment of the signal amplitude spectrum to optimize resolution and penetration for a given exploration depth. (C) 2018 Elsevier B.V. All rights reserved.

At the Ketzin pilot site for CO2 storage, from 2008 through 2013, 67,000 tons of CO2 were injected into the Triassic Stuttgart Formation, a saline aquifer at 630 m – 650 m depth. For continuous pressure and temperature monitoring over the whole depth range of the wells, a fibre optic cable was deployed in all deep injection and monitoring wells in the annulus outside the production casing. A VSP survey was acquired using a Mertz M12 vibro truck on 23 source points and using the whole length of deployed cable for recording. Seismic waves were recorded over a length of ~5900 m at a trace interval of 1 m. An inspection of the data quality shows that the signal to noise ratio is strongly affected by the coupling conditions of the cable. Within the cemented part of the casing, the coupling is reasonably good, whereas the signal to noise ratio is bad for uncemented intervals. In the lower half of the wells, coupling conditions are good enough to produce high-quality seismic shot gathers. These gathers were used for a 3D imaging of the reservoir complex used for CO2 storage in close vicinity to the injection and monitoring wells.

Monitoring and quantifying changes in CO2 saturation during the injection and post-injection phases at the Ketzin pilot site using AVO

Summary Seismic interferometry is a relatively low cost method compared with conventional seismic monitoring methods and can be performed together with microseismic and reservoir monitoring. These features make it a new potential tool for carbon dioxide storage monitoring. In this study, we acquired 6 nights of ambient noise data were recorded at the Ketzin experimental CO2 injection Site located in Ketzin, west of Berlin, Germany, in August 2013. An active survey was also acquired at the same time along of one line for a source test. Passive seismic interferometry was applied to the recorded noise data to reconstruct common shot gathers. The virtual shot gathers was processed to obtain a stacked section. The results show that the passive stacked sections are less coherent and low resolution compared with the active stacked section. However, we could also find some similar features to both the passive and active stacked sections in the shallow parts and even some agreement in the deeper parts.

To date, 3D time-lapse seismic monitoring at the Ketzin CO2 storage pilot site comprised a baseline survey conducted in 2005 and two repeat surveys conducted in 2009 and 2012. At the time of the first repeat survey (22-25 kt of CO2), the CO2 plume was found to be concentrated around the injection well with a maximum lateral extent of approximately 300-400 m and a thickness of 5-20m. Data from the 2012 survey (61 kt of CO2), show further growth and migration of the amplitude anomaly interpreted to be induced by the CO2 injection. The anomaly is similar in shape to that obtained from the 2009 survey, but significantly stronger and larger by similar to 150m in the N-S direction and by similar to 200m in the E-W direction. In agreement with the 2009 survey, the new data show a westward propagation of the plume, a trend governed by the complex structure of the reservoir. No evidence of systematic changes in the seismic signature within the overburden is observed. A quantitative assessment of the plume reveals a 15% discrepancy with the injected amount, which could be attributed to the ongoing dissolution processes. However, the estimated quantity also contains significant uncertainty. (C) 2014 Elsevier Ltd. All rights reserved.

CO2 mass estimation visible in time-lapse 3D seismic data from a saline aquifer and uncertainties

A combination of seismic and geoelectric processing was studied by means of a structurally constrained inversion approach. Structural constraints were interpreted from the seismic data and integrated into the geoelectric inversion through a local regularization, which allowed inverted resistivities to behave discontinuously across defined boundaries. This arranged seismic processing and constrained resistivity inversion in a sequential workflow, making the generic assumption that the petrophysical parameters of both methods change across common lithostructural boundaries. We evaluated the approach using a numerical example and a real data example from the Ketzin [Formula: see text] pilot storage site, Germany. The latter demonstrated the efficiency of this approach for combining 4D seismic and surface-downhole geoelectric data. In consistence with the synthetic example, the constrained resistivity inversions produced clearer delineated images along the boundary between caprock and reservoir formation. Near the [Formula: see text]-flooded reservoir, the seismic and geoelectric time-lapse anomalies correlated well. At some distance to the downhole electrodes, however, the geoelectric images conveyed a notably lower resolution in comparison to the corresponding seismic images. Both methods confirm a northwesterly trend for the [Formula: see text] migration at the Ketzin site, although a rather northerly direction was initially expected. The results demonstrate the relevance of the presented approach for the combination of both methods for integrated geophysical [Formula: see text] storage monitoring.

Seismic methods have proven to be effective for monitoring the movement and location of injected CO2 within deep saline aquifers. However, a disadvantage of seismic monitoring is the high costs ass ...

