Source arrays underwater

Part 4, Marine acquisition

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.