WAZ, RAZ, FAZ geometries
Learning objectives
- Distinguish narrow-azimuth (NAZ), wide-azimuth (WAZ), rich-azimuth (RAZ), and full-azimuth (FAZ)
- Sketch the vessel configuration for each
- Quote aspect ratios: NAZ ≈ 0.15, WAZ ≈ 0.5, FAZ ≈ 1.0
- Choose a geometry given a target (salt, sub-salt, simple basin, 4D monitoring)
Azimuth coverage is the single biggest knob on marine imaging quality. Four geometries, in increasing cost and quality:
NAZ
One vessel, streamers behind, sail lines parallel. Azimuths cluster within ±10° of sail direction. Aspect ratio (crossline-to-inline offset spread) ≈ 0.15. Good enough for simple basin imaging; fails for complex overburden and steep dip. Cost baseline: 1.0.
WAZ
One streamer vessel, 2–4 additional source vessels offset ±2–4 km to the sides. Every shot illuminates the streamer from a different angle. Aspect ratio ≈ 0.4–0.7. Standard for modern Gulf of Mexico salt imaging. Cost ≈ 2× NAZ.
RAZ
Same streamer or OBN, multiple sail directions (e.g., N–S and E–W, and maybe two diagonals). Aspect ratio ≈ 0.7–0.9. Used when WAZ isn’t enough (sub-salt flanks, complex fractured reservoirs). Cost ≈ 3× NAZ.
FAZ
OBN-based 360° coverage. One or more source vessels sail over the node patch in many directions over weeks. Aspect ratio ≈ 0.9–1.0. Best imaging available; most expensive. Cost ≈ 5× NAZ.
Why it costs what it costs
Cost grows super-linearly with aspect ratio because you’re paying for crew time in each new sail direction. Every extra azimuth pass requires turning the ship around, re-deploying any side vessels, waiting for weather — the cost scales roughly with the number of independent passes. The quality scales roughly linearly with aspect ratio in the target imaging window. Accepting a steeper cost curve for richer azimuth is the calculated bet for complex plays.
References
- Vermeer, G. J. O. (2002). 3-D Seismic Survey Design. SEG Geophysical References 12.
- Vermeer, G. J. O. (2012). 3D Seismic Survey Design (2nd ed.). SEG.
- Cordsen, A., Galbraith, M., Peirce, J. (2000). Planning Land 3-D Seismic Surveys. SEG Geophysical Developments 9.
- Bouska, J. (1995). Cube management — 3D acquisition design. The Leading Edge, 14(1), 53–57.