Advances in seismic imaging through basalts: a case study from the Faroe–Shetland Basin

New seismic reflection data have been used to image intra- and sub-basalt features beneath the Faroe–Shetland Basin in the North Atlantic, where the highly reflective top and base boundaries of flood basalts and their complex internal structure make successful seismic imaging difficult. This study demonstrates that appropriate acquisition and processing of marine seismic data from hydrophone streamers and ocean-bottom seismometers (OBS) has the potential to enhance significantly imaging of intra-basalt and sub-basalt seismic reflections. The intersection of a new seismic reflection profile recorded in 2002 with a seismic profile recorded in 1998 allows a direct comparison of advances in sub-basalt imaging over this period and an interpretation of geological structures and seismic velocities at the intersection.

To achieve better resolution of sediments below basaltic layers, surface seismic reflection data using a broad-band, low-frequency source have been recorded. By using a source wavelet with a significantly enhanced frequency spectrum centred at 10 Hz, generated from a large (167 l) airgun array tuned to the first bubble pulse, a wavelet is produced that is capable of penetrating through thick basaltic sequences. Furthermore, recording the low-frequency reflections to large offsets along a 12 000 m long streamer with 3.125 m single sensor spacing allowed effective suppression of multiple reflections, enhanced the precision of velocity analyses and produced better migration results. Seismic data along the same profile were also recorded on a dense deployment of 85 four-component OBS at 2 km or 6 km station spacing. These complementary ocean-bottom data allowed the large-scale velocity variations within and beneath the basalt to be constrained by recording at large offsets the termination of the diving rays (c. 18 km) propagating through the basalt.

Integration of seismic reflection data and OBS recordings yields good seismic velocity estimates and, therefore, increased confidence in the interpretation of intra- and sub-basalt features and lithology. Four major stratigraphic units characterized by their seismic velocities and reflection characters were identified: (a) post-basalt sediments; (b) basalt sequence; (c) sub-basalt unit; and (d) the top of presumed Cretaceous basement. The top boundary of the basalt sequence is defined by a strong reflection with a steep velocity increase from 3.5 km s−1 to 4.5 km s−1 in the c. 200 m thick layer above it. The basalt sequence here is c. 2.5 km thick. Beneath the base of the basalt there is a velocity decrease from 5.8 km s−1 to 5.2 km s−1. The sub-basalt unit is subdivided further into hyaloclastites mixed with sediments and sills, and sub-basalt sediments separated by another velocity inversion from 5.2 km s−1 to 4.5 km s−1. The Cretaceous basement with velocities above 5.5 km s−1 lies underneath these sub-basalt sediments.