The intense subsea sounds produced when conducting seismic surveys using traditional impulse sources are having a major negative impact on marine mammals. Tim Skelton looks at some of the innovative and more environmentally friendly alternatives.
The impulsive sound sources used for seismic oil exploration and monitoring include dynamite blasting and, more commonly, high-powered air guns. But they have also long been blamed for harming the environment and sea life in survey zones. Air guns in particular are responsible for some of the loudest man-made sounds in the sea, sometimes passing 250 decibels (dB). Moreover, as a side-effect they emit high frequency pulses that are not useful for surveying purposes, but which can cause injury and hearing loss in whales and dolphins, as well as disturbing essential behavioural patterns such as feeding, breeding and inter-communication. Such concerns gained wider exposure in September this year when the US Bureau of Ocean Energy Management (BOEM) released a draft environmental impact statement that concluded that oil and gas exploration seismic surveys in the Gulf of Mexico were causing significant harm. It suggested that as many as 32 million marine mammals might potentially be injured and/or harassed, including several endangered species of whale. The draft outlined a number of possible measures designed to mitigate the effects and protect marine mammals and coastal environments. These included banning seismic surveys entirely in certain ‘closure areas’, and reducing activity in others. In parts of the Gulf of Mexico, the agency recommended that energy companies conduct their surveys with observers on hand to monitor the presence of protected species. The American Petroleum Industry (API), the trade association representing the industry in North America, already believes that seismic surveys are highly regulated and says they are performed “only in government-licensed areas where operators make great efforts to prevent potential impacts on marine life.” It has also stated that surveys begin with a ‘soft-start’, gradually increasing sound levels to allow sensitive animals to leave the area. Nevertheless, the BOEM study suggests that more still needs to be done.
Good vibrations There could already be an alternative, far easier and safer solution out there that would avoid the need for drastic measures such as an outright ban. California-based GPUSA has developed an innovative new Marine Vibrator for use in seismic surveys. The company’s patent-pending MV-24 model, originally reported on in the December 2015 issue of InnovOil, has – quite literally – less of an impact on the surrounding environment whilst still providing the desired data quality, or even better. GPUSA says the answer is to keep operating frequencies at low levels that are less harmful to marine life. This means replacing the impulsive sound sources with vibratory sources. “Impulsive sources create significant broadband noise (up to 1,000 Hz), with much of it outside the useable seismic frequency band. But while frequencies above about 100 Hz are typically not seismically useful, they are still blamed for the preponderance of the harm to marine life,” CEO Jim Andersen explains. “A marine vibrator that can impart controlled vibrations over a period of many seconds, as compared to milliseconds from an impulsive source, has many advantages,” he continues. “At the most basic level it must cyclically displace large volumes of water over the seismic band of interest, but it can be limited to [a maximum frequency of] approximately 100 Hz and thus would eliminate the environmental concerns.” In addition, a precisely controlled source that is highly repeatable can even improve imaging results compared to impulsive sources that are far less repeatable. Several competing designs are currently in development as part of the Marine Vibrator Joint Industry Project (MVJIP), which is being sponsored by oil majors ExxonMobil, Royal Dutch Shell and Total, and managed by Texas A&M University’s Global Petroleum Research Institute (GPRI). The project is an ongoing five-year plan to develop, design, construct and test a commercially viable alternative to air guns, and three manufacturers were contracted in 2013 and 2014 to come up with prototypes. According to a paper presented at the American Association of Petroleum Geologists (AAPG) and the Society of Exploration Geophysicists (SEG) conference in September 2016, the parameters of this JIP were for designers to devise systems which had a five-second signal array output, had a harmonic content above 150 Hz when driven with tone in 5-100 Hz range, and which could sweep for 72 hours between maintenance, among other criteria. The three MVJIP solutions are ‘flex-tensional transducers’ (being developed by PGS), ‘dual port, doubly resonant bubble’ seismic sources (the work of Teledyne), and a so-called ‘Moving Coil Transducer’, the idea of General Dynamics subsidiary Applied Physical Sciences (APS). According to research papers, Teledyne’s solution is a “coherent seismic marine sound source technology based on the application of an underwater, gas-filled bubble as a low frequency resonator.” APS’ variation is “an underwater sound projector for producing time-harmonic waveforms comprise[d of] two pistons positioned on either side of a single electro-magnetic force generator,” according to patent filings. In comments made to the AAPG earlier this year, the man behind the JIP, David Burnett, said: “By 2017, we’ll have equipment testing in the open water environment, and the information will be public.” However, while work is now under way and participants are focused on delivering technology before 2020, all three are still several years away from being available for use aboard a typical seismic vessel. Andersen thinks the designers of these systems may be making life too hard for themselves by over-complicating their solutions. “They are trying to adapt conventional high- and mid-frequency sonar technology, and it doesn’t really fit,” he thinks. “Designing a sonar transducer to sweep from zero to 100 Hz is very different from designing one to operate at 1,000 Hz to 10,000 Hz. To get the desired amount of power at these low frequencies requires the ability to move or displace 50 litres or more of water.”
A simpler way? An alternative approach makes the GPUSA system different. According to Andersen, keeping things simple has been the key. “Our marine vibrator uses a rotating crankshaft, connecting rods and ‘pistons’ (stiff transducer face plates),” he says. The basic design is similar to a normal car engine, except that in this case the crankshaft is driven by a servo motor. Servo motor speeds can be controlled simply and precisely, as the drives incorporate positional feedback via encoders or resolvers installed within the motor. This makes different modes such as constant speed, linear ramps, non-linear ramps and stepped-ramp frequency sweeps all possible, either manually or pre-programmed. Using such accurately controlled AC servo motors has a major benefit to performance. “Since [they] have a high and uniform torque from start-up (0 rpm) up to their rated speed, this allows us to eliminate the multiple resonances that are inherent in other designs,” Andersen explains. Moreover, GPUSA’s vibrator incorporates the ability to adjust the displacement of the transducer face plate with frequency in order to maintain a relatively constant Sound Pressure Level (SPL). The company has also developed a way to vary the stroke of the connecting rods dynamically using a separate lead screw and motor assembly, and the design incorporates an internal pressure management system that minimises performance variation with depth. The transducer plates are enclosed within a high-performance rubber tyre for protection, and the vibrator system as a whole is controlled via the latest touchscreen computer technology combined with programmable logic. “Ours is the simplest and most efficient method, and I believe most experts realise it is the only technology capable of meeting the JIP specifications,” Andersen says. Whatever the outcome, the prospects for more environmentally friendly options are bright – and the benefits for operators could be great too. As Burnett remarked: “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.”