Capstone — Brazil pre-salt OBN

Brazilian pre-salt carbonates sit 5–7 km below sea level, trapped beneath a 1–3 km-thick salt canopy in 1500–2500 m of water. Streamer surveys generically fail to image the pre-salt flanks because salt-induced energy-bending deflects reflections away from the surface receiver patch. OBN on the seabed — directly below the streamer’s salt-shadow zone — recovers the flanks.

Why OBN works for flank illumination

A reflection from a steep pre-salt flank travels at shallow angles back through the sediment. From the surface, that reflection has to refract around the salt margin to reach a towed streamer — and only a narrow fraction of ray directions actually make it. A seabed node, sitting underneath the salt-margin-bending zone, intercepts those shallow-angle reflections directly. Illumination improvement for pre-salt flanks: 3–5× over streamer-only.

Programme scale

Typical: 10–30 k nodes on 400–600 m grid, deployed by one or two drop vessels, recorded simultaneously over 3–6 months. Separate source vessel sails overhead firing to an OBN patch for weeks at a time. Total programme $200–$500M. Largest to date: Petrobras Buzios campaign (2018–2023, 30 000+ drops).

Real deployments

Petrobras Lula (first pre-salt OBN, 2013), Sapinhoá, Buzios mega-field, Sépia, Mero. The pattern: after streamer-only imaging leaves unresolvable flank uncertainty for a prospect, a follow-up OBN programme is commissioned for 3–6 months, and the resulting images inform $150–$300M development wells.

References

• Berg, E., Svenning, B., Martin, J. (2010). OBN technology — recent developments. EAGE Workshop on Permanent Reservoir Monitoring.
• Vermeer, G. J. O. (2002). 3-D Seismic Survey Design. SEG Geophysical References 12.
• Cordsen, A., Galbraith, M., Peirce, J. (2000). Planning Land 3-D Seismic Surveys. SEG Geophysical Developments 9.
• Tenghamn, R., Brown, J. (2000). A new dual-sensor towed-streamer technology. SEG Annual Meeting Expanded Abstracts, 1–4.