PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Towards a realistic Approach of Sound Propagation in EIAs: How torepresent moving Sources and moving Receivers
Uwe STÖBER1; Lars O. MORTENSEN2; Frank THOMSEN2
1

DHI WASY GmbH, Germany
2

DHI A/S, Denmark

ABSTRACT
Recent guidelines for marine mammal protection like those in the US or in Denmark impose dose-based
criteria that require to accumulate the noise received by a marine mammal over time. Compared to a staticreceiver, ﬂeeing animals receive a signiﬁcantly lower noise dose because they move away from the zone ofnoise impacts. Animal movement models can therefore be useful to predict more realistic noise impacts onmarine mammals. In addition, noise sources are not always static. During seismic exploration for example,
air guns are towed behind a vessel and the locations of the air gun sounds are changing with the forward
movement of the vessel. Here we present a feasible solution for noise impact assessments that compromises
between realistic representations of mammal behavior and sound ﬁeld on the one hand and computationalresources on the other hand to provide the best possible accuracy in a realistic time frame. The approachconsists of two steps: First, the sound ﬁeld is derived from a number of detailed propagation simulations.
Then, an Agent-Based Model (ABM) simulates animal movement and noise dose accumulation. Our study
shows that noise impact assessments beneﬁt from using a more dynamic approach.
Keywords: Underwater noise, Moving source, Agent Based Model (ABM)

1. INTRODUCTION
Authorities and regulators globally have defined sound exposure criteria to protect marinemammals from man-made noise. Impact pile driving as associated with the construction of offshore
wind farms and seismic exploration undertaken for example by the oil and gas industry are known as
major high level underwater sound sources.
Criteria often permit to assess the impact based on broadband levels or individual frequencies ( 1, 2)
or even require using broadband levels (3) in order to limit the complexity of the assessment. Recent
guidelines motivate however more detailed analysis that account for the susceptibility for noise
induced hearing injury at different frequency bands (2) which requires to resolve the soundpropagation spectrally.
Many impact assessments are based on the assumption of static sources and static receivers. The
impact area in form of sound fields emanating from the source is then superimposed on mapped animaldensities (based on literature assessments or baseline monitoring). The number of animals affected inthe ‘impact zone’, is calculated and treated as a ‘take’ from the population. Regulators use the numberof takes as the basis for their decision whether or not to grant a license to operate.
Real non-moving sources like pile-driving are of course well represented by the static assumption,
but moving sources like air gun arrays are often represented by a static sound field that extrapolates thecontours of the simulated sound fields at the beginning and the end of a survey line. For theconsideration of single noise event metrics like peak sound pressure levels or single pulse soundexpose levels (SEL), this approach appears justified, but for dose-like parameters like the cumulated
sound exposure level (SEL c), the changing distance between source and receiver while the source ismoving needs to be taken into account.
The assumption of static receivers on the other hand, i.e. non-moving animals, means that animals
ignore any approaching noise or increasing noise levels (soft-start) while at the same time
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usto@dhigroup.com
lamo@dhigroup.com

experiencing effects on hearing. It may however be expected that animals try to escape that situation,and thus for moving animals, the number of affected individuals is expected to be lower than for staticindividuals. The animal movement can be modeled by agent-based models (ABM) that account for thebehavioral response of each “agent”.
Here we show how both approaches are applied in EIAs, finding a compromise between the mostrealistic description and computational constraints.

2. MOVING SOURCE MODEL
The moving source model is based on static detail simulations at a number of representativelocations. For an ideal coverage such a static model would be required at each individual air gun shot
location. Depending on the environmental conditions, especially the complexity of the bathymetry, thenumber of detail simulations can be (and for computation time has to be) drastically reduced. For a
totally flat sea floor and bot temporally and spatially constant environmental conditions, one detailsimulation would be sufficient. For typical situation with limited variability of water depth andboundary conditions, a number of three to four details simulations is considered representative.
Each detail simulation should provide a noise map related to a single air gun shot . In their projectwork, the authors use the numerical underwater noise modelling software MIKE Underwater Acoustic
Simulator (UAS) that is part of the current MIKE 2019 release. The model focuses on the noisepropagation in the far field with the aim of providing a basis for conducting a risk assessment ofenvironmental noise impacts. UAS applies the range-dependent acoustic model (RAM) algorithm
developed by Collins (4). It accounts for the change in speed of sound and volume attenuation in thewater column, and provides the option to apply the empirical model by Francoi s & Garrison (5, 6)
which is based on information on salinity, temperature and pH in the water column. Furthermore, UASincludes sound propagation in the seabed. The bottom description is based on a number of
constant-density and constant thickness layers. The sound source is modelled as an omnidirectional
point source. A detailed description of the underwater acoustic model can be found in the ScientificDocumentation for UAS in MIKE (7), including scientific model validation and model assumptions.
MIKE UAS produces 2D transects that are combined into a 3D sound field. Maximum over depth
levels are used to obtain sound maps at the representative locations.
The results of the detail simulations are mapped to individual source locations. Different mapping
options are possible, for a test case depth-weighted averages were found to yield the best results. Dueto storage and computational constraints, the ship track is re-sampled on a coarser grid (500–1000 minterval). Depending on vessel speed, each length interval corresponds to several minutes in time thatthe sound source needs to pass that length. With air gun pulse repetition rates of about 1 shot every 10s, several air gun shots will be fired during that, say 5-min period. For cumulated SEL this re-samplingis accounted for with a 5-min cumulation. The re-sampled values are in the following referred to as
5-min values for simplicity. The 5-min cumulative SEL are derived using(1)
SELcum = SEL + 10 log10 𝑛
where SEL is the single pulse sound exposure level and n is the number of pulses in the defined
period (e.g., 5 min).
For each 5-min step, sound maps with peak and rms SPL as well as cumulated and non -cumulated
SEL can be interpolated from the detail simulations. The result are sound map time series for eachsound level. The time series are then interpolated on a common regular grid for further analysis. Forsound levels that can be related to single air gun shots like the peak and rms SPL but also single -shot
SEL, the maximum with respect to time of these sound map time series is derived. For cumulated
sound levels, the 5-min SEL of the past 24 h (or any other duration) are cumulated at each grid point,then the maximum over the complete time series is computed.
The maps with maximum-over-time sound levels indicate the impact on the marine environment(e.g. disturbance or injury for different hearing groups within specific impact areas). The time series
themselves provide an input for the moving animal ABM.

