Multi-Azimuth Sub-Basalt Imaging in the NE Atlantic Margin

Juergen Fruehn, Victoria Valler, and David King
ION - GXT Imaging Solutions, Egham, United Kingdom

Early Tertiary magmatism led to the extrusion of large volumes of flood basalts, and the intrusion of widespread sill and dyke swarms in the North Atlantic Margin. In the study area, sills and dykes of two different seismic characters are observed. Shallow intrusions have a very rugose, extrusive pillow lava-like top, whilst deeper sills are thinner and are characterized by a very smooth top and high reflectivity. The challenge for sub-basalt imaging consists in defining the rugose top of the shallower basalt and finding the optimal basalt velocity. This is achieved by using a standard PreSDM algorithm and state-of-the-art multiple suppression techniques, such as 3D SRME, and advanced model-building schemes, such as gridded tomography in combination with careful horizon picking and basalt velocity estimation scans.

The primary interest for hydrocarbon exploration lies in the sub-basalt structures. Having resolved the overburden (sedimentary column and basalt geometry) as described above, sub-basalt arrivals have been identified in much greater resolution and reflection strength than previously seen in PreSTM images over this area. These events were strong enough to drive further tomographic inversions that were confined to the area below the basalt. Further efforts applying WEM and RTM migrations enhanced the sub-basalt structures.

In this study, sub-basalt imaging has also benefited from two surveys shot at oblique angles to the main survey. The migration results were stacked into a combined image after RMO correction on each individual survey. This image shows additional improvement in sub-basalt reflection continuity. Analysis of the migration results from the overlap area also showed the benefit of certain azimuths over others in illuminating sub-basalt areas.

Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery