OBN and OBC acquisition

Part 4 — Marine acquisition

Learning objectives

Describe OBN: autonomous 4C nodes deployed by ROV or rope on a grid
Contrast OBC: tethered 4C cable with dedicated recording vessel
Recognise why OBN has no free-surface ghost in the streamer sense
State typical deployment patterns: 200–400 m inline, 400–800 m crossline

Ocean-bottom seismic puts sensors directly on the sea floor. Two flavours: OBN (nodes) and OBC (cable).

OBN

An OBN station (Magseis Fairfield ZXPLR, Sercel GPR300, Inova Hawk) is a small autonomous recorder containing a 3-component geophone, a hydrophone, a 30-day battery, solid-state memory, and a precision clock. Nodes are deployed by ROV onto a pre-surveyed grid (typically 200×400 m or 400×800 m) and left there for 2–8 weeks. During deployment, one or more surface vessels sail over and fire sources on their own schedule — every shot is recorded by every node within range. At end-of-survey the nodes are retrieved, data downloaded, batteries swapped, and the fleet redeployed for the next project.

OBC

An ocean-bottom cable carries the same 4C sensors but is hard-wired via umbilicals to a surface recording vessel. No battery limit, real-time QC, but cables tangle, drag and tear in currents, and deployment is slower. The OBC market has collapsed since ~2015 as OBN matured; it survives only in shallow-water niches where ROV access is hard.

Why OBN has no streamer-ghost notch

A streamer hydrophone sits in a free-space water column: the free surface above produces a negative-polarity ghost. An OBN sensor sits on a hard (or near-hard) boundary — the sea floor. The "ghost" of the sea floor is part of the wanted signal (reflections of interest). With hydrophone + vertical geophone PZ-summation, the OBN separates up- and down-going wavefields cleanly, with flat frequency response up to the source’s upper bandwidth. Low-frequency content extends well below 5 Hz — the reason OBN dominates FWI-centric surveys.

Productivity and cost

A modern OBN project deploys 500–2,000 nodes and acquires in 2–4 weeks. Per km² cost is roughly 2–3× streamer — justified when the target requires imaging quality or 4D repeatability that streamers cannot deliver. Gulf of Mexico and Brazilian pre-salt are OBN-dominated; North Sea and shallow-shelf work is mixed.

References

Berg, E., Svenning, B., Martin, J. (2010). OBN technology — recent developments. EAGE Workshop on Permanent Reservoir Monitoring.
Vermeer, G. J. O. (2002). 3-D Seismic Survey Design. SEG Geophysical References 12.
Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.