Some fresh air on seismic surveys

Seismic Airgun Survey

I typically eschew technical excavations in our newsletter, but seismic surveys are an important tent-stake in the ocean noise discussion, so I offer the following three-minute read in hopes that you find it informative and entertaining (if not a bit nerdy).

Of the many sessions at the recent OceaNoise2017, one of the more cautiously encouraging was the session on seismic survey technologies. Marine seismic surveying is probably the most controversial noise sources in the ocean. Seismic airgun surveys – also called “airgun blasting” are really loud and can be detectable at a thousand kilometers. And they are relentless; persisting around the clock for weeks, even months at a stretch. Add this to the fact that this technology dovetails into offshore fossil fuel extraction and all the collateral planetary damage that causes – what’s not to hate about it?

But the ocean noise controversy on airgun surveys orbits around hyperbole vs. evidence: Between the specious Enviro claim that airguns are “as loud as a Saturn rocket,” and the equally fatuous harp of the Oilmen that “there is no scientific evidence that seismic airguns have any impacts on marine mammals,” there is a deep abyss where the facts can be found.

There is ample evidence of non-lethal impacts of airgun surveys such as migratory and feeding disruptions, changes in social behavior, fisheries displacement, and compromised invertebrate physiology. But with the ubiquitous presence of seismic surveys world-wide, there are still no unequivocal records of carcasses washing ashore on account of airgun exposure. So there is something to be said about the Geophysical Industry, which has been improving on their techniques (making them more effective and less damaging) for decades.

The early versions of seismic surveys amounted to tossing sticks of dynamite over the transom and analyzing the return echo of the explosion – to the accompaniment of oceans of fish carcasses floating up to the surface. The fish carcasses were a result of the extremely fast ignition speed of the dynamite, generating a forward impulse so fast that it rips up soft tissue. Great for carnage, not so good for seismic surveys (or for fish).

It turns out that slowing the impulse down (and eliminating the vestigial high-frequency noise of the signal) is a more effective use of energy for seismic surveys. Thus the “airgun” strategy that can mediate the time release of a pressurized chamber, creating a bubble that pushes acoustical energy deep into the water and seafloor below without killing the fish (or whales, or invertebrates).

This is an improvement, but that bubble also collapses at a speed that hinges on the stern laws of physics – which can’t be mediated by the equipment. So at close range the airgun signal still contains potentially damaging energy, and perhaps why the noise disrupts animal behavior and causes physical damage.

So some good news? For the last few years the geophysical industry has been developing “marine vibroseis” – a seismic-scale energy source that does not explode (or implode) and produces a continuously mediated signal which spreads out the required energy over time, “softening the blow” so to speak. If the vestigial high-frequency energy is the problem, then marine vibroseis is a good answer.

But the devil is in the details. In the far-field, airgun surveys create a distant rumbling noise suggestive of earthquakes. This (or other signal characteristics) may spook marine animals. Marine vibroseis on the other hand, would create a loud humming, or low frequency upsweeps, that while physically less damaging, may sound just a threatening to marine life as the airguns.

There will always be room for improvement. Hopefully marine vibroseis is a step in the right direction.

Spectrograph – courtesy of Bill Pramik, Geokinetcs

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