Diagnostic monitoring to identify preferential near-surface structures for CO2 degassing into the atmosphere: Tools for investigations at different spatial scales validated at a natural analogue site

• Monitoring approach using IR spectroscopy, geophysics and soil gas surveys.
• Characterization of preferential CO2 pathways at different spatial scales.
• Successful application of monitoring concept at natural analogues.
• Distinction between natural background variations and increased levels of soil gas.

Diagnostic monitoring tools applied at different scales are required to reliably detect and assess CO2 leakages from storage formations in the shallow subsurface, as well as playing an important role in helping to establish a risk assessment strategy at carbon dioxide capture and storage facilities. These tools incorporate method developments and applications that will enable large spatial areas to be consistently covered in sufficient spatial and temporal resolutions. The use of remote sensing (open-path Fourier-transform infrared (OP-FTIR) spectroscopy) in combination with regional measurements (geophysics and chamber based soil CO2 flux measurement) and local in situ measurements (Direct Push technology) enables a more reliable validation of risk, using a joint data interpretation approach. A promising tool currently in development for large-scale leakage detection and monitoring is Fourier transform infrared (FTIR) spectroscopy, which is used to determine spatial atmospheric CO2 distribution in the near-surface atmosphere. Sufficient geophysical techniques for meso-scale monitoring include geoelectrical and self potential (SP) surveys. These methods are useful for characterizing fluid flow and transport processes in permeable near-surface sedimentary layers and can yield important information concerning CO2 affected subsurface structures. The paper presents promising results achieved from measurements taken at a natural analogue site in the Czech Republic and indicates that the hierarchical monitoring approach represents a successful multidisciplinary modular concept to monitor physical and chemical processes taking place during CO2 migration and seepage. The application of FTIR spectroscopy in combination with soil gas surveys and geoelectrical investigations results in a comprehensive site characterization, including the atmospheric and near-surface CO2 distribution as well as subsurface structural features. Our data illustrate a correlation of higher CO2 concentration in the atmosphere with increased CO2 soil flux rates and soil CO2 concentrations. These soil gas anomalies coincide with structural units characterized by a distinct negative SP anomaly and a zone of decreased resistivity in the ERT result.