Passive seismic imaging of the Lower Palaeozoic in the Sudret area of Gotland, Sweden

Abstract. Passive seismic data were acquired together with active seismic data along a 2.8 km long profile in the Sudret area of Gotland, Sweden, as part of a feasibility study for storage of CO₂ below the Baltic Sea. Seismic interferometry using cross-correlation and cross-coherence was employed on the passive seismic data. Correlation was used to retrieve virtual shot gathers containing mainly surface waves, while cross-coherence was used to retrieve mainly seismic reflections. Inversion for shear wave velocity and CDP processing of the passive data result in velocity profiles and images that correlate well with borehole data, synthetic seismograms and the active seismic data. Both the passive surface wave and body wave results provide geological information which complement the active data, the surface waves providing S-wave velocity information and the body waves providing a lower frequency image. The passive data are consistent with the active data and there is no indication of any large-scale faults in the area. Furthermore, analysis of the frequency and direction of the ambient noise using power spectral density and beam forming shows that ocean waves and human activity around the island of Gotland makes the Sudret area an ideal location for passive imaging. Our results illustrate that passive seismic imaging can be an important complement to active seismic data for evaluating the subsurface with respect to CO₂ storage and monitoring in the Gotland area, Sweden, and perhaps, elsewhere.