Modeling Propagation of Seismic Airgun Sounds and the Effects on Fish Behavior

High activity of seismic surveying in Norwegian waters has caused concerns about the impact the acoustic noise from the seismic airguns may have on marine life. There is evidence that this noise can cause reactions on the behavior of the fish resulting in reduced catches. To mitigate the problem and the conflict of interest between the fishing industry and the seismic exploration interest, the Norwegian Petroleum Directorate (NPD) commissioned SINTEF Information and Communication Technology (ICT, Trondheim, Norway) and the Department of Biology, University of Oslo (Oslo, Norway) to develop an acoustic-biological model to predict the impact of seismic noise on the fish population. The ultimate goal is to develop an acoustic-biological model to use in the design and planning of seismic surveys such that the disturbance to fishing interest is minimized. This acoustic module of the model is based on ray theory and can deal with range-dependent bathymetry and depth-dependent sound-speed profiles. The bottom is modeled as a sedimentary fluid layer over a solid elastic rock and the model requires the thickness and seismoacoustic properties of the sediments layer and the rock with compressional speed, shear speed, and absorption. The model simulates the total sound field, both in the time domain and in the frequency domain, out to very large distances. Calculated sound exposure levels are compared with startle response levels for cod. Preliminary conclusions indicate a required distance in the range of 5-10 km, but dependent on the depth and the season. In additions, under certain conditions, there will appear regions with hot spots where the sound level is significantly higher due to caustics and focusing of sound. Modeled results are compared with results obtained from a joint seismoacoustic survey conducted in summer 2009 at Vesterålen-Lofoten area (Nordland VII). In this experiment, signals were recorded at fixed hydrophone positions as the seismic vessel approached from a maximum distance of 30 km toward the receiving positions. The same situation was modeled using available geological and oceanographic information as input to the acoustic model. The agreement between the real and recorded signals and the model results is good. This indicates that in the future acoustic-biological models may be used in the design and planning of seismic surveys such that the disturbance to fishing is minimized.