integrated Seismic Imaging and Modelling of Margins - the towed streamer surveys.

Zoë Lunnon1,2, Alan W. Roberts1, Philip Christie2, Robert White1, Nick Kusznir3 and Alan Roberts4

1 Cambridge University Earth Sciences, Bullard Labs, Madingley Road, Cambridge CB3 0EZ.

2 Schlumberger Cambridge Research, High Cross, Madingley Road, Cambridge CB3 0EL.

3 Department of Earth Sciences, Liverpool University, 4 Brownlow Street, Liverpool L69 3GP.

4 Badley Technology Ltd., North Beck House, North Beck Lane, Hundleby, Spilsby, Lincs. PE23 5NB.

This abstract outlines one of a two-part poster presentation describing the four year iSIMM project. iSIMM seeks to model the evolution of magmatic margins and their basins using seismic imaging with integrated towed streamer and ocean bottom seismometer (OBS) data. This poster focuses on the towed streamer data.

Intracratonic basin models can successfully predict heat-flow and subsidence from basin stretching. However, these models cannot easily be extended to magmatic margins due to melt underplating and extrusion (i.e. crustal non-conservation), and depth dependent stretching1. The North Atlantic continental margin was formed by continental break-up in the early Tertiary above the Iceland mantle plume. This led to stretching and thinning of the continental lithosphere, accompanied by massive magmatism, producing flood basalts and extensive igneous intrusion2. The seismic components of iSIMM seek to develop a structural model from seabed to Moho to image the stretched crust and the extruded, intruded and under-plated igneous material, and to use these constraints in magmatic margin basin modelling.

Basalt flows present a challenge to seismic imaging. Flows are highly heterogeneous with large impedance contrasts, and while they may have a low intrinsic attenuation, the strong multiples generated often mask the little energy penetrating to the base of the basalt and the sub-stratum. The Faroes Large Aperture Research Experiment3 used two ships to acquire 38km steamer data which penetrated the flood basalts, but did not image the Moho, or provide sufficient data to estimate the magmatic under-plating, both of which are key aims of iSIMM. Recent analyses of the Lopra borehole seismic data (Christie et al., submitted), and of wide-angle seismic data4, have defined, at least locally, certain basalt flow propagation characteristics and have assisted the survey design. A low frequency source design aids basalt penetration. Synthetic seismogram modelling prior to the acquisition was used to select shot interval and record length.

In summer 2002, iSIMM surveyed two continent-ocean transitions (COT), one close to the Faroes, the other west of Rockall-Hatton Bank (RHB). Both acquired OBS and towed streamer data: 12km Q-streamer for the near-Faroes survey and a 3km NERC streamer for RHB. Early data samples are displayed on the poster.

The Q-Marine acquisition aims to image, where possible, to the basement below the basalt. Q-Marine5 provides long offset (12km) data with high density, single sensor sampling and shot-by-shot signature monitoring. Unlike conventional streamer data, Q-data can be grouped after acquisition. A multi-streamer swath also allows some cross-line protection. Ultimately the streamer and OBS data will be integrated for deep velocity analysis and for identification of seismic mode conversions. We will follow the method of Fleidner & White6 in using pre-stack depth migration to optimise the crustal image using both streamer and OBS data.

iSIMM’s long term goal is to develop a rifted margin basin model. OBS and streamer data integration will provide an image from sea-floor to Moho and will demonstrate the applicability of Q-Marine to probe beneath basalt. iSIMM investigators are Liverpool and Cambridge Universities, Badley Technology and Schlumberger, supported by NERC, Agip UK, BP, Amerada Hess, Anadarko, Conoco, Phillips, Shell, Statoil, and WesternGeco.

1)Davis, M. & Kusznir N.J., Depth-dependent lithosphere stretching at rifted continental margins. Earth Plan. Sci. Let., In review.

2)White, R. & McKenzie, D., Magmatism at rift zones: the generation of volcanic continental margins and flood basalts. Journal of Geophysical Research, 94, 7685-7729, 1989.

3)White, R.S., Fruehn, J., Richardson, K.R., Cullen, E., Kirk, W., Smallwood, J.R., & Latkiewicz, C., Faroes Large Aperture Research Experiment (FLARE): imaging through basalt. Petroleum Geology of Northwest Europe: Proceedings of the 5th Conference, 1243-1252, 1999.

4)Richardson, K.R., White, R.S., England, R.W., & Fruehn, J., Crustal structure of the Faroe Islands: mapping sub-basalt sediments using wide angle seismic data. Petroleum Geoscience, 5, 161-172, 1999.

5)Martin, J., Özbek, A., Combee, L., Lunde, N., Bittlestone, S., & Kragh, E., Acquisition of marine point receiver seismic data with a towed streamer. Annual Meeting Abstracts, Society of Exploration Geophysicists, paper ACQ-3.3, 2000.

6)Fliedner, M.M., & White, R.S., Sub-basalt imaging in the Faeroe-Shetland Basin with large-offset data. First Break, 19, 247-252, 2001.