In recent years, global concerns about greenhouse gas emissions have stimulated considerable interest in carbon capture and storage (CCS) as a climate change mitigation option that can be used to reduce man-made CO2 emissions. This is achieved by separating and capturing CO2 from emission sources, then injecting and storing it in the subsurface. However, CCS requires the secure retention of CO2 in geological formations over thousands of years. Several geochemical and geophysical (such as time-lapse seismic) techniques allow the monitoring of the regional distribution of CO2 in the gas storage, seal integrity and the pressure evolution in response to the injection and, therefore, can be used to verify storage conformance and are valuable tools for integrity monitoring.
Example:
As part of the EU-funded DigiMon project, a CO₂ injection at the SINTEF Field Lab in Svelvik, Norway, was monitored in September 2021 using crosshole P-wave seismic tomography. A baseline survey imaged the subsurface, revealing alternating sand and silt layers and a dense clay layer at ~36 m depth. CO₂ was injected at ~65 m over six days, with daily tomography surveys tracking plume migration. The time-lapse seismic data clearly showed CO₂ accumulation beneath the clay layer, areas of movement, and changes in concentration. This dynamic monitoring provides real-time insights into CO₂ migration, helping to visualize preferential flow paths and optimize CCS projects.
Difference tomograms show development of CO2 distribution over 6 days
Limestone and other rocks can develop cavities and weaknesses over time due to weathering and karst processes. These instabilities can affect structures such as bridge piers and other infrastructure. By using seismic tomography to monitor cement injections, we can observe how the subsurface responds and ensure that stability is restored and maintained over the long term.
Example:
During the geotechnical investigation of a railway bridge, issues arose while drilling at the bridge abutment near the road. At depths of approximately 11 m and 15 m, larger cavity structures were encountered. In the immediate vicinity of the borehole, settlement and the transport of loose material into deeper cavities occurred at the slope toe, resulting in a depression directly at the abutment. The ground was subsequently backfilled. The effectiveness of the remediation was then verified using a before-and-after tomography survey.
Tomogram before cement injection (left), tomogram after cement injection (middle), differences between left (before injection) and right (after injection)