Patch, salvo, roll-along

Part 3 — Survey geometry design

Learning objectives

Define the live patch as the instantaneously-active subset of receivers
Describe the roll-along sequence: fire salvo, advance patch, repeat
Recognise fold build-up as the patch sweeps across the survey
Balance patch size against channel count and crew efficiency

A modern land 3D survey may have tens of thousands of receiver stations laid out across the survey area. The recording system can only digitise a subset at once — the live patch, typically 2–8 receiver lines wide by 6–200 receivers per line, for a total of 500–40,000 live channels at any moment.

The field sequence is:

Activate a live patch.
Fire a salvo of shots at positions inside or near the patch. Each shot is recorded by every live receiver.
Deactivate the trailing row of the patch, activate the next row ahead — roll-along.
Repeat until the patch has swept the whole survey.

Why fold builds in bands

A given bin at survey position (x, y) only accumulates traces when the patch contains receivers AND the active shot sits at positions that put the midpoint in the bin. As the patch sweeps through a band of crossline area, that band’s bins receive new contributions from each shot. Then the patch rolls forward, and the next band starts building fold while the previous band continues to receive contributions from shots lagging behind the patch. Full fold is reached behind the trailing edge of the patch.

Patch-size tradeoff

Wider patches: higher channel count, more fold per bin, more azimuth per pair, but more expensive recording hardware and more complex cable management. Narrow patches: cheaper, simpler, but lower fold — which may require re-shooting areas to reach the fold target.

Modern nodal systems (Fairfield ZLand, SmartSolo IGU) eliminate the patch concept entirely — every node records autonomously, always live. The "patch" becomes whatever geographic extent the shots during one download window illuminate. Roll-along becomes a purely logistical operation (redeploy nodes from the back of the survey to the front).

Salvo design

Shots within a salvo may be fired in sequence or simultaneously (§1.6 blended acquisition). Simultaneous blending doubles or triples crew rates. The constraint: the patch must remain live long enough to record the full listening time of every shot in the salvo, typically 6–10 s.

References

Cordsen, A., Galbraith, M., Peirce, J. (2000). Planning Land 3-D Seismic Surveys. SEG Geophysical Developments 9.
Stone, D. G. (1994). Designing Seismic Surveys in Two and Three Dimensions. SEG Geophysical References 5.
Vermeer, G. J. O. (2002). 3-D Seismic Survey Design. SEG Geophysical References 12.