Marine noise: swell, shipping, barnacle, biological
Learning objectives
- Identify the five dominant marine noise families by their spectral signatures
- Explain why streamer-borne noise (tow, swell-mass-loading) dominates at low f
- Describe shipping noise as harmonic lines + weather dependence
- Connect noise family to processing remedy (f-k filter, matching decon, etc.)
Marine traces carry a rich mix of interfering waves, many of them low-frequency. The processing flow (§2.5 of the Processing book) spends much of its effort on these. Five dominant families:
Surface swell (0.1–2 Hz)
Ocean surface waves produce a massive pressure field at depths of tens of metres. Even though the band is below the seismic useful band, the amplitude can be so large that dynamic range of the digitisers is eaten. Processing removes it with a 2 Hz low-cut filter after ingestion.
Streamer swell (1–5 Hz)
The streamer itself is a long, slender, nearly-neutrally-buoyant body. Surface waves make it oscillate in depth at ~1–3 Hz, producing a large spurious pressure signal that contaminates the first cycle of every low-frequency reflection. The cure is keeping streamer depth tight (active depth control via birds) and, in processing, notch filters.
Shipping (5–30 Hz harmonic)
Propeller shafts rotate at ~10–20 Hz; their blade-passing frequency is that multiplied by the blade count (5 at 20 Hz → 100 Hz, with harmonics above). Shipping noise shows as narrow lines on the spectrum, tracks the ship’s passage, and can be separated by its distinctive ship-speed-dependent dominant frequency.
Cable drag / tow noise (broadband)
Turbulence in the water-cable boundary layer generates broadband noise modulated by vessel speed. Historically called "barnacle noise" because biofouling used to make it worse. Broadband, so not removable by narrow-band filters; statistical methods (SVD-based denoising, multi-trace coherency) suppress it.
Biological (impulsive)
Snapping shrimp produce 2–5 kHz impulses, mostly above the seismic band. Whale calls (particularly baleen whales at 30–250 Hz) overlap the seismic band. Impulsive spikes are easy to clip out; sustained harmonic calls are harder but rarer.
References
- Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.
- Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.
- Yilmaz, Ö. (2001). Seismic Data Analysis: Processing, Inversion, and Interpretation of Seismic Data (2 vols.). SEG Investigations in Geophysics 10.