Airguns have long been the industry standard sound source used by crews to acquire seismic data in the marine environment offshore.
The driving mechanism for airguns is supplied by compressed air, and the devices create considerable noise when large volumes of air are emitted. These intense pulses of acoustic energy act as a signal, traveling downward through the water and ultimately triggering a series of refractions and reflections off the varied horizons present in the target subsurface area.
This popular and effective sound source technology has remained essentially static through the years.
Change, however, is afoot.
The technology has drawn criticism from environmental activists and federal policymakers and regulators for its supposed impact on sound-sensitive marine life like whales and dolphins. The validity of those criticisms has been a continued source of debate, particularly with the passage of the Marine Mammals Protection Act (MMPA) in 1972 and its various amendments in the decades since.
What isn’t debatable, however, is the political and social pressure on the seismic industry, and as a consequence, the industry had to begin re-thinking its surveying methodology, specifically regarding noise creation in the marine environment.
Building a Better Acoustic Source
But developing acceptable source alternatives to the airgun is a complex endeavor.
In addition to the necessity to reduce noise levels, companies must address their own needs as well. Ensuring signal quality comparable to or better than the proven airgun method in order to arrive at an equal or, ideally, superior end-product ranks high on the list.
With the MMPA in place, regulators eventually moved to enforce varied requirements on the data gatherers.
These included placing a marine mammal observer onboard working boats to be on the lookout for any and all marine life, along with monitoring systems to pick up on any and all mammal activity within the shoot area.
So, industry participants are busy working to develop, among other options, a marine vibrator that is commercially and technically viable to serve as a preferred sound source for many marine seismic survey operations.
The big differentiator here is that airguns with their loud blasts of noise are impulsive sound sources, as opposed to the controlled-source marine vibrators.
An independent joint industry project (JIP) is focused on development and testing for an acceptable substitute for airguns.
Sponsored by three of the industry majors, ExxonMobil, Shell and Total, the JIP has received considerable attention, along with individual companies engaged in this type research.
After reviewing a range of vendors, the JIP contracted with three of them – PGS, Applied Physical Science, and Teledyne Webb Research – each charged with designing and building a prototype marine vibrator suited for use in offshore data acquisition programs.
In so doing, the vendors must adhere to a set of specifications for the prototype established by the founding sponsors.
The JIP is managed by Texas A&M University’s Global Petroleum Research Institute (GPRI).
The output specs and reliability requirements for the marine vibrator are comparable to the kinds of airgun arrays now being used, according to Mike Jenkerson, geophysical adviser for marine seismic at ExxonMobil.
He and a couple of industry colleagues succinctly emphasized the motivation for the project:
- Mitigating some of the environmental objections to seismic surveying in certain parts of the world.
- Delivering additional geophysical and operational benefits, such as shallow water operations, improved bandwidth control and signal encoding capabilities.
Jenkerson noted that many of the techniques now used to improve operational efficiency for land vibrators could be adapted for marine vibrators.
A Smaller Footprint
David Burnett, technical director of GPRI, who serves as technical administrator for the program, elaborated further on the effort.
“The goal is to create an energy source for offshore sites and transition zone waters that has more directed acoustic signal,” Burnett said. “If we cut down on the energy level, then the acoustic footprint is smaller and you get better resolution on your seismic work.
“The ultimate goal is to find a place where you have a sensitive environment and you don’t want to come in and run the seismic boats (with airguns),” he noted.
“We anticipate that with a marine vibrator having a smaller environmental footprint, it’s going to be easier to get access and less expensive overall to run these things.
“The majors want the technology to be developed and to get far enough along so the industry will adopt these measures and utilize,” Burnett added. “They’re taking some of the risk out of research that the offshore seismic people would have to do.”
The program is making steady progress.
“We’ll have performance and reliability test data this year,” Burnett said. “By 2017, we’ll have equipment testing in the open water environment, and the information will be public.
“We want all to get tested and hope they test out well enough that we get three really good (systems).”
Burnett pointed out some of the nuances in the designs, including weight, power and technological complexity.
He envisions that when offshore exploration picks up again, some of the companies will have an option to choose either marine vibrators or airguns for data gathering.
“This offers a future for offshore exploration that meets more expectations,” he said. “You’re a better neighbor, so it’s better to have.”