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

Abstract

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.

Mafic intrusions, hydrothermal venting, and the basalt-sediment transition: Linking onshore and offshore examples from the North Atlantic igneous province

“…Evidence for prolonged intereruption hiatuses (e.g., Jolley et al, 2012) within the main tabular sequence is also recognized from the presence of deep erosional channels cut down (approximately 20 m) into the underlying flows, which have subsequently been filled by ponding of later lavas and require enough time to develop erosional drainage systems on the lava surface (e.g., Schofield and Jolley, 2013;Ebinghaus et al, 2014). The lava pile at Neist Point therefore presents a complex and typically heterogeneous lava sequence (Figure 3b), which would cause significant scattering and attenuation of seismic energy, and therefore create imaging challenges in the subsurface (e.g., Gallagher and Dromgoole, 2007;Davison et al, 2010).…”

“…In the subsurface, large intrusions may often be easily imaged as the highest amplitude reflectors in a sedimentary package (e.g., Planke et al, 2005); however, imaging sills may become more challenging at greater depths, with poor-quality data and where the sill thickness reduces (e.g., Smallwood and Maresh, 2002;Schofield et al, 2015). In the case of extrusive volcanic sequences, their highly heterogeneous velocity structure can cause extensive scattering of seismic waves, and it is therefore notoriously difficult to image volcanic rocks along with the stratigraphy and structures present beneath the basalt cover (Gallagher and Dromgoole, 2007;Davison et al, 2010). In this context, the presence, significance, and implications of intrusions and associated HVCs (e.g., Svensen et al, 2004) within the basalt-covered basinal regions of volcanic rifted margins are to date poorly constrained.…”

A fresh approach to ditch cutting analysis as an aid to exploration in areas affected by large igneous province (LIP) volcanism

“…One of the major issues that have inhibited exploration in areas affected by LIP sequences is the difficulty in acquiring reliable seismic data from beneath basalt cover (Roberts et al 2005;Gallagher & Dromgoole 2007). In recent years detailed investigations into the geophysical signatures of different flood basalt facies from down-well logging tools have been undertaken (Planke 1994;Bartetzko et al 2001;Helm-Clark et al 2004;Boldreel 2006;Nelson et al 2009).…”

“…Exploration in areas affected by large igneous provinces (LIPs) such as much of the offshore West of Shetland North Atlantic region has many associated difficulties. In the West of Shetland Basin (WSB) region these may include deep water, difficulties in seismic imaging, regular bad weather, difficult formation properties and a relative lack of regional down-well data with which to predict the extent and volume of volcanic rocks (Archer et al 2005;Close et al 2005;Gallagher & Dromgoole 2007). With additional difficulties come additional costs and, as such, it is a matter of no small significance to gain the absolute maximum in information from completed wells.…”

Improved signal processing for sub-basalt imaging

“…While accepting that carefully parameterized acquisition can be used to provide a greater richness in low-frequency signal, Gallagher & Dromgoole (2007) and Hardwick et al (2010) conclude the sub-basalt image is primarily dependent on the careful retention and enhancement of low-frequency signal at all stages of the processing.…”