Passive time-lapse of valhall subsurface from correlation of ambient seismic noise

We used two "vintages" of ambient seismic noise recorded at the Valhall Life of the Field Seismic network in 2004 and 2005 to perform a passive time-lapse of the subsurface. First, the cross-correlations between each pair of stations separated by less than 1 km are computed for each year. Second, the relative velocity variations between the 2004 and 2005 cross-correlations are computed using the Moving-Window Cross-Spectral technique. Finally, the relative velocity variation measurements are regionalized using a tomographic technique. The results show a strong decrease of velocity on the eastern side of the Valhall network as well as large patch of increased seismic velocity on the southern part of the network. This increase of velocity can be attributed to the exploitation of the southern flank of the Valhall reservoir with new wells and is in good agreement with a previous time-lapse study using Scholte waves from an active experiment.