Depth imaging of basalt flows in the Faeroe-Shetland Basin

We use a grid of long-offset seismic profiles to map the thickness of subsurface basalt flows that extend southeastward across the Faeroe Shelf. Offsets of up to 38 000 m were achieved using two seismic acquisition vessels towing multichannel streamers and shooting alternately. Wide-angle reflections and refractions from the basalt and underlying strata allow us to build a velocity model down to the basement Lewisian crust. The velocity models give a first-order picture of the geology of low-velocity sediments of presumed early Palaeocene and Mesozoic age beneath the basalt flows. They also provide a control for pre-stack depth migration of the entire seismic data set, thus allowing us to produce good depth images of the structure. Pre-stack depth migration of selected wide-angle arrivals, such as those from the base of the basalt and the underlying basement, produce strong reflection images that allow these interfaces to be identified unambiguously on the higher-resolution images produced from the entire data set: in particular, we can distinguish between the primary and multiple reflections. We thus use the very long-offset arrivals to 'tag' the velocities of the geological interfaces from which they are returned, and hence to improve both the imaging and the interpretation of the basalt flows and the underlying structure.