Since 1995, the geophysical industry has become accustomed to mitigation protocols that aim to minimize the risk of injury to marine mammals. The protocol by the Joint Nature Conservation Committee (JNCC) in the United Kingdom is the most widely recognized example1, though an increasing number of nations have implemented similar guidance. The industry also independently applies best practices in jurisdictions without such regulations2.
Operational mitigation
The basis of these ‘operational’ mitigation protocols is to monitor for marine mammals and other protected species within proximity to a seismic vessel, and more specifically, the source. Trained Marine Mammal Observers (MMOs) equipped with binoculars and Passive Acoustic Monitoring (PAM) systems monitor for the presence of protected species prior to the initiation of the survey, and the start of acquisition is delayed if marine life is in the area. Then, the source amplitude is gradually increased over 20-40 minutes. If protected marine life is detected during acquisition, the source is temporarily shut down in some jurisdictions until it is confirmed that the animals have moved beyond a pre-determined distance deemed to be safe.
Exclusion zones are typically established and monitored around the acoustic source array at 500m as an additional precautionary measure. This further reduces the risk of potential physical harm to marine life. Within this range, sound levels from the source are expected to fall below those likely to cause auditory injury, or ‘Permanent Threshold Shift’ (PTS). Since inception of operational mitigation, significant work has been undertaken to better understand the thresholds for PTS and temporary auditory damage known as Temporary Threshold Shift (TTS) (which is akin to the ringing ears a human might experience from a loud environment like a concert) across different groups of species or ‘functional groups’ determined by the broad range of frequencies used by marine mammals of different types to utilize sound. These exposure criteria are widely recognized by regulators and stakeholders as the thresholds against which to verify the validity of exclusion zones3.
Some regulators have applied increased precaution based on exposure to sound over time. Often based on unrealistic assumptions of static sources and animals (which does not reflect the reality of a survey operation), these assumptions can lead to much larger exclusion zones. These zones can be hard to manage and have unintended consequences for environmental exposure based on additional time spent in the field while not providing additional protection in real-time. This will be the subject of a further article looking in more detail at these issues.
Disturbance
While the scientific consensus is that the risk of PTS is well managed by operational mitigation, the potential for disturbance to different species by a variety of impulsive and continuous sound sources requires a different approach. In Norway, operational mitigation has focused only on the ‘soft-start’ process but has otherwise included annual advice regarding fish spawning areas and cetacean feeding areas, resulting in spatio-temporal restrictions for survey activities4. Driven by conservation objectives such as the achievement of ‘Favourable Conservation Status’ (FCS) under the EU Habitats Directive5 within a European context, the focus is shifting more toward these types of spatio-temporal restrictions in order to limit the exposure of species or areas of habitat that they occupy frequently to underwater sound not just from geophysical surveys but the broader range of sources of underwater radiated noise (URN).
Since 2020, the UK has implemented area-based thresholds (as opposed to limits based on sound levels) within the Southern North Sea Special Area of Conservation (SAC), an area designated for the protection of harbour porpoises. Here, an activity cannot expose more than 20% of the SAC to impulsive sound in a 24-hour period, or more than 10 % over a 6-month season6. A similar approach has also been agreed by the EU, with implementation across EU member states still in development7.
Reducing underwater noise
The direction of travel for the management of URN is toward reductions overall, consistent with objectives of, for example, the Marine Strategy Framework Directive (MSFD)8. As for tackling other issues such as marine litter, the Oslo-Paris Commission (OSPAR) is developing a Regional Action Plan (RAP) with Contracting Parties committed to ‘reducing anthropogenic underwater noise to levels that do not adversely affect the marine environment’. As such, collective actions to reduce URN from all sources are being developed, including looking at what means of abatement exist for different types of sources, as well as overall targets which states will be looking to implement. As with operational mitigation guidance in the past, such developments will inform policy developments in other regions, making this an important topic for industry to continue to focus on, to ensure evidence-driven policy developments.
Looking ahead
Underwater noise continues to be a key environmental issue for industry, with policy having developed over time from being focused on the potential for injury to a more holistic approach considering disturbance to species and the potential impacts of URN on the ecosystem more broadly. While many sources of underwater noise are a by-product of another activity, achieving geophysical objectives requires a certain signal. Industry continues to develop tools and technologies that reduce sound levels, particularly in frequency bands not relevant to a given application. However, it is important that regulators ensure a pragmatic approach, recognizing the excellent track-record of industry in reducing risk to negligible levels and ensuring the continued compatibility of operations and marine biodiversity.
References:
1. JNCC. JNCC Guidelines for Minimising the Risk of Injury to Marine Mammals from Geophysical Surveys.
2. IOGP/IAGC. Recommended Monitoring and Mitigation Measures for Cetaceans during Marine Seismic Survey Geophysical Operations Scope.
3. Southall, B. L. et al. Marine Mammal Noise Exposure Criteria: Updated Scientific Recommendations for Residual Hearing Effects. Aquat Mamm 45, 125–232 (2019).
4. Sivle, L. D. et al. Seismic surveys and the role of scientific advice in Norway. Mar Policy 183, 106918 (2026).
5. European Union. Council Directive 92/43/EEC of 21 May 1992 on the Conservation of Natural Habitats and of Wild Fauna and Flora. (1992).
6. JNCC. Guidance for assessing the significance of noise disturbance against Conservation Objectives of harbour porpoise SACs (England, Wales & Northern Ireland).
7. Sigray, P. et al. Setting EU Threshold Values for Impulsive Underwater Sound.
8. European Union. Directive 2008/56/EC of the European Parliament and of the Council. (2008).
