IHA Application: Seismic Surveys in Cook Inlet by Marathon Oil ...

Power-down Procedures
A power down involves decreasing the number of airguns in use such that the
radius of the 180-dB (or 190-dB) zone is decreased to the extent that marine
mammals are not in the safety zone. During a power down, one airgun is
operated. The continued operation of one airgun is intended to alert marine
mammals to the presence of the seismic guns in the area. In contrast, a shut down
occurs when all airgun activity is suspended.
If a marine mammal is detected outside the safety radius but is likely to enter the
safety radius, the airguns may (as an alternative to a complete shut down) be
powered down before the mammal is within the safety radius. Likewise, if a
marine mammal is already within the safety zone when first detected, the airguns
will be powered down immediately if this is a reasonable alternative to a complete
shut down.
Following a power down, airgun activity will not resume until the marine
mammal has cleared the safety zone. The animal will be considered to have
cleared the safety zone if it:
• Is visually observed to have left the safety zone, or
• Has not been seen within the zone for 15 min in the case of pinnipeds, or
• Has not been seen within the zone for 30 min in the case of whales.
Shut-down Procedures
The operating airgun(s) will be shut down completely if a marine mammal
approaches or enters the applicable safety radius and a power down is not
practical. The operating airgun(s) will also be shut down completely if a marine
mammal approaches or enters the estimated safety radius of the source that would
be used during a power down. The shutdown procedure should be accomplished
within several seconds (of a “one shot” period) of the determination that a marine
mammal is within or about to enter the safety zone.
Airgun activity will not resume until the marine mammal has cleared the safety
radius. The animal will be considered to have cleared the safety radius if it is
visually observed to have left the safety radius, or if it has not been seen within
the radius for 15 minutes (beluga, harbor porpoise, killer whales, seals, and sea
lions).
Ramp-up Procedures
A “ramp up” procedure will be followed when the airgun array begins operating
after a specified-duration period without airgun operations. NMFS normally
requires that the rate of ramp up be no more than 6 dB per 5 minute period. Ramp
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