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.