Seismic surveys are created using bursts of acoustic energy that are referred to as “marine sound, or noise, depending on your perspective.” With that thought, John Young, director of the sound business line for CSA Ocean Sciences, introduced a recent panel discussion that included seismic innovators working on new sound sources designed to produce better subsurface images as well as scientists and regulators concerned about the environmental impact of that noise. At that session and others at the recent annual meeting of the Society of Exploration Geophysicists (SEG), there was discussion about multiple ways to move away from the intense pulses of acoustic energy produced by air guns. The industry standard emits both useful sound for seismic imaging and higher-frequency noise that dissipates in the ground.
One sign that this talk may lead to alternatives to air guns for marine seis¬mic is a joint industry project by three major oil companies backing a new generation of offshore seismic sound sources designed to reduce noise and improve the seismic signal.
Their goal is to “derisk” vibrator technology, said Mike Jenkerson, geo¬physical advisor for marine seismic at ExxonMobil, who represented the Marine Vibrator Joint Industry Project (JIP) at the conference. The JIP managed by Texas A&M University is supporting development and testing to determine if there is an alternative to air guns that is effective and reliable even with a smaller acoustic signal.
The signal is the critical difference between sounds created using controlled vibrations and air guns, which are called impulsive sound sources and create loud, sharp booms as a large volume of air is emitted. Marine science and regulatory experts on the SEG panel indicated that research on the impact of subsea noise on sea life is likely to gradually lead to increasing limits on seismic surveys, where noise regulation is already a costly fact of life.
“A big concern is that regulators will makeit more difficult to use impulsive sources in many areas,” said Rune Tenghamn, vice president geoscience and engineering for PGS, which is one of three companies chosen by the JIP to develop an alternative. “This is the main driver for using marine vibrators.”
Regulations, based on laws passed decades ago when whales were the main concern, are evolving to protect a growing range of marine life from outside noise sources. The US National Oceanic and Atmospheric Administration’s (NOAA) regulations are based on multi¬ple mandates designed to protect marine life and places such as fishing grounds. Leila Hatch, a marine ecologist for Stell- wagen Bank National Marine Sanctuary, said the goal is to expand protection “beyond species to an ecosystem level.”
The regulatory pressure is pushing new air gun designs that narrow the sound frequencies emitted. They have also inspired lower-intensity survey ideas, such as the “popcorn method” from BP. This approach could significantly reduce the peak energy of air gun arrays, which are normally fired in unison, by setting each gun off in succession, like corn popping.
Dolphin Geophysical supports an air gun designed by the man who invented it, Stephen Chelminski, who says his new design can send out more useful frequencies with less environmental impact. Shuki Ronen, external and collaborative research manager for Dolphin Geophysical, said the seismic company is looking for better seismic sources because, “We do have regulators looking at us and asking what we are doing.”
“Noise does represent a 21st century environmental problem,” Hatch said. Air guns add to the noise from cruise ships, freighters, and workboats, among other things, in complex industrialized areas of the ocean, said Hatch, who cochaired the NOAA working group that developed tools to map the many sources of under-water noise created by human activity.
This long-term program to better understand background noise in US waters is not related to NOAA’s effort to revise a key measure of the effect of noise on the hearing ability of marine animals. The noise study, though, is an example of the growing number of large, wide- areas data sets being created by environmental researchers. Over time, more data matter because permits are based on detailed estimates of the number of animals affected and the noise they experience. Different databases can lead to different outcomes and new models for determining the impact of noise during a permit review.
The US Bureau of Ocean and Energy Management (BOEM) is currently writing an environmental impact statement covering seismic surveys in the Gulf of Mexico and has asked NOAA to consider rule changes based on that report. This is likely to change the regulatory approach taken in the Gulf of Mexico to align it more with the Arctic and Atlantic oceans, said Sarah Courbis, principal scientist at Smultea Sciences, an environmental consulting firm.
This may cause problems for some companies seeking permits for wide-area surveys. The method used to estimate the number of animals affected by sound could result in totals that appear high based on standards permitting activi¬ty that has a “negligible impact” and affects only “small numbers” of animals, said Courbis.
“In large-scale seismic projects, small numbers can be a confusing metric to evaluate, and NOAA has had difficulty with lawsuits associated with negligible impact findings in data-poor situations,” Courbis said.
Sound sources with a smaller acoustic signal might help reduce such concerns as the industry moves to multiple sound source surveys.
“The less sound in the environment, the lower the number of marine mammals that would be expected to be harassed,” she said, adding that if the number of animals affected proves to be lower—which will need to be verified when marine vibrators have been built— that could make “it more likely that permits can be issued.”
Multiple Options
Marine vibrator work goes back decades, to sonar system projects, providing a technical foundation for seismic system development.
Tenghamn was among those working on vibrators back in the 1980s, several employers before he was hired by PGS. A diagram explaining how the device works, which appeared in a recent presentation, is nearly identical to one that appeared in a PGS publication in 2005. A big difference now is the PGS device is one of three with development support from the Marine Vibrator JIP.
The three companies that founded the JIP in 2010 represent a critical element that has been missing for those developing marine vibrators—customer demand for an alternative to the air gun. The JIP chose three companies to develop marine vibrators: PGS; Applied Physical Science (APS), which is part of aerospace and defense company General Dynamics; and Teledyne Webb Research. They were chosen from among 36 initially contacted by the JIP, which is managed by the Texas A&M Engineering Experiment Station.
The APS and PGS devices use electromagnetic force to create sound by moving metal parts. The APS device activates a piston that vibrates a metal endcap, and the PGS device vibrates a spring that activates an outer shell. The Teledyne design produces a controlled sound by com¬pressing an air bubble inside a cylinder. While the devices vary in patent- protected ways, all use precise, electric control systems to create “coherent sound.” The controlled output of a vibrator can be crudely compared to a bass speaker, while an air gun is like a big bubble popping.
The word coherent is used to describe lasers because the photons are directed into narrow beams. The output of a coherent sound source is described by Andrey Morozov, chief scientist for Teledyne, as waves with a predictable, fixed- phase relationship, and the signal spectrum as “clean, harmonic content.”
“The motivation is from the environmental side. But I am not sure that is the main strength,” he said during an SEG presentation. “A coherent source can change dramatically the quality of the data. No matter how you design an air gun, it cannot produce coherent sound.”
Vibrators reduce peak sound by delivering the same amount of energy an air gun would emit in a millisecond, and spread it out over seconds. The JIP is requiring that devices emit an evenly distributed signal over bandwidth from 5 Hz to 100 Hz. This would increase the amount of signal at the bottom end of the range, which is valued by geophysicists for determining rock properties and imaging deep layers. Past 100 Hz, the JIP，s specifications require a rapid drop in those higher-frequency sounds.
The novel compressed bubble approach was chosen by Teledyne because past work suggested it was better suited to producing ultra-lower frequency sound, said Morozov.
The test of whether the devices can deliver on their promise could begin this year.
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