Migration aperture & design rules
Learning objectives
- Derive the migration aperture A = z · tan(θ_max) from diffraction geometry
- Explain why survey extent = target outline + 2A on each of four sides
- Recognise θ_max as the steepest dip you want to image
- Estimate aperture cost: extra shooting area + extra fold, both scaling with z · tanθ
A point diffractor at depth z produces a diffraction hyperbola spreading across the surface. To focus that energy back to a point during migration, the survey must record the hyperbola out to the steepest dip θ_max we care about. Past that angle, the hyperbola is either attenuated (Q, spherical divergence) or outside our offset / azimuth range. The half-width of the usable hyperbola is the migration aperture:
For a 2 km target and θ_max = 45°, A = 2 km. For θ_max = 70° (salt flanks), A = 5.5 km. For a flat shallow target at z = 500 m and θ_max = 30°, A = 290 m. The numbers vary over two orders of magnitude.
4 sides of a 3D survey
In 2D you add A to each end of the target line — twice. In 3D the aperture extends in every horizontal direction. The survey outline = target outline + 2A in each of four directions. Survey area = target area + 2A × perimeter + 4A². For a 10 × 10 km target with 2 km aperture, survey area becomes 14 × 14 = 196 km², almost double.
When to cheat
If you know the dip direction is predominantly along one axis (e.g. a single thrust system dipping east), you can shoot less aperture perpendicular to it. This is called an asymmetric aperture and can save 20–30% of shooting. It requires reliable dip knowledge in advance — mistakes show as under-migration along the cheated direction.
Aperture vs fold: two independent knobs
Aperture sets the survey extent; fold sets the density of traces per bin. A big-aperture / low-fold design costs more in land area; a small-aperture / high-fold design costs more in channels. Target-specific: shallow / well-illuminated targets can skimp on aperture; deep / steeply-dipping targets cannot. Salt-body imaging projects routinely design for θ_max = 70–80° and pay for it with huge surveys (§10.2 capstone).
References
- Vermeer, G. J. O. (2002). 3-D Seismic Survey Design. SEG Geophysical References 12.
- Bouska, J. (1995). Cube management — 3D acquisition design. The Leading Edge, 14(1), 53–57.
- Yilmaz, Ö. (2001). Seismic Data Analysis: Processing, Inversion, and Interpretation of Seismic Data (2 vols.). SEG Investigations in Geophysics 10.
- Vermeer, G. J. O. (2012). 3D Seismic Survey Design (2nd ed.). SEG.