Using shear‐wave resonances induced by earthquakes for cost‐effective CO 2 ‐monitoring

We show how CO 2 ‐storage sites could be monitored using fibre‐optic cables as sensors and local earthquakes as natural sources. The method uses shear‐wave resonances, which are observable in sedimentary layers with strong shear‐wave velocity contrasts often occurring for shallow sand layers embedded in shale. The method has been demonstrated using distributed acoustic sensing data acquired offshore the Svalbard archipelago to determine average shear‐wave velocities within sediment layers beneath the seabed. Application to CO 2 ‐storage sites is demonstrated using simple models based on the Sleipner CO 2 ‐storage site and a shallow offshore North Sea gas field to simulate fluid saturation changes. The modelling demonstrates that there is a potential to use this technology to monitor changes that are likely to occur in a CO 2 ‐storage project. We think there is a good potential for the detection of pore pressure changes, both within the CO 2 ‐storage volume and overburden layers. The method is well suited to the situation where existing seabed fibre‐optic cables are used to record the data, and natural earthquakes are used as sources to generate the shear‐wave resonances. Apart from the cost of the deployment of fibre‐optic cables, this is a very cost‐effective monitoring tool. Furthermore, in areas close to offshore wind turbines (where fibre‐optic cables often are deployed), the method could offer an excellent and very cost‐effective monitoring method.