A Seismic Shift Towards Quieter Surveys
Since the earliest days of offshore oil and gas exploration, the need to “shoot” seismic surveys has been helping companies to find the hydrocarbons they can then produce.
Seismic data helps geophysicists and geologists understand the rock formations in the earth, what might be happening in them and, crucially, if they might contain oil and gas. Similar to acquiring seismic data onshore, it means emitting sound energy (a source) then detecting its return and interpreting that information to image the subsurface.
While the industry moved away from use of dynamite as a source in the 1960s, the sound energy created, now mostly by air guns, can still impact marine life. As such, it’s use is highly regulated. Many countries use UK-based Joint Nature Conservation Committee guidance, which will see activity delayed if a marine mammal is detected within 500m. The Australian National Offshore Petroleum Safety and Environmental Management Authority has a 1-3km zone, while in Brazil and Ireland it’s 1km. There are also other bodies with their own guidance as well as country specific regulations. Most mean having onboard mammal observers and towed passive acoustic monitoring (with PAM onboard unmanned vessels also emerging). Some countries have even banned seismic exploration altogether (Italy has had a temporary ban in place pending new assessments).
But, there’s still pressure to protect marine life so there’s also work to rethink the source. Since 2011, a group comprising of Total, Shell and ExxonMobil has been working on alternatives, based on marine vibroseis technology, under a joint industry project managed by Texas A&M University. Marine vibroseis works by emitting a continuous lower level energy. “Instead of a clap, one instantaneous instance of noise with high peak to peak pressure ratio, we propose a quiet hum in the background, but is designed to have the same seismic energy as the clap,” says Andrew Feltham, Research Geophysicist – Acquisition, at Total. “We’re substituting high amplitude with a longer duration. The idea is to have a quieter instantaneous peak to peak pressure level and significantly reduce, if not eliminate, potential harm to the marine environment.”
The marine industry has been trying to use this technology this since the 1960s, by taking land vibroseis technologies offshore, but with “with limited success”. Because of this, the JIP was formed, contracting with three different companies. Some haven’t worked, but one has, says Feltham; Applied Physical Sciences (APS), part of General Dynamics, which has developed the Marine Vibrator - Integrated Projector Node (MV-IPN).
“It has a piston that moves in and out, dynamically coupled to the water, creating sound waves from the source into the surrounding water and subsurface,” he says. This reduces the peak to peak ratio, which is of most concern in terms of impact on marine life and the environment, but also eliminates high frequency sound waves that come out of instantaneous sources and hard to control. If these can be avoided, it means reducing impact on mid-water hearers (mammals) that hear down to about 100Hz and higher frequency hearers.
In testing in Lake Seneca, New York, the system was used with a 5-100hz linear sweep - a frequency band commonly used. Test data suggests a marine vibrator array should reach a peak 205dB signal and the higher frequency signals otherwise emitted by instantaneous source were avoided. “By eliminating sound above 100Hz we’re having no impact on the mid and high frequency hearers and reduced peak to peak pressure,” says Feltham. “Now, it’s just the low frequency hearers we need to make sure are well protected.”
According to modelling, this system would also greatly reduce the area in which, if animals enter, operations would have to stop. For a large gun array, with 259dB peak to peak ratio pressure, you’d have to be about 2.8km from the source before the peak to peak pressure reduces to 190dB (a level deemed appropriate according general rules for limiting impact on marine mammals). For the marine vibrator technology, the distance would only need to be 67m from the centre of the source - for a full array covering 18m x 18m (with single units only having a radius of 4m) – and potentially far less.
“If we were to use marine vibroseis systems, we could reduce the peak to peak pressure level, which is where a lot of the concern for marine mammals and the impact of seismic surveys on their environment exists. If we could reduce the region of impact to potentially to the size of source itself, we could greatly improve seismic surveys around the world,” says Feltham.
Additional benefits include greater control over the sound energy - being able to create specific signatures, potentially improving imaging and enabling multi-source surveys. No longer using compressed air also means the source isn’t limited by being connected with umbilicals, enabling dispersed source arrays, to cover greater areas.
There’s still work to be done, working with environmental regulations in different countries and testing the technology. But, Feltham is positive. “In terms of background anthroprogenic noise, we believe we can significantly reduce the environmental footprint by using marine vibroseis rather than a conventional system. We’ve designed a system that we think limits impact on mid and high frequency hearers and significantly reduces the size of the mitigation zone that would need to be around the source while operating in the environment.”