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.