Seismic noise-based time-lapse monitoring of the Valhall overburden

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 imaging of the subsurface. First, the cross correlations between each pair of stations were computed for both vintages to extract Scholte waves. Second, the relative velocity variations between the 2004 and 2005 cross correlations were measured on the ballistic waves using the Moving-Window Cross-Spectral technique. Finally, the best quality relative velocity variation measurements were regionalized using a modified eikonal tomography technique. The results, albeit noisy because of the short duration of the available records, show a large patch of increased seismic velocity in the southern part of the network and a weaker anomaly in the northern part. The southern 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.