Receiver deployment: cables vs nodal

The receiver spread is the most expensive and most labour-intensive part of a land survey. Until about 2010, every receiver was a copper-wire daisy-chain back to a central recorder — the “Cable system”. Since then, autonomous battery-powered nodal systems have swept large parts of the market. The two are not interchangeable: each has deployment rates, channel-count ceilings, and QC consequences that favour different surveys.

Cable systems

A cable system (Sercel 428XL, INOVA G3i, DMT SUMMIT) is a hierarchical network. Geophone strings (typically 6 or 12 phones) plug into a cable segment. Segments plug into line-takeout modules (LTUs). LTUs connect via main-line cable back to a central recording truck or doghouse. Data streams in real time — a QC engineer watches every trace on every shot.

Advantages: instantaneous data, immediate noise monitoring, live fold and coverage maps, and the crew can stop shooting the moment a problem appears. Disadvantages: telemetry bandwidth caps channel count around 20,000–30,000 live channels (with modern fibre backbones); deployment is slow because every cable must be laid out, connected, and QC’d; and cables are vulnerable to weather, wildlife, and farm equipment.

Nodal systems

A nodal system (Sercel WING, INOVA Hawk, Magseis MASS) is a distributed recorder: each node contains a geophone (or 3C), a 24-bit ADC, a 1–2 week battery, GPS time synchronisation, and local flash memory. Nodes are dropped, GPS-located, left in place, then harvested; data is downloaded in the harvest lab.

Advantages: deployment is 2–4× faster than cable (drop and go); channel counts are effectively unlimited (50,000–100,000 is routine, and 1M-channel pilot surveys have been done); no cable damage; and nodes can be left in place for weeks–months for passive seismic monitoring. Disadvantages: no real-time QC — you find out after harvest if a node died; battery life and memory limit survey duration; and per-channel equipment cost is higher.

Deployment rates

A disciplined 20-person crew can lay out roughly 250 cable channels per person per day in open country, dropping to 100–120 in forest or wheat stubble. The same crew drops 700 nodes per person per day in open country, 500 in forest. For a 50,000-channel survey in open country: ≈3.6 days nodal vs 10 days cable. In forest: ≈5 days nodal vs 21 days cable — a four-fold gap that is the real reason nodal has taken over large-channel-count land 3D.

When each wins

Cable wins for: short jobs where real-time QC catches problems quickly; high-risk areas (complex geology) where you want to adapt on the fly; classical narrow-azimuth 3D. Nodal wins for: dense/wide-azimuth 3D (the channel count is simply unreachable with cable); long-duration passive monitoring; difficult terrain (drop by helicopter); and any survey where the deployment cost dominates the crew day rate. Modern mega-surveys are nodal-only; most regional exploration jobs are still cable.

References

• Mougenot, D. (2013). MEMS-based 3C accelerometers for land seismic acquisition. The Leading Edge, 32(4), 388–396.
• Cordsen, A., Galbraith, M., Peirce, J. (2000). Planning Land 3-D Seismic Surveys. SEG Geophysical Developments 9.
• Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.
• Berg, E., Svenning, B., Martin, J. (2010). OBN technology — recent developments. EAGE Workshop on Permanent Reservoir Monitoring.