2 Seismography at Sleipner
Seismic surveys of the Utsira saline aquifer in the North Sea before (left) and at intervals during injection of carbon dioxide reveal how CO2 fluid – which is less dense than water – moves upwards and sideways over time. A million tones of CO2 per year are injected through a long horizontal well to the injection point (IP). It is hoped that the natural seals above and below the IP will stop the CO2 escaping from the aquifer. The vertical scale shows the time for an acoustic wave to travel from the surface to the reflector and then back to the surface, which corresponds to depth. The different colours show acoustic impedance (AI), which is the product of the density of the medium and the velocity of the sound wave. As carbon dioxide is added, the overall density is reduced and thus the AI contrast is enhanced, showing the location of the CO2. Image: Andy Chadwick, British Geological Society