High-resolution marine seismic data have been interpreted in an attempt to gain detailed understanding of the shallow section in a basalt-covered area, where a subsea tunnel is planned for construction. Problems encountered are analogous to exploration problems in areas with high acoustic-impedance contrasts. The testing of a previously postulated strike-slip fault has been one of the main issues of this study. It will be demonstrated that at least two coherent intra-basaltic reflectors, beneath the water bottom, appear to represent significant unconformities, and that these, because of their extent, tend to refute the hypothesis of faulting in the study area. We interpret relatively thick sedimentary layers to be associated with these unconformities – or intervolcanic hiatuses – and since sediments usually have higher porosities (and permeabilities) than volcanic rocks, these reflectors might well represent zones of streaming fluid. The dip of the basalt flows is small – about 2 or 3 degrees – and therefore fluid could migrate several hundred meters to kilometers until it reaches an excavated tunnel. Incoherent portions of some of the interpreted seismic reflectors, as well as lowvelocity zones detected in previous studies in the uppermost few tens to about 120 m, indicate rock deterioration in this zone.
Creating seismic images of horizons and other geological features within and beneath basalt layers can be a great challenge for geoscientists in both industry and academe. Massive basalt is a high-impedance rock, which normally implies that a large proportion of the seismic energy is reflected back upon incidence at a boundary between a low-impedance medium (such as water or sediment) and the basalt itself. This study relates to the planning of a subsea road tunnel (Figure 1) in the Faroe Islands to be drilled through flow basalts and deals with aspects that are analogous to exploration problems in areas with high-acousticimpedance layers. High-resolution 2D marine reflectionseismic data have been interpreted and visualized for the purpose of obtaining an optimal understanding of the uppermost 100-150 m of rock below the water bottom. Certain intra-basaltic features beneath the water bottom have been recognized, which has served to answer crucial questions related to the study area. The testing of a previously postulated strike-slip fault, which is inferred to have a vertical displacement of 200-300 m, and a lateral displacement of 4-6 km, is an important issue of this work.
Sheriff and Geldart (1995, pp. 150-152) state that most reflections mark unconformities and/or time boundaries that most commonly correspond to distinctive lithologic changes. However, since the geology in the study area is almost entirely made up of flow basalts with mostly thin strata of intercalated volcaniclastic sediments, as well some organic and reworked sediments (Passey et al., 2007; Passey, 2009), significant lithologic changes are uncommon. However, unconformities have been established onshore (e.g. Rasmussen and Noe-Nygaard, 1970; Passey, 2009) and one of them corresponds to the ‘C horizon’, which is also a key feature in the interpretation of the data in question. Migrated seismic data have been used in the interpretation of all the features.
