Exploring the Shetland–Faeroes Basin using wide-angle seismic technology

Abstract

Densely sampled wide-angle seismic profiles using ocean-bottom seismometers have been used to obtain detailed velocity models of the Shetland-Faeroes Basin in both two and three-dimensions. We demonstrate that the wide-angle (refraction) technique, which employs dense spatial sampling to long offsets, may be successful in areas such as the Atlantic margin, where deep water and/or volcanic horizons mitigate against successful profiling using conventional methods. We employ a series of ray-trace and tomographic inversion procedures on travel time data from profiles across the basin axis to give detailed in situ velocity models and accompanying resolution and uncertainty estimates. Comparison of the in situ velocity of the Mid-Faeroe Ridge with deterministic lithology-velocity data from nearby boreholes strongly implies that CretaceousTertiary shale and sandstone sequences extend to depths of 4.5 km. The presence of pre-Cretaceous sedimentary lithologies at 4.5-8.0 km depth is supported by in situ velocities of 4.0-4.5 km s" 1 which are best explained by well-consolidated sediments and are incompatible with likely constituents of the basement. The southeastern flank of the Mid-Faeroe Ridges is marked by a prominent fault marking a lateral velocity variation of about 0.5 km s" 1 . A rapid increase in velocity at 8 km depth suggests transition into the basement, whose velocity is similar to high-pressure measurements of basalts from the Faeroe Islands. Data from an area] grid of shots and receivers centred over the Mid-Faeroe Ridge have been inverted to give a fully three-dimensional velocity model of a small volume to depths of 10+ km. Variations in the depth of isovelocity surfaces from this model correspond closely with the geometry of the Ridge, and the position of the flanking normal fault and its adjacent basin to the southeast.

Reconciling gravity and seismic data in the Faeroe–Shetland Basin, West of Shetland

“…The faulted northwest margin of the basin, concealed by Tertiary volcanics from conventional seismic interpretation, NW P P' SE should be amenable to the gravity modelling method especially if supported by refraction or wide-angle reflection seismic data (Barton et al 1999). In particular, we believe that important exploration parameters such as the total thickness, age a n d depth distribution of the sedimentary fill, can be better constrained by detailed gravity modelling.…”