Pressure Recordings at Ekofisk - Experience and Route Forward

The installation of the Optowave PRM system at the ConocoPhillips operated Ekofisk field was completed end of October 2010 (Nakstad et al, 2011). The seismic cable network covers 60 km2 and consists of 200 km cables with 3966 4C stations. All stations are trenched at ~1.5 m except for a few stations that are only rock-dumped. The spacing between the stations along the cable lengths is 50 m with spacing of 300 m between parallel cables. Four seismic acquisitions have been performed by the end of 2012 (LoFS1 – LoFS4). During the first survey (LoFS1), which was completed end of January 2011, some hydrophones were initially observed and reported by ConocoPhillips to provide reduced pressure recordings. This paper discusses in some detail this observation which has been identified to be related to the pressure transfer from the water column to the hydrophone sensing element within the trenched station. This pressure transfer function is affected for many hydrophones installed at Ekofisk due to the combination of the actual mechanical protection (housing) of the hydrophone element and the surrounding sediments.