Source arrays underwater
Learning objectives
- Describe a marine source-array layout: 2–3 sub-arrays × 6–8 guns each
- Explain the depth–ghost-notch trade-off: shallower tow pushes notch above the usable band
- Recognise the cross-line symmetry of the source and its role in azimuth directivity
- Quote typical total volumes (3,000–4,500 cu-in) and firing pressures (~2,000 psi)
A production marine source is almost always a cluster of air-guns arranged in 2–3 parallel sub-arrays, towed at 6–9 m below the surface. Each sub-array is a straight line of 6–8 guns ranging from ~20 cu-in up to ~350 cu-in. Total cluster volume is typically 3,000–4,500 cu-in at a firing pressure of 2,000 psi. The primary pulses from all guns add coherently; the bubble oscillations (§1.4) are spread in period across the cluster and so mostly cancel.
Why multiple sub-arrays
Three reasons:
- Total energy: a single sub-array maxes out at ~1,500 cu-in; three sub-arrays deliver ~4,500 cu-in without making any single tow cable too heavy.
- Bubble cancellation: each sub-array has its own bubble-period range; spread across 3 sub-arrays the cancellation improves.
- Cross-line symmetry: the source’s directivity pattern cross-line depends on sub-array spacing; 8–10 m spacing gives a modestly directive pattern that is OK for streamer-sailing direction.
Tow depth and the ghost notch
The free-surface ghost notch sits at fₙ = V_water / (2h) ≈ 750/h Hz. Shallower tow (4–6 m) puts the first notch at 125–190 Hz — well above the useful seismic band. Deeper tow (10–15 m) moves the notch to 50–75 Hz, inside the useful band, but buys much better sub-10 Hz response (what FWI needs). The choice depends on the target: depth imaging of sub-salt or deep basin drives deeper tows; high-resolution shallow work stays shallow.
Signature measurement
Near-field hydrophones on every gun record that gun’s individual firing, and the far-field signature is computed by combining the near-field recordings with the known geometry. This is the signature used by processing — not the nominal "designed" signature — because every shot varies slightly and deconvolution needs the true wavelet (§2.8 of Processing).
References
- Dragoset, B. (1990). Air-gun array specs: a tutorial. Geophysics, 55(11), 1426–1440.
- Ziolkowski, A. (1970). A method for calculating the output pressure waveform from an air gun. Geophysical Journal International, 21(2), 137–161.
- Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.