Passive recordings from an array of 2500 four component receiver stations at the Valhall reservoir in the Norwegian North Sea were made available from February 2004 and January 2005 by BP. Analysis of some of the raw hydrophone records shows that the bulk of the records do not yield obvious, crisp events. Some noise trains that look like approximately 7 second envelopes of a shot record emanating from the location of the production/drilling facilities are observed occasionally. Crisp water-velocity hyperbolic events are sometimes seen which are also centered at the location of the platforms within the array.
Simple correlation does not yield interpretable synthesized shot gathers from the raw data. Dividing the correlations by the power spectrum of the traces correlated reveals several interesting features. There are a few complete hyperbolas in the data that may correspond to the desired events analogous to active seismic data. Dominating the gathers however, is a single event from a source probably 40 km to the West of the array traveling at roughly 1450 m/s. The same event is present in both the 2004 and 2005 data. The event locus is exactly over the Ardmore field in British waters, operated by Tuscan Energy (Scotland) Limited.