Capstone: OBN deep-water imaging in salt

Ocean-bottom nodes (OBN) are the premium acquisition choice for sub-salt exploration in the Gulf of Mexico, offshore Brazil, and similar complex environments. Nodes sit on the sea floor; they record the full 4-component wavefield and give clean low-frequency content (down to ~1.5 Hz) that streamer surveys cannot match. That low-f content is FWI's oxygen.

Project setup

Gulf of Mexico deep-water Miocene target under a 2–3 km thick salt canopy. 2 km water depth; node layout 400 m node spacing, 1500 shot positions per node. Wide-azimuth data because the node layout and the shot grid together provide full-circle illumination. Target depth: 6–8 km below sea level.

The pipeline

Why this project needs the full physics-based toolkit

The salt canopy has Vp ≈ 4500 m/s against sedimentary Vp ≈ 2500 m/s; a ~2× velocity contrast at the salt boundary bends rays severely. Multiple arrivals, turning waves, and prism reflections at the salt flanks all contribute. Kirchhoff PSDM misses all of them — its single-ray travel-time table cannot represent multi-path physics. One-way WE misses the turning waves. Only full two-way RTM correctly images the sub-salt reflectors.

Salt is also strongly anisotropic (5–15 % ε and δ in typical Gulf salts). An isotropic RTM mislocates sub-salt reflectors by 200–500 m laterally. Anisotropic (TTI) RTM is mandatory; add TTI FWI to jointly update the velocity and anisotropy parameters.

Low-frequency FWI is the control variable

FWI at 1.5–3 Hz is what makes this project feasible. Streamer data at 5 Hz+ would be fine for imaging away from salt but cycle-skips through salt. The ~6 dB of extra low-f bandwidth OBN delivers translates into an FWI that actually converges on the sub-salt model.

Cost profile

• Acquisition: $50–150M per 4000 km² (node cost dominates).
• FWI processing: 3–6 months on a 200-GPU cluster.
• RTM: 2–4 months on the same cluster (50 TFLOP-hours per shot).
• Human interpretation + QI: 6–12 months after imaging.

Total project cycle 18–30 months. The reward is a new play in a mature basin — a single successful sub-salt discovery can justify a decade of sub-salt exploration investment.

Where this goes next

§10.3 covers a different deep-water scenario: a wide-azimuth marine streamer where the primary deliverable is a high-resolution velocity model for exploration mapping, not a specific sub-salt target.

References

• Yilmaz, Ö. (2001). Seismic Data Analysis (2 vols.). SEG.
• Etgen, J., Gray, S. H., Zhang, Y. (2009). An overview of depth imaging in exploration geophysics. Geophysics, 74, WCA5.
• Baysal, E., Kosloff, D. D., Sherwood, J. W. C. (1983). Reverse time migration. Geophysics, 48, 1514.
• Virieux, J., Operto, S. (2009). An overview of full-waveform inversion in exploration geophysics. Geophysics, 74, WCC1.