Exploring below the basalt, offshore Faroes: a case history of sub-basalt imaging

Discoveries in the Faroe–Shetland Basin to date have been made in areas not covered by extensive basalt flows as these flows seismically obscure the geology and structure of the underlying section. The industry has been tackling this problem for many years and it is known that high frequencies penetrate only a short way into the basalt before being scattered, generating high frequency noise. Recent seismic data in Faroes Licence 006 show a dramatic improvement in sub-basalt imaging in this problem area. The improvement comes primarily from processing the data, removing high frequencies (dominantly noise) early in the processing to concentrate on the low frequency data and using careful multiple removal at all stages of the processing. Velocity analysis must be performed as an iterative process and take into account the geological model. Deep towing the source and cable also improves the data by avoiding the higher frequencies that generally cause noise and thereby enhancing the lower frequency signal. Applying these processing techniques to earlier 2D surveys also shows significant improvement in imaging the sub-basalt structure. Data examples show that the sub-basalt structure can now be mapped on the improved data and sub-basalt prospects defined.