Lessons Learned in Assessing Underwater Noise Potential Impacts for an Offshore Seismic Survey in Southern Adriatic Sea
Authors: De Stefano Marilena , Rusmini Marco , Benson Thomas , Torchia Giovanni , Sordini Eugenio , Nucci Maria Estella , Bertolini Paola 
Published: March 2020

Within the Environmental Impact Assessment of an offshore 3D Seismic Survey in Southern Adriatic Sea, a comprehensive approach to address potential underwater noise impacts was developed to protect marine fauna. The study was based on applicable international guidelines about underwater noise such as ACCOBAMS, JNCC, and IAGC. The approach allowed to define, and agree with local authorities, consistent mitigation measures to protect marine fauna species in the study area.

Primary data on marine mammals, sea turtles and fish species were collected and integrated with secondary data from literature. An underwater sound propagation model based on the Range dependent Acoustic Model (RAM), producing 3D sound maps, was used to simulate the noise propagation of the 5085 cu. in. seismic array, consisting of 24 airguns at a depth of 8 m. Broadband metrics of cumulative SEL (Sound Exposure Level) and SPL (Sound Peak Level) were used to assess the distance from the source within which TTS (Temporary Threshold Shift) and PTS (Permanent Threshold Shift) might have occurred. The simulation considered potential impacts on cetaceans, sea turtles, pinnipeds and fish species, based on internationally recognized sound thresholds for TTS and PTS (i.e. NOAA). The model indicated an exclusion zone of 700 m to be implemented around seismic sources to avoid permanent injuries or deaths of the considered species. A model sensitivity analysis as well as a measurement test in the field were performed prior to the commencement of the seismic survey in order to validate the calculated exclusion and verify the validity of the model in other periods of the year. The field validation test was performed prior to the survey by recording airguns emissions along a 7 km streamer. The measured sound levels were lower than those predicted confirming the validity of the previously identified 700 m exclusion zone.

Targeted mitigation measures were implemented in line with all considered guidelines, including Marine Fauna Observers and Passive Acoustic Monitoring. Starting from a deep analyisis of the international guidelines for the protection of the marine fauna, the selection of the most suitable mitigation measures was then driven by the local operational context and the national legislation. Local authorities and scientific bodies were involved in the mitigations design process; an Emergency Response Plan, including a Stranding Action Plan, was drafted and approved by competent authorities to prevent and manage potential incidents.

The novelty of this study lies in the use of a robust modelling tool as a basis to define a series of mitigation measures based on a rigorous precautionary approach, such as the development of an Emergency Response Plan and a Stranding Action Plan. This approach was developed in close collaboration with in-country authorities and scientific bodies. The consistency of the approach reassured local and international stakeholders regarding the mitigation of potential underwater noise impacts of the seismic survey on marine fauna species in the area.