Modelling sound propagation in the Southern Ocean to estimate the acoustic impact of seismic research surveys on marine mammals

TitleModelling sound propagation in the Southern Ocean to estimate the acoustic impact of seismic research surveys on marine mammals
Publication TypeJournal Article
Year of Publication2010
AuthorsBreitzke, M., and Bohlen T.
JournalGeophysical Journal International
Volume181
Pagination818-846
ISBN Number0956-540X
EndNote Rec Number1164
Keywordsacoustic properties, air gun, Antarctica, Bellingshausen Sea, broad-band calibration, computational seismology, continental-margin, controlled source seismology, exposure, scholte-wave dispersion, variability, wave propagation, Weddell Sea, West Antarctica, whales physeter-macrocephalus
Abstract

P>Modelling sound propagation in the ocean is an essential tool to assess the potential risk of air-gun shots on marine mammals. Based on a 2.5-D finite-difference code a full waveform modelling approach is presented, which determines both sound exposure levels of single shots and cumulative sound exposure levels of multiple shots fired along a seismic line. Band-limited point source approximations of compact air-gun clusters deployed by R/V Polarstern in Polar Regions are used as sound sources. Marine mammals are simulated as static receivers. Applications to deep and shallow water models including constant and depth-dependent sound velocity profiles of the Southern Ocean show dipole-like directivities in case of single shots and tubular cumulative sound exposure level fields beneath the seismic line in case of multiple shots. Compared to a semi-infinite model an incorporation of seafloor reflections enhances the seismically induced noise levels close to the sea surface. Refraction due to sound velocity gradients and sound channelling in near-surface ducts are evident, but affect only low to moderate levels. Hence, exposure zone radii derived for different hearing thresholds are almost independent of the sound velocity structure. With decreasing thresholds radii increase according to a spherical 20 log(10) r law in case of single shots and according to a cylindrical 10 log(10) r law in case of multiple shots. A doubling of the shot interval diminishes the cumulative sound exposure levels by -3dB and halves the radii. The ocean bottom properties only slightly affect the radii in shallow waters, if the normal incidence reflection coefficient exceeds 0.2.
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