Report of the Round Table Session

Young, J.1*, Pajala, J.2*, Bellman, M.3, Wochner, M.4, Sipilä, T.5, Jasny, M.6, Houégnigan, L.7, Mougenot, J.M.8, Cato, D.9, Hedgeland, D.10, and Nimak-Wood, M.11


1 CSA Ocean Sciences Inc., USA
2 Finnish Environment Institute, Finland
3 ITAP, Germany
4 AdBm Technologies, Australia
5 Technical Research Center of Finland, Finland
6 Natural Resources Defense Council, USA
7 Laboratory of Applied Bioacoustics, Technical University of Catalonia, BarcelonaTech (UPC), Spain
8 TOTAL, France

9 Defence Science and Technology Organisation & University of Sydney, Australia

10 BP, UK

11 Gardline Environmental Limited, UK

* Session Chairs and Corresponding Authors; E-mail:


This report can be referenced as:

Young, J., Pajala, J., Bellman, M., Wochner, M., Sipilä, T., Jasny, M., Houégnigan, L., Mougenot, J.M., Cato, D., Hedgeland, D., and Nimak-Wood, M. (2015). Report of the Mitigation Session, Oceanoise2015, Vilanova i la Geltrú, Barcelona, Spain, 10-15 May. (Editors Michel André & Peter Sigray). Retrieved from



Concern continues to grow around the issue of if and how underwater anthropogenic noise maybe affecting marine life. Past studies (e.g., Southall et al., 2007) indicate potential impacts may not solely depend on the level of the noise from a given source. For example, other characteristics such as, frequency, pitch, type (pulsed or non-pulsed), along with factors such as how the noise was perceived by the animal (context) and what activity the animal was performing at that time (feeding, breeding, migrating, etc.) may be of equal or greater importance than noise levels. Therefore, in thinking about mitigation measures we must consider not only an individual localized noise source but also the overall spatial and temporal situation, or in other words, the accumulation of all noise sources and other pressures over longer periods of time and over larger spatial/ecosystem scales. Further consideration should be given to the fact that not all noise sources are equal. For example, in some cases such as seismic and sonar, the sound is intentionally generated and in other cases such as pile driving or shipping it is an unwanted by-product. Mitigations measures may vary depending on the type of operation generating the noise.

Regulatory Landscape

Following the principals of the European Marine Strategy Framework Directive (2008)/56/EC programmes of mitigation measures are established after; a) the definition of what constitutes good environmental status; b) the establishment of environmental targets; and c) the development of monitoring programs. The EU is currently at the step of developing and implementing monitoring programs in order to gather the necessary data to allow them to determine protective and cost effective mitigations. Some member states are also exploring economic incentives, which make it in the economic interest of those using the marine ecosystems to act in ways that help to achieve the good environmental status objectives.

The European Marine Strategy Framework Directive (MFSD) obviously does not extend to non-EU countries but many countries do have their own country regulations, in particular, for seismic and pile driving. In other words, today a single unified set of international underwater noise regulations does not exist for such activities as seismic surveys, pile driving and shipping. This is a topic of discussion that continues to be debated and may be considered for the next Oceanoise meeting in 2017. In the meantime, ocean noise producers, regulators, the scientific community and conservation communities are left with trying to determine the best course of action for noise mitigation by following national requirements, guidelines, etc.


One should note there are minimal regulations concerning man-made underwater noise from shipping but in 2012 the International Maritime organization (IMO) published voluntary guidelines and proposals for reducing underwater noise from commercial ships. In addition, several ship classification organizations have developed “Silent” or quiet-vessel class notations. The aim of which is to encourage and demonstrate reductions in ship noise. Some green certification organizations are beginning to develop their own standards for ports, terminals and shipping lanes. A number of efforts are underway to collect acoustic signatures of existing commercial ships, for the purposes of monitoring and mitigation.

The main sources of ship noise are the machinery inside the ship and propeller cavitation outside the ship. Reduced ship speed, anti-vibration machinery mountings and low cavitation propellers reduce the shipping underwater noise significantly. The primary question is, in the absence of international regulations, how low is acceptable from an environmental and commercial point of view?

