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Sound generators used in scientific seismic surveys - calibration and modeling

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Breitzke, M. (2006): Sound generators used in scientific seismic surveys - calibration and modeling , International workshop "Impacts of seismic survey activities on whales and other marine biota", 6-7 Sept., Dessau, Germany. .
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Academic research in the Southern Ocean comprises both high-resolution reflection seismic surveys to study - for instance - the depositional history of fine-scale sedimentary structures and lower-resolution, deep-penetrating reflection and refraction seismic surveys to study - for instance - large scale crustal structures. These studies are usually embedded in research programs focussing on topics like the geodynamic evolution and the plate tectonic, paleoceanographic and climatic history of the Southern Ocean. Single airguns or airgun arrays of small size and volume and single- and multi-channel streamers are usually used as sound sources and receivers for high-resolution reflection seismic surveys, whereas airguns and airgun arrays of larger size and volume and ocean-bottom-hydrophones and seismometers and single- and multi-channel streamers are usually applied for lower-resolution , deep penetrating reflection and refraction seismic surveys. To ensure that these research activities do not affect marine wildlife and particularly marine mammals in the Southern Ocean adversely knowledge of the sound pressure field of the seismic sources is essential. Therefore, as an example, a broadband marine seismic source calibration study conducted with R/V Polarstern at the Heggernes Acoustic Range, Herdlefjord, Norway in October 2003 is presented here. The objectives were (1) to determine the spatial distribution of the sound pressure levels emitted by the airguns and airgun arrays available at the Alfred-Wegener-Institute for Polar and Marine Research, Bremerhaven, Germany in October 2003, (2) to determine the frequency bandwidth, the spectral peak level and the amplitude decay at higher frequencies, and the cumulative and total energy of the different source signatures, (3) to determine the theoretical, back-projected nominal source levels at 1 m distance from far-field measurements assuming a spherical amplitude spreading, (4) to determine radii, within which according to the presently applied thresholds and the current scientific knowledge marine mammals might possibly experience behavioral or physiological disturbance or physical injury due to the received sound pressure levels.Up to now thresholds defined by the National Marine Fisheries Service (NMFS), USA have often been used. According to these regulations received levels greater than 180 dBrms re 1 µPa might possibly cause hearing effects like temporary threshold shifts (TTS), and received levels greater than 160 dBrms re 1 µPa might possibly lead to behavioral disturbances like avoidance of the sound source for cetaceans. For underwater pinnipeds received levels are allowed to be 10 dBrms higher. Furthermore, recent studies on mid-frequency cetaceans like bottlenose dolphins (Tursiops truncatus) and white whales (Delphinapterus leucas) have shown that in addition to the (rms-) sound pressure level the signal duration and energy plays an important role whether and to what extent TTS is induced. Therefore, a dual criterion which takes both the maximum peak pressure level and the total energy flux level (= sound exposure level (SEL)) into account is recommended as improved, science-based mitigative tool, and a 90% energy approach is recommended for the derivation of the signal duration. During the 2nd meeting of the Marine Mammal Commission in 2004, the Noise Exposure Criteria Group introduced first levels for such a dual criterion which take the different characteristics of impulsive signals (e.g. seismic airguns) and quasi-monofrequency tones (e.g. sonars) into account, and defines that TTS is potentially induced if either a peak pressure of 224 dB re 1 µPa or a SEL of 183 dB re 1 µPa2s for impulsive signals or 195 dB re 1 µPa2s for quasi-monofrequency tones is exceeded. For underwater pinnipeds levels are defined to be 20 dB lower. To give a complete overview, here radii are presented for both the rms-level and the dual peak pressure and SEL-based criterion, using the presently known thresholds. It is worth to mention that the 195 dBSEL threshold for quasi-monofrequency tones is based on many consistent TTS measurements on bottlenose dolphins and white whales and is therefore rather well established, whereas the 224 dB0-pk and the 183 dBSEL threshold for impulsive signals presently relies on only one measured TTS induced in a white whale by a watergun signal and is therefore possibly subject to change in future, when additional data and/or new scientific knowledge is available.To determine the spatial distribution of the sound pressure levels during the calibration survey each airgun (array) was shot along a line of 2 - 3 km length running between 2 hydrophone chains with receivers in 35, 100, 198 and 263 m depth. A GI-Gun (2.4 l), a G-Gun (8.5 l) and a Bolt PAR CT800 (32.8 l) were deployed as single sources, and 3 GI-Guns (7.4 l), 3 G-Guns (25.6 l) and 8 VLF-Guns (24 l) as arrays. The measurements are complemented by a modeling approach for an 8 G-Gun (68.2 l) and an 8 G-Gun+1 Bolt PAR CT800 array (100.1 l). The data analysis was based on the "SEG Standard for Specifying Marine Seismic Energy Sources" and includes a determination of the peak-to-peak, zero-to-peak and RMS-amplitudes, sound exposure levels and amplitude spectra as function of source-receiver distance.The amplitude vs distance graphs, analyzed for the 4 hydrophone depths, show the typical directivity of marine seismic sources. Due the destructive interference of the direct wave and the ghost reflection, amplitudes almost vanish close to the sea surface and are highest in several hundred meters depth ("Lloyd mirror effect"). A comparison between the amplitudes recorded during approach and departure reveals a shadowing effect of Polarsterns's hull. Amplitudes recorded at the same source-receiver distance are lower during approach than during departure indicating that the ship's hull deflects sound propagation forward the ship. Mitigation radii derived from the amplitude vs distance graphs of the deepest hydrophone for the 180 dBrms level vary between 200 - 600 m for the measured single airguns and between 300 - ~1300 m for the measured and modeled airgun arrays. Extrapolated source levels range from 224 - 239 dB0-pk re 1 µPa @ 1 m for the single airguns and from 232 - ~250 dB0-pk re 1 µPa @ 1 m for the airgun arrays. Spectral peak levels occur below 100 Hz, amount to 182 - 194 dB re 1 µPa/Hz @ 1 m and decrease by ~30 dB re 1 µPa/Hz within the 1 kHz range, and by ~50 - 60 dB re 1 µPa/Hz within the broadband range up to 96 kHz. A first modeling approach of source directivities based on the assumptions of deep water and a homogeneous water column shows slight differences between the amplitude decay curves of the single G-Gun signals recorded at the 2 deepest hydrophones and more pronounced discrepancies for the recordings at the 2 shallow hydrophones. One possible explanation for these discrepancies is a stratification of the water column. Further studies which replace the assumption of a homogenous water column by a depth-dependent sound velocity profile in the modeling approach are necessary here.

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