Real-time field QC — diagnosing the shot

Part 6 — QC during acquisition

Learning objectives

Identify five common trace-level problems: dead, noisy, reversed, low-amp, clipped
Explain why field QC catches problems hours not weeks before processing
Apply the “stop shooting now” threshold: ≥ 5% bad-channel rate or any mechanical
Recognise modern systems auto-flag most problems; humans still review

Every shot a modern crew fires is displayed within two seconds of the source fire on a QC engineer’s screen. The engineer stares at a variable-area wiggle gather and scans for five trace-level problems: dead (no signal), noisy (broadband hash), reversed polarity, anomalously low amplitude, and clipped (square-topped). Each of these maps to a physical cause — a broken phone, a microphonics fault, a miswired cable pair, a poor plant, or an over-driven preamp.

Why real-time?

If a fault is detected within the shot, a repeat shot costs the crew 60–80 seconds and a few litres of fuel. If it is caught at the end of the day, it may require a night-shift crew to replant the station before tomorrow’s shooting. If caught a week later, the crew has moved, the template has rolled, and the reshoot is economically impossible. Field QC exists to compress the detection-to-fix loop to minutes.

The five problem classes

Dead — a flat line with tiny jitter. Usually a broken geophone, a disconnected cable pair, or a failed node. The fix is physical: replace or reseat.
Noisy — high-amplitude broadband hash across the whole trace. Wind on the phone, a loose spike, nearby cultural noise (pumpjack, vehicle), or an electrical fault.
Reversed polarity — waveform flipped vs its neighbours. A miswired cable pair or a node installed upside-down.
Low amplitude — wiggles are a fraction of the surrounding traces. Poor coupling (soft mud, poor plant) or a damaged element in a multi-phone group.
Clipped — the trace tops flatten at a fixed value. Over-driven preamp gain, or a proximity-triggered clip on a very-near-offset channel.

Stop-shooting thresholds

Crew policy: if more than 5% of live channels go bad in any shot, halt, diagnose, repair, re-shoot the affected VPs. If two consecutive shots show the same channel bad, that channel goes on the repair list for the end-of-line station visit. Modern systems (Sercel 428XL, INOVA G3i, Hawk) fire automatic alerts; a human QC engineer reviews every shot during the first hour of the day to calibrate the auto-flagger to the day’s site conditions.

References

Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.
Cordsen, A., Galbraith, M., Peirce, J. (2000). Planning Land 3-D Seismic Surveys. SEG Geophysical Developments 9.
Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.