Marine vibrators and emerging sources

Air-guns have been the marine workhorse since the 1970s. They are simple, robust, and well-understood. But two pressures are pushing the industry to alternatives: environmental regulation (peak SPL limits and marine-mammal protection zones) and FWI’s appetite for sub-5 Hz energy that air-guns struggle to deliver.

Why marine vibrators?

A marine vibrator is essentially a submerged vibroseis — a piston-driven low-frequency source that sweeps for 6–10 s. At the same total energy per shot, it has peak SPL roughly 10–20 dB below an equivalent air-gun array. For whales and porpoises, that’s the difference between a startling impulse and a long low hum they can swim past.

Marine vibrators also extend down to ~1 Hz, an octave below air-gun arrays’ practical low-frequency limit. Sub-5 Hz energy is gold for FWI, which uses it to break out of cycle-skipping and converge to the correct model.

Why they’re not the default (yet)

• Reliability: mechanical sweepers in seawater are harder to build than compressed-air devices.
• Cost: R&D amortised over few surveys.
• Operational downtime: repair at sea is hard.
• Proven track record: air-gun arrays have 50 years of field validation.

Several vendors (PGS, Geokinetics, BGP) have commercial marine vibrators operational; the trend is toward increased use, particularly for 4D monitoring and environmentally-sensitive basins.

References

• Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.
• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.
• Berkhout, A. J. (2008). Changing the mindset in seismic data acquisition. The Leading Edge, 27(7), 924–938.