Correlation-based static correction of 4D seismic data with a demonstration at the Ketzin CO2 storage site, Germany

The Common Reflection Surface, or CRS technique offers a comprehensive workflow for improving seismic processing, imaging, and reservoir characterisation in time and depth, which has been demonstrated in a project of geological CO2 storage at the Ketzin site in Eastern Germany. In applications to the 3D seismic baseline data that was acquired before CO2 injection, the results of the CRS time processing chain are compared to a previous conventional processing. The CRS noise suppression and regularization in the prestack data result in the so-called CRS gathers where acquisition related data gaps and fold variation are compensated for using the lateral event continuation of the CRS method. Both, the data reconstruction and the enhanced prestack signal quality lead to an increased resolution of the subsurface image, and strongly improve the tie to the well data. In the mapping of shallow gas at extremely low fold at this time level, CRS-based AVO resolves a well defined outline and inner structure of the gas zone, and clearly discriminates high-amplitude events outside the gas zone. The CRS technique thus proves to be a versatile tool for improved structural assessment and reservoir monitoring in both, storage and exploration projects.

The prior knowledge of the rock mass behavior along a projected roadway is fundamental for planning activities and safety measures at a construction site. However, pre-investigations are often costly and time-consuming. To generate high-resolution images of geotechnically important structures and changes in the rock mass, the Integrated Seismic Imaging System (ISIS) was developed at the GFZ. Seismic measurements offer detailed information on the rock mass, especially if the data acquisition takes place on-site during tunneling. However, to be of importance for the decision making on-site, the data needs to be processed and interpreted within a small timeframe. To meet this requirement the interpretation process needed to be automated. In the ONSITE project, a first step towards automating this process has been done by developing adapted routines with self-learning algorithms for rock mass classification based on seismic measurements. For the classification, the widely known RMR and RQD have been used so that a general idea about the rock mass behavior and not only single parameters can be gained from the results Based on the RMR, two rock mass classes were determined along seven seismic profiles from the Faido adit that belongs to the Gotthard base tunnel. The boundary between those classes was at 60 RMR which separates "fair" from "good" rock in the classification scheme. The RQD was separated into 3 classes, based on the number of occurrences, with either values in the range " excellent" (RQD > 90), " good" (RQD 75 to 90) or " lower" (RQD < 75). Both classification approaches using SVMs showed good training and testing accuracies, though the RQD was not as sensitive to the seismic velocities as had been expected.

Capture and geological storage of CO2 are an option to reduce greenhouse gas emissions. At Ketzin (Germany), the first European onshore pilot scale project was initiated in 2004. For this project, 3D time-lapse surveys are an essential tool for high resolution reservoir characterization. A baseline and a smaller repeat survey were acquired in Autumn 2005 before the injection and in Autumn 2009 after 22 kilotons of CO2 had been injected respectively. Seismic differences between the baseline and the monitor survey were previously interpreted as a fluid saturation effect only. However, for some fields undergoing injection both fluid and pressure changes have approximately the same degree of impact on seismic data. Landro (2001) described a method based on AVO analysis to discriminate the both types of changes directly from 3D time-lapse data for an oil-water contact. In this study we apply this method to a case of CO2 injection into a saline aquifer. We were able to detect the CO2 propagation in the reservoir, but currently a low signal-noise ratio prevents a clear discrimination between changes caused by fluid saturation and by pore pressure.

As a response to global warming, mainly caused by increasing atmospheric concentration of carbon dioxide, and growing energy consumption world-wide, Carbon Capture and Storage (CCS) is regarded as one option to ensure safe energy provision and mitigation of climate change in the near future. Although the technology of geological storage of carbon dioxide has been applied in the framework of oil and gas exploration for over a decade now, it has not yet reached a mature state for, e.g., coal fired power plants or energy intensive industries. One of the first pilot sites for onshore storage of CO2 in a deep saline aquifer has been deployed at the town of Ketzin (Germany). The characteristics of the site are not typical for future industrial demonstration projects which will be operated in much larger dimensions and in deeper reservoirs. However, the storage operations at the Ketzin site are combined with a wide range of geophysical, geochemical and microbial monitoring methods which are tested for their applicability on larger storage sites. Seismic monitoring plays a crucial role in this context as it has proven to deliver the most comprehensive information on the spatial distribution of the injected CO2 in the reservoir. The first 3D seismic repeat survey was acquired after approximately 14 months of injection and delivered high resolution images of the lateral distribution of the injected CO2. A mass estimation of the CO2 imaged by the seismic measurements, using petrophysical and borehole logging results showed that the seismic surveys were able to image approximately 93-95% of the injected CO2. The remaining 5-7% are assumed to be undetected within the reservoir, or dissolved in the reservoir brine and thus undetectable for seismic measurements.

To monitor the migration of the injected CO2 in the Ketzin project (Germany) a permanently buried multi-component seismic array has been installed in August 2009. Since then the array has been continuously recording passive seismic data. Additionally an active seismic survey resulting in a 230 m long 2D seismic line roughly passing over the monitoring well Ktzi 202/2007 has been acquired in November 2009. This relatively small set-up is meant as a pre-screening study to demonstrate the added value of the buried sensors before installing a larger 3D array.A first analysis of the active seismic data shows, that the use of an array of buried receivers results in (time-lapse) seismic data that offer a superior S/N ratio suppressing both ambient noise and surface related coherent noise. This setup is expected to lead to an increased repeatability. This paper describes the experimental set-up and first results of the acquired baseline data using the buried sensors. (C) 2011 Published by Elsevier Ltd.