Abstract
A seismic survey conducted in the proximity of the nearshore feeding grounds of gray whales Eschrichtius robustus off northeastern Sakhalin Island, Russia, required the develop- ment of a comprehensive monitoring and mitigation plan. Prior to the execution of the seismic survey, the sound levels from the airgun array source were modelled for all seismic acquisition lines. This yielded estimated shoreward boundaries for an assumed disturbance threshold of 156 dB re 1 µPa 2 -s sound exposure level (SEL) that allowed an assessment of which lines would require more stringent mitigation. To enable prediction of the sound front under variable propa- gation conditions, a set of acoustic footprints with a realistic range of parameters was pre-com- puted. In the field, an acoustic monitoring network of 9 bottom-deployed stations with tethered radio buoys transmitted full waveform data to a shore-based monitoring post. The signals were processed to yield pulse levels for comparison to the model predictions. The appropriate footprint was selected on the basis of the pulse levels received during the initial minute of a seismic line and communicated to observation teams. To further ensure the sustained accuracy of the selected foot- print during a line acquisition, the modelled levels were compared in real time to the measured readings as the source moved past the line of monitoring stations; they were consistently found to match the incoming received sound levels within an accepted tolerance of 3 dB. This selection approach resulted in the best available estimation of acoustic exposure even through significant temporal changes in the hydrological conditions.
Highlights
Airgun arrays are the primary technology employed for offshore hydrocarbon exploration
The depth of planning, extent of validation and complexity of infrastructure and analytical processing associated with the acoustic monitoring of the 2010 Astokh 4-D seismic survey are likely unmatched by any other operation of its kind executed to date, a fact justified by the small population size of the primary species of concern and made possible by the conformation of the region where the activities took place (N owacek et al 2013, Bröker et al 2015)
Having the opportunity to characterize the sound field from an airgun array operating in the same region well ahead of the seismic survey operation provided significant information about the end to end accuracy of the source and propagation modelling at varying azimuthal angles
Summary
Airgun arrays are the primary technology employed for offshore hydrocarbon exploration. These arrays emit impulsive pressure signals, the reflected signals of which are used to image sub-bottom seafloor layers. The. A population of gray whales is known to feed during summer in the waters where Sakhalin Energy Investment Company (SEIC) carried out its 2010 4-D Astokh seismic survey, N E Sakhalin Island, Russia. It was recognized that airgun sounds from the 2010 survey would have the potential to cause disturbance to the gray whale feeding activities. A detailed acoustic monitoring study formed part of the monitoring and mitigation plan designed by SEIC and the IUCN ’s Western Gray Whale Advisory Panel (WGWAP) through its Seismic Survey Task Force to minimize survey impacts (IUCN 2008b, 2009, 2010, Bröker et al 2015). The acoustic monitoring infrastructure included a line of telemetric real-time bottom-anchored buoys transmitting full waveform data to shore, an evolution of a conceptually similar design used to monitor a seismic survey in an adjacent region in 2001 (Rutenko et al 2007)
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