Seeing below the basalt – offshore Faroes

Abstract

Exploration in the basalt covered areas of the Faroes offshore has always suffered from poor seismic imaging below the basalt. Long offset 2D and 3D seismic data were acquired and a significant improvement in the seismic image below top basalt has been achieved. Deep towing of the source and receiver cables helped by extending the seismic bandwidth towards lower frequencies. Bubble‐tuned rather than conventional peak‐tuned source arrays gave little, if any, incremental benefit. The improvement in the imaging comes primarily from the approach to processing the data. High frequencies (dominantly noise) are filtered out of the data early in the processing to concentrate on the low frequency data. Careful multiple removal is important with several passes of demultiple being applied to the data using both Surface‐Related Multiple Elimination (SRME) and Radon techniques. Velocity analysis is performed as an iterative process taking into account the geological model. Reprocessing legacy 2D surveys, acquired with wide‐ranging parameters, using these processing techniques improved these datasets significantly, indicating that sub‐basalt imaging seems to be more sensitive to processing than to the choice of acquisition parameters.

Coal seismic surveying over near-surface basalts: Experience from Central Queensland, Australia

“…These findings are all consistent with the relatively successful imaging experiences within thin basalt covered areas such as in the Foinaven and Schiehallion fields of the UK (Gallagher and Dromgoole, 2008). It is therefore to be expected that the thicker basalt at the second site is more difficult to manage.…”

“…The use of this technique for subbasalt imaging has been demonstrated by authors including Fruehn et al (2001), Silva and Corcoran (2002), Droujinine et al (2008), Gallagher and Dromgoole (2008), Chand et al (2009), Rao andSuryanarayana (2010), and Flecha et al (2011).…”

“…This is a simple extension to conventional seismic surveying and can be implemented in the field with only a nominal increase in cost. This approach has been suggested by many authors (Ryu, 1997;Emsley et al, 1998;Fruehn et al, 1998;Wombell et al, 1999;Fliedner and White, 2001;Fruehn et al, 2001;Hu et al, 2003;Lau et al, 2007;Spitzer et al, 2008;Gallagher and Dromgoole, 2008;Masoomzadeh et al, 2010). Long-offset recordings are less affected by shot noise and forward modeling indicates large amplitude reflection events are generated at long offsets (Wombell et al, 1999;Dobson et al, 2003).…”

High-resolution coherency functionals for velocity analysis: An application for subbasalt seismic exploration

“…Our aim is to improve the identification of the stacking velocity of the weak subbasalt signals at an early stage of the processing. In fact, Gallagher and Dromgoole (2008) underline the importance and the difficulty of knowing the velocity model: They showed that even with good multiple suppression techniques, the velocity analysis can be difficult to interpret. Indeed, multiple removal is a thorny issue because demultiple operations, which are often performed repeatedly, can also lead to the loss of the weak subbasalt primaries.…”

“…Zhou et al, 2012). Gallagher and Dromgoole (2008) perform a comprehensive study of various kinds of marine seismic data, acquired with different sources, streamer lengths, and source and streamer depths. They conclude that fine-tuning of the acquisition is indeed beneficial, but a processing sequence, focused on the low frequencies of the spectrum and on accurate multiple removal and velocity analysis, is crucial.…”