Interpretation of high-resolution seismic data in a flow basalt area in the NE Atlantic Ocean

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 low‐velocity zones detected in previous studies in the uppermost few tens to about 120 m, indicate rock deterioration in this zone.