Report of the Impulsive Sounds Session

Racca, R.¹*, Lucke, K.²*, Bellmann, M.³, Booth, C.⁴, Cheong, S-H.⁵, Matthews, M-N.¹, Peng, C-M.⁶

¹      JASCO Applied Sciences, Canada
²      JASCO Applied Sciences, UK
³      Institute for Technical and Applied Physics (itap), Germany
⁴      SMRU Consulting, UK
⁵      National Physical Laboratory, UK
⁶      OceanSound Co., Taiwan

* Session Chairs and Corresponding Authors; E-mail: roberto.racca@jasco.com – klaus.lucke@uba.de (Klaus Lucke is now with Umwelt Bundesamt, Germany). Note: Stephen Robinson at NPL, UK, was originally co-chair with R. Racca and contributed to organising the session.

This report can be referenced as: Racca, R., Lucke, K., Bellmann, M., Booth, C., Cheong, S.-H., Matthews, M.-N., and Peng, C.-M. (2023). Report of the Impulsive Sound Session, OCEANOISE2023, Vilanova i la Geltrú, Barcelona, Spain, 22-26 May. Retrieved from https://2023.oceanoise.com

__________________________

Introduction

In the Impulsive Sounds session of the previous edition of the conference, OCEANOISE2017, the speakers had addressed in their presentations four main topics: our understanding of the underwater noise production mechanisms of impact pile driving, and how this knowledge could be used to regulate and mitigate noise; current numerical modelling capabilities to estimate acoustic exposure from impact pile driving, and their experimental validation in the field; looking beyond the water column at potential effects of pile driving on benthic fauna; and the relevance of particle motion to the complete assessment of effects on marine fauna from impulse noise sources. The panel round table that had followed was opened by a lead-in by Christ de Jong about the then newly published first international standard for measurement of radiated underwater sound from percussive pile driving (ISO 18406: 2017), which was an important first step but still left many topics to be addressed. The discussion had then tackled two main issues: the maturity of modelling tools available for estimating potential effects of piling noise (it was agreed that modelling codes had been shown to yield generally consistent and reliable results in relatively controlled test scenarios, but accuracy under real-life variability in conditions might not be nearly as good); and whether the present understanding of sound emission from driven piles did enable accurate forecasting of the pathways of sound coupling so that the most appropriate mitigation technologies could be planned (there was consensus that much work remained to be done in modelling the efficacy of mitigation measures such as bubble curtains especially when deployed at larger ranges from the pile, whereupon the dynamics of the bubble sheet and the propagation of sound through water and seafloor might be uncertain). The round table’s final discussion had focused on the recommended way forward in scientific research efforts: aside from the already mentioned need for comprehensive modelling of the acoustic pathways from driven pile through water, seafloor, and mitigation barriers, emphasis had been placed on better estimation of low frequencies and on the modelling and field measurement of particle motion from pile driving, including an extension of the ISO 18406 measurements standard to include that metric.

The pandemic-prolonged hiatus of six years that followed that conference saw an unprecedented growth in, and planning for, large scale pile driving in the ocean as the movement toward decarbonizing energy production compelled coastal nations to develop bold plans for offshore wind farms development. That in turn triggered a surge in regulatory requirements and the consequent evolution of methods to estimate the sound field from pile driving and its management through mitigation barriers, and to assess realistic levels of exposure for motile marine species dwelling in the area. Furthermore, the surge in use or planned use of large patches of the seafloor to install wind turbines and connecting electrical transmission infrastructure led to the encroachment with areas of post-wartime dumping of unexploded ordnance (UXO) and the consequent need to safely clear it, creating another source of underwater impulse noise if in-situ exploding were the method of disposal. These factors shaped and broadened the range of topics represented in the Impulsive Sounds session at OCEANOISE2023.

Session summary

The six presentations in the session covered three main subjects: strategies for piling noise monitoring during large-scale offshore wind farms construction; numerical modelling approaches for estimating the effectiveness of sound mitigation barriers and for computing the realistic exposure of moving animals in a pulsed sound field from multiple sources; and the understanding, monitoring, and mitigating of noise emissions from disposal of UXO at sea through detonation technology and/or the use of sound barriers. This represented a considerable expansion of scope compared to OCEANOISE2017, reflecting not only the evolution of modelling and monitoring requirements associated with the ambitious development globally of energy production from offshore wind and its related environmental permitting and oversight, but also the collateral need to address in some regions the safe and responsible neutralizing of a historically dormant threat now come to the fore.

