Capstone: Marine WAZ FWI project

Wide-azimuth (WAZ) marine acquisition uses multiple source vessels firing at shifted azimuths relative to a single streamer vessel, giving each image pixel illumination from a range of directions rather than the single shot-receiver line a conventional narrow-azimuth survey provides. WAZ was developed for sub-salt imaging (azimuth diversity helps FWI avoid cycle-skipping) and has become the standard for large exploration programmes.

Project setup

5000 km² WAZ survey in a deep-water frontier basin. 10 km streamers, 50 m streamer separation, 3 source vessels at ±30° and 0° azimuths. Primary deliverable: an FWI v(x,z) volume at 12.5 m lateral / 10 m vertical sampling, calibrated to a dozen regional wells. Secondary deliverables: pre-stack depth-migrated volume using the FWI velocity as input.

The pipeline

FWI as deliverable

In many modern workflows the FWI velocity model IS the deliverable, not just a step on the way to migration. Interpreters use the velocity volume directly to map facies (clastic vs carbonate vs basalt), identify gas clouds (low-velocity anomalies), and set up reservoir models. A well-calibrated FWI v(x,z) at 12.5 m resolution is a geology-grade product by itself.

The ML acceleration stage

ML-based low-frequency reconstruction (§9.3) is a recent addition to WAZ workflows. Even OBN-grade low-f data starts at 1.5–2 Hz; reconstructing plausible 0.5–1.5 Hz content from the measured spectrum gives FWI another octave of headroom. The reconstruction is adversarial-network-based and needs careful QC, but when it works it lets FWI converge with much more forgiving initial models.

Multi-scale schedule

Typical cascade:

• 3–5 Hz band: 100–200 iterations; converges from tomography starting model.
• 5–8 Hz band: 100 iterations; preserves the 3 Hz-scale structure, adds intermediate-scale features.
• 8–12 Hz band: 50–100 iterations; reservoir-scale detail emerges.
• 12–15 Hz band: 30–50 iterations; fine-scale thin-bed resolution.

Where this goes next

§10.4 moves to a 4D-monitoring-focused project where the deliverable is a time-lapse difference rather than a static image.

References

• Virieux, J., Operto, S. (2009). An overview of full-waveform inversion in exploration geophysics. Geophysics, 74, WCC1.
• Pratt, R. G. (1999). Seismic waveform inversion in the frequency domain, Part 1. Geophysics, 64, 888.
• Tarantola, A. (1984). Inversion of seismic reflection data in the acoustic approximation. Geophysics, 49, 1259.
• Etgen, J., Gray, S. H., Zhang, Y. (2009). An overview of depth imaging in exploration geophysics. Geophysics, 74, WCA5.
• Yilmaz, Ö. (2001). Seismic Data Analysis (2 vols.). SEG.