EXPLORATION IN THE SHETLAND-FAEROE BASIN USING DENSELY SPACED ARRAYS OF OCEAN-BOTTOM SEISMOMETERS

Recent exploration activity in the peripheral regions of the Shetland- Faeroe Basin, offshore northwest Scotland, has led to the discovery of some of the largest oil reserves on the United Kingdom (UK) continent al shelf. We present results from two ocean-bottom seismometer profile s acquired by Mobil North Sea Ltd. across the center of the Shetland-F aeroe Basin. These data provide a powerful tool for delineating long-w avelength velocity variations and thus have potential for reducing the nonuniqueness associated with conventional seismic exploration method s. Analysis of the first-arrival traveltime data using both forward an d inverse ray-based techniques produces a well constrained velocity-de pth model of the basin fill. We estimate that the uncertainty in the v elocity structure is +/-5% from a series of trial and error perturbati ons applied to the final models. The velocity structure of the Faeroe Basin has three principal layers: (1) a near-surface layer with veloci ties in the range 1.6 to 2.2 km/s, (2) a 3.0-3.2 km/s layer which is c haracterized by a northwards structural pinch out in the center of the basin, and (3) a deeper laterally heterogeneous layer with velocities in the range 3.8 to 4.2 km/s. In the northwestern portion of the basi n, a high velocity (5.0 km/s) basaltic layer is imaged dipping toward the southeast at a depth of 2-3 km. The basement is mapped at a depth of 7-9 km in the center of the basin. Gravity modeling provides indepe ndent corroboration of our models through the application of a velocit y-density relationship obtained from a synthesis of physical property measurements. Reflections from the Moho indicate a crustal thickness o f 18 +/- 3 km, suggesting that the basin is underlain by highly attenu ated continental crust, but the velocities in the basement are closer to those of the Faeroe Islands basalts than the expected Lewisian gnei ss, suggesting that it may be highly intruded.