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Faeroe sub-basalt seismic imaging: a new iterative time processing approach
- Source: First Break, Volume 20, Issue 10, Oct 2002,
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- 01 Oct 2002
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Abstract
The main problem for seismic imaging in the Faeroe Basin is the presence of a widespread basalt body of variable thickness (up to several thousand metres) overlying the possible sedimentary basin that is the exploration target. Conventional seismic reflection methods (based on P-wave reflections) have difficulty in obtaining reliable images of the sub-basalt units due to the very high absorption, scattering and reflection capacity of this volcanic formation for P-wave energy. To overcome this problem, different approaches have been suggested. One of them is the use of converted waves to image beneath the basalt (Joppen & White 1990; Li & MacBeth 1996; Masotti et al. 1996). Converted waves generated at the interface between formations with strong velocity contrast may contain more energy than the standard P transmitted waves, especially at long offsets. As a basic requirement, data sets suitable for such an approach need ultra-long offsets, typically in the range of 10–20 km. Nevertheless, recently, Silva & Corcoran (2002) demonstrated the use of an iterative pre-stack depth migration approach employing conventional seismic data (< 4 km offset) for imaging beneath the basalt. The data set used in our analysis (courtesy of Veritas DGC) has been acquired with a single vessel towing a 12 km offset cable. Here, the major problems which are related to dual-vessel acquisition (gaps, noise, different feathering, etc.) are eliminated. The total length of the line was approximately 93 km (Fig. 1). Figure 2 shows the time migrated PP section processed in a standard manner using the first 6 km of offsets. A tentative interpretation has been made, but as shown in Fig. 2, no coherent energy is visible beneath the top of the basalt. The sub-basalt sequence has been interpreted using knowledge of the regional settings. The scope of this paper is to present the ENI-Agip approach to processing such challenging long offset seismic data sets.