N° 7 2021 
Mitigation strategies for impulsive noise 
Mitigation measures have so far been well described for different 
impulsive noise sources like seismic surveys and construction 
work (Genesis Oil and Gas Consultants, 2011; OSPAR Commission, 
2014; Feltham et al., 2017; Long & Tenghamn, 2018; Thomsen et 
al., 2019; Verfuss et al., 2019; Bellmann et al., 2020; Koschinski 
& Lüdemann, 2020). Several options are available depending on 
source, site of activity and species of concern. 
and bandwidth compared to airgun arrays, they produce longer 
duration signals with short inter-signal periods and there are 
concerns regarding potential disturbance (Matthews et al., 2021). 
For the clearance of unexploded ordnances (UXO) at sea, alternatives 
to high order detonation such as low order deflagration*® (Cheong 
et al., 2020) are likely to result in less noise and could be a game- 
changer in the clearance of several thousand UXOs littering the 
seabed in the North Sea in particular. Adoption of these by offshore 
operations is increasing, providing an opportunity to gather more 
evidence on their effectiveness and safety. 
To avoid emissions of impulsive sound in the first place, alternative 
methods have been considered across many industries. In offshore 
construction, there are installation procedures that do not need 
to be carried out by impact hammer, including the use of quieter 
systems such as gravity foundations® and suction buckets* 
(Koschinski et al., 2020). However, the installation choice will be 
driven primarily by cost, geology and logistical considerations. For 
some seismic explorations, techniques like vibroseis*® (Feltham 
et al., 2017; Long et al., 2018) could provide an alternative, and 
industry has made significant progress in the last few years in 
developing fully commercial marine vibrators (Feltham et al., 2017). 
However, more evidence is needed regarding the effects on marine 
arganisms. Although these sources have a lower sound pressure 
Temporal and spatial restrictions are used alone or as complements 
to other measures, and are particularly appropriate when source 
mitigation is more challenging and for areas/times that may be 
more sensitive to noise. Sensitive areas include fish spawning 
grounds, marine mammal calving/breeding grounds and areas of 
persistent high densities of marine mammals, while restrictions 
can be either year-round or seasonal (OSPAR Commission, 2009b). 
in the United Kingdom, for example, recent noise management 
advice for harbour porpoise marine protected areas is structured 
around the use of area and time limits to noisy operations UNCC 
et al., 2020). 
ROUTE 
CHAHGE 
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Figure 9. Temporal and spatial restrictions can be used to avoıd aregs/times that may be more sensitive to noise e.g. for spawning, calving/breeding or 
migration, with activities carried out outside those restrictions 
5 Gravity foundations are support structures that use their own weight to hold them in place 
7 Suction buckets use a pressure difference during installation, pumbina water out of the 
bucket to force them to sink into the seahed 
48 \Äbroseis is a seismic technique where a vibration source is used to generate controlled waves 
49 Low order deflagration is where the explosive within the ordnance is burnt, resulting in 1ts 
deactivatian and aualidina detanatiann