Pile Driving

The maximum noise limit set by the German authority (BSH) is 160 dB re 1µPa2.s for Sound Exposure Level (SEL) and 190 dB re 1µPa for Peak Sound Pressure Level (SPL), which must be complied with at a distance of 750 m to the construction site. At least that is the case in the German North and Baltic Sea. The noise reduction is accomplished by a variety of mitigation techniques such as; secondary mitigation measures such as coffer pylons or bubble curtains as well as primary mitigation measures like reduced pile driving energy. The latest mitigation techniques have resulted in pile driving noise reduction of an additional 10 dB to 15 dB re 1µPa with one mitigation system and up to 20 dB re 1µPa with a combination of mitigation measures. Difficulties with the existing noise mitigation measures are currently the water depth. All listed results were produced in water depths of up to 30 m. Results of the bubble curtain in water depths of greater than 40m show that noise reduction ability decreases slightly with increasing water depth. This is a problem for the future since some offshore wind farm projects will be located in water depths between 30 to 60 m.


To minimize the risk of the potential impact of sound from seismic sources, a so-called soft-start or ramp-up method is applied. Ramp-up begins with the smallest airguns in the seismic source array then; the source array output is increased by adding additional source elements over a period of time e.g., 30 minutes. Although ramp-up is widely used, given its practicability, there is little information to show how effective it is. It should also be noted that ramp-up is only one of several mitigation techniques currently being used. Others may include MMO’s, PAM, etc.

After reviewing current ramp-up techniques and in consideration of marine mammal auditory perception, it has been suggested that, in many cases, the rate of increase in sound exposure level (SEL) may be too gradual to be noticed as increasing loudness by the animals and thus may not have the desired mitigation effect. Larger increases in level per step, as used in some ramp-up procedures, may be more effective.

Characteristic to an air-gun array is also the question of potential impact from horizontal directivity. In other words, the radiated sound level varies with direction from the array because of the interference in the acoustic arrivals from different air guns.   Joint Industry Programme on Sound and Marine Life (JIP) modelling suggests that this may be a significant effect on the received level, although the variation in level experienced by an animal as the direction changes is likely to be small, given the slow rate at which the direction is likely to change.

A potential alternate seismic source is under development today and is called “marine vibrator”. Other source development efforts (e.g., Popcorn) are also underway which use existing or modified airgun technology. Marine vibrator technology is not new but improvements in materials and the need for additional types of sound mitigation have been driving the recent development. The output of marine vibrators differs from that of airguns in that they spread the energy out over time in a swept fashion instead of an instant release of energy. Moreover, they offer a substantial improvement in control of the energy at frequencies outside the range of geophysical interest. The hope is that by doing so, there will be less potential impact to the environment by having better control of the source characteristics (e.g., amplitude, frequency, rise-time, etc.).

It is yet to be determined whether marine vibrators have less impact on the environment and whether they can become a viable seismic exploration tool. The interested reader may want to review the OGP Marine Sound JIP report “Environmental Assessment of Marine Vibroseis” by LGL Ltd. and Marine Acoustics Inc. This report can be found at the OGP Marine Sound JIP website,

It should be noted that understanding the environment related questions about marine vibrators is an area of interest for the oil and gas industry Marine Sound JIP, while the oil and gas industry Marine Vibrator JIP will focus on assessing the viability of the marine vibrator as an exploration tool.

Passive Acoustic Monitoring

Today passive acoustic monitoring (PAM) is often used as a mitigation tool, for animals that vocalize, during seismic surveys and pile driving operations. However, it should be noted that PAM has other uses such as when conducting animal density surveys, gathering baseline data, etc.. Having information about animal density, animal location and animal type, allows one to use this information to reduce noise exposure to the animals, making it an effective mitigation tool or to build an overall mitigation strategy. Over the last decade PAM has moved from the research and academic realm to the commercial side. In doing so, significant improvements have been made in DCL (Detection, Classification and Location) capabilities.

Technology Development

Challenges remain with respect to technology developments such as real-time or near-real time PAM, marine vibrators, modified airguns, etc. Continued progress on the development of a particular mitigation technology will depend on factors such as being able to operate safely, improve the ability to mitigate potential environmental impacts be capable of demonstrating operational integrity, and to be cost effective.


Mitigation tools and techniques will be further developed as time passes and as we learn more about the potential impacts to the environment. Not one mitigation tool or technique will likely suffice but instead a “tool box” approach will be required to properly protect the environment while remaining cost effective. Additional scientific studies, cooperation among concerned parties and smart regulations will lead us to a protective and balanced approach.