3. MOVING RECEIVER MODEL
Using ABM, the movements of a number of agents (animals) in response to noise and otherenvironmental factors can be modelled and results post-processed statistically. Representing
avoidance responses of individual marine mammals in relation to sound is th us achievable on fine1899

spatiotemporal scales (8).
DHI’s own software ABM lab (for more information, please see (https://www.mikepoweredbydhi.com/products/abm-lab) handles typical marine mammal behavior during, for example during
migration and feeding. For the simulation of migration, a number of agents is continuously released.
The agents have a defined movement speed that depends on the group type, e.g., mother and calf,
single individual. During feeding, the movements of the agents can be set to a correlated random walkprinciple (e.g., 9).
ABM permits to set environmental conditions like the time series of the sound field and define abehavioral reaction to the sound field. Two responses are typically defined: avoidance anddisplacement. Avoidance behavior assumes that marine mammals will try to avoid the noise, but will
return 1 hour after the noise ends (e.g., 10). The avoidance can, for example, be set to a sigmoid
dose-response curve (e.g., 11, 12). The number of takes is calculated by post processing of the ABM
output, where agent occurrence in one of the impact zones is calculated as a take. Displacement istreated in the same way as avoidance, but here the mammals are assumed to stay away from the noisefor 24 hours, which corresponds to the lower estimate of displacement time due to pile driving for avery sensitive species, the harbor porpoise (Phocoena phocoena; 10).
In the dynamic assessment, the number of takes is derived by the agents entering a zone of impact.
Once an individual agent enters a zone of impact, this is counted as a take. The counting is undertaken
for the entire simulation period when the noise takes place. Individual agents can only be ‘taken’ once
during the noise period (e.g., one continuous seismic survey or impact pile-driving of one pile). Thus,
individual agents can be taken on subsequent noise periods but not more than once in the same noise
period.

4. TEST CASE PHASE
The moving source model has already been applied in projects providing a more detail to thedescription of the actual processes like the proper accumulation of the noise dose for changing
distances to the sound source. An ABM Demonstrator is currently developed for the North Atlantic
right whale (Eubalaena glacialis) using simulated pile driving near a planned offshore wind farm site
off the US East Coast. An assessment of the number of takes will be carried using the most recent setof impact criteria. Both the static and the dynamic (ABM) approach will be applied following several
scenarios. The goal is to compare the number of takes between the two approaches.
One could argue that it is premature to use ABM simulations using data on avoidance anddisplacement as information of response to sound is limited in right whales and many other cetacean
species. Yet, despite the scarcity of data, EIA investigators have already begun to use impact criteria toestimate a behavioral response zone (see for example 13). Furthermore, Skjellerup et al. (1) and NMFS
(2) recommend the use of response movement for marine mammal EIAs. This new paradigm is alreadyshifting towards dynamic approaches, and industry as well as regulators might want to use the mostcomprehensive and reliable technology when applying it. Furthermore, thanks to a variety of recent
studies, data on behavioral response of marine mammals to underwater noise exposure are rapidly
increasing (14). The way forward might be to apply a more holistic approach using the static and the
dynamic approach in EIAs to provide regulators with a range of outco mes and scenarios including
uncertainty quantification. Then, whenever data become available, the static approach can be
gradually phased out to make room for more validated animal response models.

5. SUMMARY AND CONCLUSIONSMoving source and moving receiver models have been introduced. Both models promise a more
realistic representation of the noise transmission and accumulation process than the static approaches.
While the application of the moving source model focusses on seismic studies of the oil & gas industry,
the moving receiver model useful for any type of noise that might impact marine life including seismicstudies but also construction work with impact pile driving.

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