One of the most ubiquitous sounds in the sea is the noise generated by trans-ocean shipping. Cavitation and turbulence generated by propellers is the dominant noise source, but also transmission of mechanical noise through a ship’s hull. Some great work is being done by Chris Clark’s lab at Cornell and by John Hildebrand’s lab at Scripps to understand the impacts of this problem, and the International Maritime Organization (IMO) has come up with noise standards and recommendations to quiet down the merchant fleet, but as long as there is international commerce in goods there will be shipping noise.
Shipping NoiseOcean Enterprise
Seismic Airgun Surveys
Since the end of “easy oil” the fossil fuel industries are driven ever deeper into offshore oil and gas deposits. The industry as a whole contributes a substantial amount of noise to the sea. Second only to shipping noise in ubiquity is the noise of seismic airgun surveys. The surveys are conducted by trawling arrays of airguns behind a ship which explode every 10-15 seconds. The noise permeates the seafloor and the signal that reflects back is deciphered to portray the substrates below. These surveys can often be heard thousands of miles from the source and are known to disrupt whales and fish in various ways, including killing them. But it is more likely that the common disruptions from airguns are harder to determine – such as degradation to hearing, habitat displacement, stress, and migratory disruptions. The current global survey fleet includes close to 100 vessels, perhaps half of them working at any time around the world.
Seismic airgun bubble pulse and oscillation:
Seismic Airgun SurveysOcean Enterprise
Seafloor fossil fuel processing
As fossil fuel exploration and production moves into ever deeper waters, developing technologies are moving much of the processing down to the sea floor. Hydrocarbon deposits are not simple pools of oil or pockets of gas; rather they are typically an untidy mix of oil, gas, brine, and solids. These need to be separated, the useful product extracted and the waste products dealt with in some manner. Deposits are also often under enormous pressures. The wellhead pressure of the BP-Horizon disaster was in excess of 13,000 psi (and why stopping the flow was difficult). Handling multi-phase materials (solids, liquids, and gasses) at enormous pressures is not a formula for “quiet.” Equipment placed on the seafloor to handle the tasks of separation, reinjection, and flow control all produce some attendant noises. While none of these processes have yet been characterized or measured, seafloor equipment is being deployed that are akin to setting up small cities on the seabed. If these were in an airborne environment the noises would be attenuated within a reasonable distance. That they are in the sea means that whatever noises they generate will be heard at far greater distances
Seafloor fossil fuel processingOcean Enterprise
Low-frequency Active Sonar came out of the work of the brilliant physical oceanographer Walter Munk. He had been working on the transmission of ‘phase coherent’ long wavelength signals since 1978. Taking advantage of the efficiency with which water transmits acoustical energy, and the far reach of low frequency, long wavelength signals, the idea was to create sounds in the sea that could be heard at long distances and carry decipherable information. The concept went through a number of iterations including the Herd Island Feasibility Test (HIFT, 1991) which was the first sound heard around the world. The HIFT results were relayed into Acoustic Thermography of Ocean Climate (ATOC, 1995), and finally Low Frequency Active Sonar (LFAS). From a geo-physics perspective the idea is great, and the ability to transmit information around the globe would be particularly useful in surreptitious communication to tactical submarines. From a biological standpoint it may not be that great because these noises may mask important communication and navigation cues for baleen whales. These programs seem to have run their course, and while there may be future needs for a full scale LFAS program, currently (as best we can tell) marine basin-scale communications programs are not seriously compromising the ocean bio-acoustic habitat.
Low-Frequency SonarOcean Enterprise
Mid frequency military sonar has proven one of the most troubling acoustical assaults on marine life, particularly associated with catastrophic strandings of marine mammals. The most common victims have been odontocetes – toothed whales and dolphins, but Minke whales have also washed up on the shore after the use of these sonars. “Mid Frequency” is considered between 1 kHz and 10 kHz – within the human auditory band. When noises have been correlated to events they have been ranging sonars – extremely loud chirps and tones used for navigation and surveillance. But increasingly more complicated signals are being deployed that may have varying impacts on various animals.
Mid-Frequency SonarOcean Enterprise
The ocean is largely opaque to radio frequency energy so communications underwater either needs to be transmitted by wire, or by sound. Increasingly as military and industrial processes take place in the ocean they will use various acoustical communication signals. Some of these signals are designed to transmit text, others to transmit control and condition data. Because most of this data transmission is “digital” it uses a code of “ones” and “zeros.” These data streams do not sound pleasant at any volume, but at ear-shattering volumes they are even more problematic. Mid frequency signals (between 1kHZ and 10kHz) are designed to work in settings up to 25km. High frequency (20kHz – 65 kHz) are used up to 10 km. Higher frequency (100kHz and up) are used in close proximity up to 2 km. Mid frequency signals are in the range of human hearing, the higher frequencies are in the hearing range of dolphins, porpoises, and seals.
Communication SignalsOcean Enterprise
Offshore Energy: Wind and Tidal Swings
Increasingly energy is being converted from natural sources such as wind and tidal swings. While these sources are marked improvement over fossil fuels in terms of global impacts, they come with their own set of problems; noise being one. From installation through operation we can anticipate various noises to be generated. Installation often requires anchoring some structure to the seafloor. This is most often done with piles driven into the substrate. The piles are typically hammered in, transmitting a significant amount of acoustical energy into the surrounding environment. One of the big concerns with wind turbines is the noise of the gearbox being transmitted down the mast into the water. There are ways of decoupling this, but at some cost. Another concern is the “thwap” noise generated by the tip vortices as they pass by. While the noise of one turbine may not be problematic, wind farms are being planned and installed which will include hundreds to thousands of turbines. These large aggregations spread across hundreds of square kilometers of the ocean may create soundfields that distract or disrupt acoustical adaptations which have evolved over millions of years. Tidal technologies may bring their own sets of noises: The creaking of thousands of hinges, cavitation behind countless propellers, the thrashing of hundreds of swell-capture devices will all have some acoustical impacts. It remains to be heard whether they are a problem or not. But we do know that fossil fuel is killing the planet and we must weigh in as to what (or how) we are willing to sacrifice to keep up with our energy demands.
Offshore Energy: Wind and Tidal SwingsOcean Enterprise
Acoustic Harassment Devices
Marine aquaculture is a rapidly developing field. The protein conversion inefficiencies notwithstanding, the sea-pens and enclosures are also targets for marine life – mostly seals and sea lions, that don’t miss the fact that if they can broach the barriers they could be on “easy street.” In order to protect their enterprises marine fish farms employ “acoustic harassment devices” to discourage predation by the resident carnivores. As to be expected these devices sound nasty to the target predator. These devices displace the normal habitat of any animal that can hear them. There is naturally spill-over, so in addition to marine fish farms competing with wild animal food sources to feed their stock, they also disrupt wild animal’s acoustic habitats.