Large-aperture seismic: Imaging beneath high-velocity strata

Seismic imaging beneath high-velocity basalt requires new strategies. A two-ship experiment from the Faroe-Shetland region - using ultra-wide offsets and other features specifically designed for sub-basalt imaging - proved highly successful

Juergen Fruehn, Robert S. White, Moritz Fliedner and K. Ruth Richardson, (University of Cambridge); Ed Cullen, Christopher Latkiewicz, Wayne Kirk and John R. Smallwood, Amerada Hess Ltd.

Acquiring marine seismic data with apertures of more than 10 km enables the use of high-amplitude, wide-angle, reflections and refractions to image interfaces that may be faint or impossible to see on near-vertical data. This article discusses the acquisition of two-ship seismic data, along a profile near the Faroe Islands in the North Atlantic, Fig. 1 [high resolution version, 277K], with continuous offsets up to 38 km. It also shows the advances in imaging through high-velocity layers using this technique.

After a brief geologic history of the survey region, the Faroes Large Aperture Research Experiment (FLARE) is described. It shows how data acquired in this large-aperture manner can be used in imaging beneath basalt flows that lie above sediments.

Application of ray tracing and traveltime inversion on "supergathers" (shot gathers with the full offset range of 38 km) that were synthesized from the raw data shows the presence of a low-velocity layer beneath eastward-thinning lava flows. The low velocities of 3.5 to 4.5 km/s indicate a 2.5- to 3-km-thick sedimentary section. Seismometers placed on the Faroese island of Suduroy show that this sub-basalt sediment section pinches out beneath the island.