ERM's Mike Fraser and Steve Mitchell describe how noise propagation modelling was key in assessing how a seismic survey could impact marine fauna in the North Sea and helped obtain consent for the procedure
A 3D seismic survey is to be carried out in UK and Norwegian waters to explore for hydrocarbons in the south of the Statfjord licence block. Written consent to undertake the survey is required from the secretary of state under the Offshore Petroleum Activities (Conservation of Habitats) Regulations 2001, as amended in 2007, and an application has to be made to Decc to obtain this.
Information regarding environmental issues must be supplied including the effects of underwater noise. Potential disturbance and damage to the hearing of whales, dolphins, porpoises and turtles from underwater noise generated by air guns was a key issue. ERM prepared the necessary environmental impact assessment (EIA) justification for Statfjord Licence Group.
Our approach
In line with UK guidelines, a study was carried out to identify whether any marine species were likely to be significantly affected by the seismic survey. In particular, it examined whether the survey would cause deliberate disturbance, injury or death of any marine species protected under European legislation.
The study provided a description of the proposed survey, the baseline conditions and an assessment of the potential impacts from planned and accidental events, including: emissions to air; discharges to sea; the physical presence of the survey vessel and support vessels; and noise generated by vessel engines and air guns. Most potential impacts were identified as not significant, but further assessment was needed on underwater noise disturbance.
Marine noise exposure modelling
Underwater noise propagation from the seismic air gun array was predicted using ERM’s marine noise exposure model.
The model has been developed to simulate the propagation of sound taking into account the local marine environment, including water temperature, salinity, bathymetry, seabed absorption and the underlying bedrock characteristics, all of which affect the propagation or reflection of sound waves.
Sound propagation is modelled down to frequencies of 10Hz to address the hearing range of all relevant species. After defining the local environment, the model produces transmission-loss results for a set of vertical transects through the study area. These are used to compute frequency-weighted sound levels as a function of water depth around the noise source.
This process yields instantaneous noise levels. However, to consider disturbance and potential damage effects fully, it is necessary to also consider the dose of noise an animal will experience over time.
To do this ERM has developed a noise exposure model that sums the noise levels as an animal moves through the water. In this case a “swim away” model was used (see figure above) assuming the animal moves away from the noise source at a given speed at a depth where the noise level is highest.
This process allowed a cautious, but robust, calculation of the noise dose and the impact on the various species known to be in the area of the survey activity.
The likelihood of animals being close enough to the noise source to be significantly affected was then estimated from known animal sightings and population data for the area. A suitable soft-start process to the seismic survey was adopted to give sufficient time for animals to move away from the exclusion zone so that the potential for significant effect was minor.
Outcomes and lessons learned
In keeping with other projects, the modelling indicated that noise from seismic operations can potentially lead to impacts over a wide area unless mitigation measures are adopted.
It is important to model the noise impact zone taking into account site-specific factors that affect noise propagation to ensure that effects are realistically assessed.
It is also necessary to consider animal movement within the sound field, which affects the accumulated noise exposure and potential for injury.
To assess the potential for an offence, the likelihood of animals being sufficiently close to the airgun array must be taken into account. There is inherent uncertainty in this process even if a detailed study of the marine baseline is available. It was critical for marine biologists and acousticians to work together so that robust assumptions could be made.
With appropriate good practice mitigation consent was given in time for the survey to proceed on schedule.
Mike Fraser ([email protected]) is a principal consultant at ERM and Steve Mitchell ([email protected]) is a technical director at ERM.
