Imaging sub-basalt structures using locally converted waves

In many parts of the world large-scale basalt flows associated with continental rifting have blanketed pre-existing sedimentary basins. In recent years considerable attention has turned to exploration beneath these extensive flows. A region of particular interest is the Atlantic margin of northwest Europe which experienced early tertiary magmatism associated with the interaction of the Iceland plume and the opening of the Atlantic Ocean (Barton and White, 1997). Imaging sub-basalt structures is notoriously complicated and there are several reasons why few successful case histories exist. The basalt flows are characterised by rough boundary topography and finely-layered internal structure. Therefore, scattering and attenuation are probably significant. Additionally, basalt can be considerably higher in velocity than the surrounding sediments. Hence, the point of critical P-wave reflection is reached at small incidence angle and only a small amount of P-wave energy penetrates the basalt layer. Finally, short-offset data is contaminated with multiples, again due to the high-impedance contrast with surrounding sediments. The Rockall Trough which lies in deep water west of Northern Ireland is the site of extensive intrusive and extrusive dolerites and basalts. Seismic exploration in this area presents both acquisition and processing challenges. We present a methodology for processing long-offset data recently acquired in this region. Our technique builds on the approach of Emsley et al. (1998) who use locally P-to-S converted waves recorded at far offsets to penetrate the basalt. The use of local mode conversions extends the range of useful offsets for imaging beneath interfaces with large velocity contrasts (Tatham et al., 1983; Tatham and Goolsbee, 1984). We improve upon the approach of Emsley et al. (1998) by also employing the τ-p transform to further isolate converted waves from multiples and head-waves and to increase the signal-to-noise ratio of the extracted waves. In addition, the τ-p transform enables us to isolate arrivals with specific incidence angles, thereby facilitating the distinction between converted-wave and pure-mode energy even further (Tatham et al., 1983; Tatham and Goolsbee, 1984).