Marine acquisition: airguns, streamers, OBC/OBN

Part 1 — Acquisition & the data we process

Learning objectives

Describe the standard towed-streamer acquisition setup and the roles of each component
Compute the first ghost-notch frequency from tow depth and explain why deeper tows move notches down
Distinguish towed streamer from OBC and OBN and give a case for each
Recognize why ghost de-ghosting (source + receiver) is a standard processing step on marine data

Marine surveys look deceptively simple: tow a source and a long cable of receivers behind a boat. Physically, the setup is cleaner than land; computationally, it introduces a signature noise pattern that shapes the bandwidth of every trace and requires a dedicated processing step to remove. That pattern is the ghost.

1. The standard towed-streamer setup

Source. An array of airguns towed at 5–12 m depth. Each gun releases compressed air into the water, producing a bubble that collapses and re-oscillates. Arrays are tuned (sub-arrays at different depths, different volumes) so the bubble harmonics destructively interfere and the effective source is a short broadband pulse.
Streamers. Long neutrally-buoyant cables (5–10 km) holding pressure-sensitive hydrophones at 6–15 m depth. A modern survey tows 8–20 streamers in parallel, 50–100 m apart.
Recording. Each hydrophone digitizes 2–8 s of data after each shot. Thousands of channels × thousands of shots → terabytes per survey.

2. The ghost

The sea surface is an almost perfect mirror for seismic waves: acoustic impedance of air is essentially zero, so the reflection coefficient at the water–air interface is close to R = −1. Every pulse emitted by the source travels downward and upward; the upward one bounces off the surface and comes back down, inverted in polarity, delayed by the round-trip time through the water above the source. That delayed inverted copy is the source ghost.

The same thing happens at the receiver side: every upcoming wave arrives at a hydrophone, and a delayed, inverted copy — having bounced off the surface — arrives shortly after. That is the receiver ghost.

3. Ghost operator in the frequency domain

In the frequency domain, the source ghost multiplies the pulse by

G_{s}(f) \;=\; 1 - e^{-i\,2\pi f \cdot 2z_{s}/c}

where zₛ is the source depth and c is the speed of sound in water (≈ 1500 m/s). The magnitude is

|G_{s}(f)| \;=\; 2\,\bigl|\sin\!\left(\pi f \cdot 2z_{s}/c\right)\bigr|

which is zero at

f_{n} \;=\; n\,\frac{c}{2 z_{s}},\qquad n=0,1,2,\dots

The receiver ghost does the same at the streamer depth, creating its own notch family. The total ghost response is the product of the source and receiver ghosts.

Drag the tow depths. Shallower tow (e.g. 5 m) pushes the first source notch up to 150 Hz — well above the useful seismic band — so you keep most of your bandwidth, but you lose the low frequencies. Deeper tow (e.g. 20 m) pulls the notch down to 37 Hz, punching a hole right in the middle of your signal band. Every survey designer weighs this trade.

4. Why low frequencies matter

Low frequencies are the first thing you lose to the ghost. They are also the hardest to recover by deconvolution because the ghost is zero there — you cannot divide by zero in the frequency domain without amplifying noise. And low frequencies are exactly what FWI and broadband impedance inversion need most (§§6.2 and 7.4). This is why modern survey design agonizes over the first notch.

5. Solutions: deeper streamers + de-ghosting

Over/under cables. Two streamers at different depths; the pair’s ghost notches do not coincide, so you can combine them to fill in the notch of either.
Multi-sensor streamers. Measure both pressure and particle velocity at each hydrophone. Pressure is a scalar and includes the ghost additively; velocity is vector and the ghost has opposite polarity. Combine: ghosts cancel, primaries add. Broadband recording without tuning the tow depth.
Wave-equation de-ghosting. Forward-model the ghost and invert it out in the processing flow.

6. OBC and OBN: put the receivers on the seafloor

If you lay receivers on the seafloor instead of towing them, you kill the receiver-side ghost almost entirely (the seafloor absorbs the surface-reflected wave). Also, you can record three-component or four-component data (pressure + three velocity components), enabling shear-wave processing and PS imaging.

Ocean-Bottom Cable (OBC). Receivers on a cable laid from a boat. Limited length, repeated deployments.
Ocean-Bottom Node (OBN). Autonomous pod receivers dropped on the seafloor by ROV, retrieved days or weeks later. Deeper water, better density, but expensive.

OBC/OBN surveys cost far more than towed-streamer per square kilometer, but they are the standard for subsalt imaging and high-value reservoir monitoring (time-lapse). Part 10 has a capstone on Gulf of Mexico OBN imaging through salt.

7. The processing impact

Every marine processing flow has these acquisition-specific early steps:

Source signature deconvolution (remove the bubble tail).
De-ghosting (source and/or receiver side, depending on acquisition).
De-multiple (surface-related multiples — covered in Part 4).
De-noise (swell noise, tidal noise — covered in §1.4).

**The one sentence to remember**

Marine data has notches at n·c/(2z) from the source and receiver ghosts; tow-depth choice is a bandwidth-vs-low-frequency trade; multi-sensor streamers and OBN are the modern routes around it.

Where this goes next

Section §1.3 is the bookkeeping section — SEG-Y format, trace headers, and how shots and receivers turn into CMP bins. Not glamorous, but every processor spends at least a week per year debugging header problems, and this is where you learn what to check.

References

Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge UP.
Yilmaz, Ö. (2001). Seismic Data Analysis (2 vols.). SEG.
Claerbout, J. F. (1976). Fundamentals of Geophysical Data Processing. McGraw-Hill.