Shell moves to protect marine mammals offshore Alaska
In addition, Shell designed a marine mammal monitoring and mitigation program with scientific tools, including vessel-based marine mammal observations, aerial overflights to spot marine mammals, and two different networks of seafloor recorders: 28 on-bottom hydrophones (OBHs) in the Chukchi Sea recorded ambient sound, the sounds produced by Shell’s activities, and the vocalizations of marine mammals. A network of 35 directional autonomous seafloor acoustic recorders deployed in the Beaufort Sea provided similar information as the OBHs, as well as triangulated positions of the vocalizing marine mammals that offered more detail on the migrations of marine animals past Shell’s operations.
Monitoring, mitigating effects of exploration
On-board marine mammal observers (MMOs) provided the most basic and essential monitoring and mitigation. Trained observers (including both scientists and Inupiat personnel) observed from the deck of all vessels during daylight. They recorded data on the presence and behavior of marine mammals near operations. They also had the authority to direct vessel movements and activities if there was concern that marine mammals could be affected. If marine mammals were observed within sound “safety zones” around vessels where sound exceeded regulatory thresholds, MMOs could direct either mitigative measures, which include evasive maneuvers, or shutdown of operations. Shell validated safety zones around the seismic vessel’s source of airguns within which received pulse levels were ≥ 180 dB re 1 μPa (rms) for cetaceans (whales and dolphins) and ≥ 190 dB re 1 μPa (rms) for pinnipeds (walrus, sea lions, and seals). These criteria assume that seismic pulses received at lower levels will not injure these animals or impair these animals’ hearing abilities, but that higher received levels might have some such effects. MMOs had the authority to shut down airgun operations if marine mammals were seen too close to the seismic vessel and source. By stationing at least one MMO on the bridge of the vessel, MMOs could communicate directly with the captain to insure protective measures occurred as quickly as possible.
Ramp-ups, shut-downs
A ramp-up of the array of seismic airguns occurred whenever airguns start firing, such as at the beginning of the season, after a shut-down, or if they have been turned off for any reason. A ramp-up provides a gradual increase in sound levels, and involves a step-wise increase in the number and total volume of airguns firing until the full volume is achieved. Ramp-up occurs at a rate not greater than 6 dB per 5-minute period and must be preceded by at least 30 minutes of marine mammal-free observation by the MMOs. The purpose of a ramp up is to warn marine mammals in the vicinity and to give them time to leave the area.
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Volume 68 Issue 9
September 2008
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