SmartSolo Nodes Enable Passive Seismic Imaging for CO₂ Storage Feasibility Study in Sweden
Introduction to the paper
A study conducted in the Sudret area of Gotland, Sweden, explored the use of passive seismic imaging as a complement to active seismic surveys for a carbon capture and storage (CCS) feasibility study. The research, a collaboration between the Chinese Academy of Geological Sciences, Uppsala University, the Geological Survey of Sweden, and Lund University, aimed to characterise the subsurface geology for potential CO2 storage. While active seismic surveys are commonly used for this purpose, they are often expensive and do not allow for continuous monitoring. This project demonstrated that passive seismic methods, which utilise ambient noise as a seismic source, offer a cost-effective and environmentally friendly alternative or supplement to traditional methods.
Challenge
The primary challenge was to accurately image and characterise the Lower Palaeozoic sedimentary successions in the Sudret area of Gotland to assess their suitability for geological CO2 storage. This included identifying potential porous sandstone reservoirs and the overlying sealing strata, as well as confirming the absence of large-scale faults. The investigation was part of a broader program by the Geological Survey of Sweden to find suitable offshore.
CO2 storage sites, with the onshore Gotland site serving as a crucial analogue due to its similar geological structure. Active seismic surveys had already been conducted, but the study aimed to determine if passive seismic imaging, a more affordable and less invasive method, could provide comparable or complementary high-quality data to confirm the findings and offer a continuous monitoring option.
Solution
To address the challenge, the researchers deployed a total of 329 SmartSolo IGU-16HR 1C nodal units with 5Hz geophones. A 2.8 km long profile was laid out with a receiver spacing of 10 meters, enabling both active and passive seismic data acquisition. For the passive seismic portion of the study, the SmartSolo nodes were used to record 14 hours of continuous ambient noise data. The high number of nodes and their close spacing enabled dense data collection along the profile, which was crucial for the subsequent seismic interferometry processing. Using the recorded ambient noise, seismic interferometry with both cross-correlation and cross-coherence was applied to retrieve virtual shot gathers. This process effectively transformed the ambient noise recordings into usable seismic data, simulating an active seismic survey without the need for a controlled energy source.
Results
The passive seismic data, collected by the SmartSolo nodal units, yielded high-quality subsurface images that were consistent with and complemented the active seismic data and borehole logging results.
- Shear Wave Velocity Profile: The passive surface wave data, retrieved using cross-correlation, allowed for the creation of a detailed shear wave velocity model of the subsurface. This model correlated well with the acoustic logging data from the Nore-1 borehole, particularly in identifying a high-velocity layer between 480 m and 580 m depth.
- Seismic Reflections: Passive body wave data, retrieved via cross-coherence, produced a seismic reflection image that was validated against a synthetic seismogram and the active seismic data. Three key reflections at 170 ms, 280 ms, and 360 ms were clearly identified and corresponded to known geological boundaries.
- Deeper Insights: The passive data also provided reliable deep geological information that the active seismic survey could not capture due to its limited source energy. This demonstrated the passive method’s ability to offer a lower-frequency image that complements the higher-frequency active data.
- Geological Confirmation: The results from both the passive and active surveys confirmed the presence of the three target Cambrian sandstone reservoirs and the overlying sealing strata, and importantly, showed no indication of any large-scale faults in the study area.
Significance
This case study highlights the remarkable effectiveness of SmartSolo nodes in passive seismic imaging for geological characterisation. By leveraging the low-power consumption and autonomous nature of the IGU-16HR 1C 5Hz nodes, the researchers were able to conduct an extended, 14-hour continuous recording session with minimal operational overhead. The project demonstrates that passive seismic imaging, particularly when facilitated by nodal technology, can be a vital and cost-effective tool for subsurface evaluation and continuous monitoring in various applications, including CCS. The ability to utilise ambient noise as a source not only reduces environmental impact but also provides a more economical solution for large-scale or long-term monitoring projects. This project highlights the importance of utilising advanced seismic technology, such as SmartSolo nodes, to gain comprehensive and multifaceted insights into the subsurface, which can be crucial for the safe and successful implementation of projects like geological CO2 storage.
Figure 1: A) The experimental site marked by a red star is located in southern Gotland, Sweden. B) Inverted shear wave velocity from passive surface waves and the Nore-1 borehole section. C) Top: Passive seismic reflection section overlaid with its power spectrum. R1 and R2 represent reflections from the Silurian, R3 from the top of the Ordovician, R4 from the base of the Cambrian, and R5 represents a possible reflection from within the Precambrian or a multiple from the top of the Ordovician. Bottom: Active seismic reflection stacked section overlaid with its power spectrum; the dashed box indicates reflected energy that can potentially be correlated with the R5 reflection in the passive stack
Reference: Wang, Z., Juhlin, C., Hedin, P., Erlström, M., and Sopher, D.: Passive seismic imaging of the Lower Palaeozoic in the Sudret area of Gotland, Sweden, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-1325, 2025.