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 seismic 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 “de-risk” vibrator technology, said Mike Jenkerson, geophysical 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 make it 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 multiple mandates designed to protect marine life and places such as fishing grounds. Leila Hatch, a marine ecologist for Stellwagen 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 underwater 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 activity 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 compressing 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. “The JIP is showing progress in meeting its objective of demonstrating the viability of the marine vibrator technology,” Jenkerson said. The goal is to complete the project in 3 to 4 years.
APS is moving toward testing a full-scale model of its design. “A quarter-scale model of our device has been successfully demonstrated and a full-scale version will be fabricated and tested in 2016,” said Jim McConnell, APS’ chief engineer.
This competition is not designed to pick a winner. The JIP hopes the effort results in multiple competing marine vibrators. Competition could help ensure that there is a financially attractive alternative to air guns, which are relatively inexpensive and reliable.
To ensure that marine vibrators can stand up to hard use common on a long seismic shoot, the JIP’s standards set a minimum service time between maintenance.
The three companies involved in the JIP are not the only ones working on new sound sources. Another made by Geokinetics was recently tested in the Gulf of Mexico. “We were very successful at getting back all the energy intended to put in the ground, so it was working,” said Bill Pramik, vice president of acquisition technology at Geokinetics.
Full-scale survey work will require arrays of 10 or more units that fit into current survey systems. “I doubt anyone (developing a marine vibrator) will have an array ready in 2 years,” he said.
The tests in US waters require a permit under rules for new and unusual technology, said Chad Vaughan, a BOEM geologist. He said the permit is commonly referred to using the acronym: a NUT. Despite the acronym, Pramik said the agency is supportive, and the permitting process is not much more difficult than that required for a traditional survey.
Less Impact
Before launching the marine vibrator project, the JIP commissioned an extensive environmental review to see if a vibrator would present less risk than an air gun. It was done by the E&P Sound and Marine Life Program, whose 100 backers include the companies in the Marine Vibrator JIP.
The program’s 2011 study, based on information available at the time, concluded that lower peak volume and controlled signals “should, in most respects, have less environmental impact than surveys using air gun arrays.”
But the coauthor of the study, Bill Ellison, chief scientist for Marine Acoustics, pointed out the available data did not include field testing because no one has created an array capable of producing the sound needed for a test simulating a seismic survey.
The lower sound intensity of a marine vibrator is expected to be less likely to startle marine animals or damage their hearing.
Significantly reducing the intensity of the sound above 100 Hz limits the number of animals that are thought to hear and communicate in that range. Those in that large category include whales with teeth, such as sperm whales, and dolphins, as well as many fish and invertebrates. There are exceptions. Some baleen whales, whose food is collected in a strainer-like mouth, are sensitive to sounds in the lower frequencies used for seismic—though only one species, the Bryde’s whale, is rarely found in the Gulf of Mexico.
There is no final answer to the questions created by seismic noise regulations. The system now uses data from laws written decades ago to protect whales and other large mammals. Over time, the underlying law and science have gradually evolved. It is a slow process that ultimately will reflect a more detailed, complex view of ocean noise and its impact on the marine ecosystem.
“NOAA regulations are evolving,” Courbis said. “There is always new science.”
During the SEG panel discussions, it was pointed out that the sounds emitted by vibrators may also cause problems. “The potential impact of marine vibrators is unknown. I cannot tell if impact is negative or positive,” said Gerrit Blacquiere, a senior research scientist at Delft University of Technology. He pointed out that vibrating sound sources reduce the peak energy by spreading the signal over a longer period, but that may not help if the animals are affected by the cumulative energy.
“It will be imperative that industry model and test the output of this technology in order to scientifically support the technology as a less impactful alternative to air gun arrays,” Courbis said.
While the hum of the seconds-long signal of a vibrator may interfere with communication or mask environmental sounds used to sense danger or find food, the problem is not unique to vibrators. The lower-frequency sounds created by an air gun shot travel a long way and contribute “to the more chronic ‘hum’ under water,” Hatch said. The control offered by marine vibrators could offer an opportunity to better balance environmental concerns and industry needs.
Measuring whether a whale is bothered by a particular sound is complicated by the inconsistent reactions of intelligent creatures, whose movements and motivations are hard to observe and understand. There are various theories as to why dolphins are often seen swimming behind seismic survey boats. But the sort of test done to judge what humans hear and if a loud sound causes temporary or permanent deafness cannot be done on animals in the wild, Courbis said.
“We need to design good experiments that are sensitive to what we want to measure,” said Douglas Nowacek, an associate professor of conservation technology in the engineering school at Duke University.
A recent test of a marine vibrator in the Gulf of Mexico by Geokinetics involved towing the company’s prototype unit on a catamaran behind a boat equipped to power it. When the vibrator was shut down, a pod of marine mammals approached the device. Pramik said, “I think they wanted to find out why it stopped.”
As scientific research adds to the list of possible problems caused by marine noise, the industry is moving toward survey methods using multiple sound sources for more shots per day. Onshore, where vibrators are the standard seismic source, surveys using as many as 10 onshore vibe trucks can multiply the number of shots per day. The result is better surveys done in fewer days.
“As soon as we get marine vibrators, when we go to lots and lots of (sound) sources, that will raise environmental issues,” said Ray Abma, a senior research geophysicist at BP.
Marine Vibrator Joint Industry Project
Members: ExxonMobil, Shell, Total
Goal: Develop and test multiple marine vibrators with a goal of proving it is a viable sound source.
Management: Texas A&M University
Outreach: 36 companies contacted and 19 asked for information
Companies Chosen: Applied Physical Sciences, PGS, Teledyne Webb Research
Status: The next step is building full-scale prototypes for testing, which could occur in 2016.
Vibrator Specifications Output: From 10–100 Hz: 200 decibels (dB) re: 1 µPa/Hz at 1 m; From 5–10 Hz: 190 dB re: 1 µPa/Hz at 1 m
Noise reduction: Rapid reduction in output above 100 Hz, dropping by 40 Hz at 150 Hz
Durability: 72 sweep hours between minor maintenance; 720 hours between pulling in and switching out the vibrators
Applications: Built for offshore service in towed and stationary modes; Able to operate in shallow water where air guns do not
