Progress towards a Passive Shallow-subsurface Continuous-monitoring System at Valhall Using the LoFS Array

The Valhall Life of Field Seismic (LoFS) OBC seismic array was installed in 2003 for the purpose of regular long-term 4D monitoring of the Valhall oil field. With academic collaborators, BP has investigated whether it could be used to synthesize virtual sources using passive seismic interferometry, allowing continuous monitoring of the field. The ambient noise background consists of Scholte waves, which propagate with a limited depth of penetration below the subsurface. Furthermore, although the noise is broadband, it only has the necessary statistical properties required for interferometry over a frequency range of about 0.18 to 1.75 Hz. At such low frequencies, however, Scholte waves do penetrate the top few hundred meters. Preliminary results give hope that it may be possible to use the Valhall OBC array to monitor the shallow subsurface in near real time.