Source signature QC
Learning objectives
- Compare pilot vs measured waveforms for vibroseis and air-gun sources
- Recognise harmonic distortion, baseplate decoupling, bubble drift, gun mistiming
- Apply acceptance thresholds: correlation ≥ 95%, distortion ≤ 5%
- Describe how failures trigger re-shoots or truck/gun servicing
Every source fires with an expected signature. For vibroseis it is the pilot sweep; for air-guns it is the modelled far-field signature produced by the gun array at depth. The measured output — ground-force plate on a vibrator, near-field hydrophone on an air-gun array — should match that expectation to within tight tolerances. Signature QC catches physical problems at the source side before they smear data downstream.
Vibroseis signature
Two common faults. Harmonic distortion: when ground coupling is non-linear (hard rock that slips, frozen ground that cracks), the plate emits 2f and 3f energy in addition to the fundamental sweep. The spectrum shows clear bumps above the sweep band. Above 5% distortion the correlated record begins to show cycle-skipping in shallow reflectors. Baseplate decoupling: the plate lifts briefly off the ground during the sweep, producing a flat-lined gap in the ground-force record. Even a 20% decouple reduces correlated signal by several dB.
Air-gun signature
The signature is a sharp primary impulse followed by a damped oscillation of the air bubble. The bubble period depends on gun volume, depth, and pressure: a 150 cu-in gun at 6 m, 2000 psi has a ~80 ms bubble period. Drift in any of the three parameters shifts the period measurably. A single mistimed gun in a multi-gun array produces a double-humped signature and notches in the composite spectrum.
Acceptance criteria
Vibroseis: pilot-measured correlation ≥ 95%, harmonic distortion ≤ 5%. Air-gun: bubble period within ±10% of expected, gun firing time within ±1 ms of tool time. Failed shots trigger an immediate reshoot; repeated failures on the same truck or gun pull the unit for servicing. A good crew logs every signature to a signature database so drift trends can be spotted over days.
References
- Ziolkowski, A. (1970). A method for calculating the output pressure waveform from an air gun. Geophysical Journal International, 21(2), 137–161.
- Dragoset, B. (1990). Air-gun array specs: a tutorial. Geophysics, 55(11), 1426–1440.
- Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.
- Sallas, J. J., Weber, R. M. (1982). Comments on the digital-filter equivalent of a vibrator. Geophysics, 47(11), 1577–1582.