OceanSound presented an overview of the challenges that Taiwan had to meet in the development in recent years of several large offshore wind farms, involving the formulation of a monitoring standard for underwater piling noise referring to ISO 18406 but also complying with a variety of local regulations and directives in individual jurisdictions. Although regulatory criteria and monitoring protocols may differ substantially in other world regions, Taiwan’s experience in their practical implementation on a large scale may provide a reference for countries at an earlier stage of offshore wind energy development. JASCO Applied Sciences (UK) described an agent-based modelling approach to estimating the exposure range of animals to a given level of sound from concurrent piling operations, which can be used to define monitoring zones for mitigation in a manner that is realistically accurate because it includes the behaviour of the animal. Still on the topic of mitigation planning, JASCO Applied Sciences (Canada) discussed recently developed sound propagation modelling methods able to estimate the effect of acoustic barriers such as large bubble curtain rings on the impact ranges from pulse sources including pile driving and UXO detonations, in a manner that considers waterborne sound propagation as well as seafloor borne energy that could bypass the attenuation from a barrier deployed in the water column.

Regarding the acoustic impact of UXO disposal, the Institute for Technical and Applied Physics (itap) presented results from the project NAVESS, aimed at characterizing the underwater noise caused by conventional (high order) detonation of ammunition both unmitigated and mitigated using various layouts of single and double Big Bubble Curtains as noise abatement systems, to assess with accuracy their effectiveness under different conditions. Exploring a different pathway to mitigation, the UK National Physics Laboratory (NPL) discussed an extensive field study at UK windfarm sites and a trial in Dutch waters of the acoustic output produced during low order disposal operations, in which deflagration charges induce a slower rate burn of the UXO charge instead of detonating it.

Whether generated by pile driving or an explosion, a pulse sound tends to lose the sharpness of its transient peaks as it propagates to greater distances, in other words, become less impulsive. SMRU Consulting gave a presentation on the project RaDIN (Range Dependence of Impulsive Noise), aimed at providing a more comprehensive understanding of how the impulsive characteristics of sounds from impact pile driving and unexploded ordnance detonations change with increasing distance from the source. The findings of the project are intended to be incorporated into a future tool for impact assessments, as the impulsiveness of a sound can affect its influence on the hearing of a marine animal.

Questions and panel discussion

To set the stage for the panel round table following the presentations, the Chairs gave a brief flashback of what the main topics of the talks and of the panel discussion had been in the same session at OCEANOISE2017 and drew a comparison with the broader scope of issues covered in the current session, in particular the specialized modelling approaches that had evolved for mitigation planning and the newly emerged relevance of UXO disposal noise. Although a few indicative topics for discussion had been preselected and were shown, to better engage a dialogue between the panellists and the audience it was opted to open the floor to questions and let them steer the conversation.

The first theme broached was the definition of impulsive and non-impulsive sounds. The panel acknowledged the difficulty of defining a universal criterion, noting that parameters such as integration time, inter-pulse period, kurtosis, crest factor and several others were relevant and might have to be considered on a case-by-case basis. The key point was that there should not be a sharp demarcation, which regrettably has been the regulatory status quo for far too long (witness the 120/160 dB re 1µPa behavioural criteria as an example) and should be rectified. The question was raised about whether the choice of kurtosis as a measure of impulsiveness did reflect an observed correlation of that metric with auditory impact; some evidence points to it being reasonable, but there is no clear answer for now. It was also asked whether studies about impact of impulsiveness had looked at auditory injury (TTS); some answer may come from the large body of literature and expert opinion being considered in RaDIN.

The panel was then asked to comment on what data gaps had to be filled to enable improvement of modelling methods for offshore wind farms construction piling and especially UXO disposal, to facilitate planning of mitigation strategies and regulatory approval. Responses from panellists included: controlled measurements of (double) Big Bubble Curtains effectiveness using multiple receivers up and downrange, to characterize depth and range dependence of attenuation in the full propagation environment; impulse response of noise attenuation systems depending on environmental factors for more accurate modelling; frequency spectrum of acoustic signal originating from explosive disposal (both high and low order) of different kinds of UXO; and vibration measurements in the seabed.