Cable positioning & navigation

Part 4 — Marine acquisition

Learning objectives

Define feather angle and identify the industry tolerance (± 5° production)
Explain compass-birds: clamp-on cable control surfaces that apply lateral force
Describe tail-buoy GPS and acoustic ranging for absolute cable positioning
Recognise that binning must use real (not nominal) positions to be correct

An ocean current flowing across the sail line pushes streamers sideways. Drag along the cable means the drift is greatest at the tail and (approximately) zero at the head. The result is a parabolic bend called feathering; the angle between the streamer’s local tangent and the sail line is the feather angle.

The numbers

In open-ocean moderate current (0.3–0.5 m/s cross-track), feather angles run 5–12°. Industry tolerance for production 3D is ± 5° — beyond that, midpoint bins fill unevenly and the survey may need to be re-shot. Excursions to 15° in unusual conditions (strong loop currents, spring tides) are survived by post-acquisition interpolation of missing bins but reduce image quality.

Compass-birds

A compass-bird is a hydrodynamic control surface clamped around the streamer every ~300 m. It carries a compass, an acoustic transducer, and a servo-driven pair of fins. The fins apply lateral force to pull the cable back toward its nominal position. Under good control 70–90% of the drift is corrected; what remains is the residual feather the survey lives with.

Tail buoy + acoustic network

Every streamer ends in a tail buoy carrying a GPS receiver and an acoustic transducer. The buoy’s absolute position (GPS) plus the chain of acoustic ranges (each bird to its neighbours) gives the full cable shape in real time. That shape is written into every trace header. Modern processing bins using the TRUE shape, not the nominal straight line — anything less would shift every midpoint by tens of metres.

Deep-water edge cases

In deep-water basins with strong loop currents (Gulf of Mexico, Brazilian pre-salt), feather angles can exceed 15° and birds saturate. Acquisition plans then call for re-shooting affected sail lines (expensive) or sailing cross-wind for a couple of lines until the current shifts. In the worst case, the vessel parks and waits.

References

Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.
Vermeer, G. J. O. (2002). 3-D Seismic Survey Design. SEG Geophysical References 12.
Yilmaz, Ö. (2001). Seismic Data Analysis: Processing, Inversion, and Interpretation of Seismic Data (2 vols.). SEG Investigations in Geophysics 10.