Monitoring the gray whale sound exposure mitigation zone and estimating acoustic transmission during a 4-D seismic survey, Sakhalin Island, Russia

A seismic survey conducted in the proximity of the nearshore feeding grounds of gray whales Eschrichtius robustus off northeastern Sakhalin Island, Russia, required the development 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 mu Pa-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 propagation conditions, a set of acoustic footprints with a realistic range of parameters was pre-computed. 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 footprint 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.