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Federal Register / Vol. 86, No. 109 / Wednesday, June 9, 2021 / Notices
surveys, which generate underwater sound levels at or more than or 160 dB
between 125 Hz and 38 kHz. Mitigation zones must include all in-water areas where work-related sound received by sea otters will match the levels and frequencies above. Mitigation zones will be designated as follows:

Exclusion Zones EZ will be established with the following minimum radii: 500 m 0.3 mi from the source for the full seismic array and 100
m 328 ft for the single bolt airgun 655
cm3 or 40 in3.
A Safety Zone SZ is an area larger than the EZ and will include all areas within which sea otters may be exposed
to noise levels that will likely result in Level B take.
Both the EZ and SZ will be centered on the sound source the seismic array.
The radius of the SZs are shown in Table 3 as calculated based on modeling techniques described herein and in Appendix A of NSF/LDEOs application.

TABLE 3ESTIMATED RADIAL DISTANCES FROM THE SEISMIC SOUND SOURCE TO THE 160-dB ISOPLETH
The area within the isopleth is designated as the Safety Zone SZ

Source and volume
Water depth m
Single Bolt airgun, 40 in 3

>1,000 m
1001,000 m <100 m <1000 m
1001,000m <100m

4 strings, 36 airguns, 6600 in 3

Predicted distances in m to the 160 dB
received sound level 1 431
2 647
3 1,041
1 6,733
4 9,468
4 12,650

1 Distance
is based on LDEO model results.
is based on LDEO model results with a 1.5 correction factor between deep and intermediate water depths.
is based on empirically derived measurements in the GOM with scaling applied to account for differences in tow depth.
4 Based on empirical data from Crone et al. 2014; see Appendix A of the NSF/L2012;DEO IHA application for details.
2 Distance
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3 Distance
13. PSOs must conduct visual monitoring of the entire EZ and the visible SZ continuously during all seismic work occurring in daylight hours.
14. Prior to seismic work, a rampup procedure must be used to increase the levels of underwater sound at a gradual rate.
A ramp-up will be used at the initial start of airgun operations and prior to restarting after any period greater than 30 minutes min without airgun operations, including a powerdown or shutdown event.
Visual monitoring must begin at least 30 min prior to and continue throughout ramp-up efforts.
During geophysical work, the number and total volume of airguns will be increased incrementally until the full volume is achieved.
The rate of ramp-up will be no more than 6 dB per 5-min period. Ramp-up will begin with the smallest gun in the array that is being used for all airgun array configurations. During the rampup, the applicable mitigation zones based on type of airgun and sound levels produced must be maintained.
It will not be permissible to rampup the full array from a complete shutdown in thick fog or at other times when the outer part of the EZ is not visible.
Ramp-up of the airguns will not be initiated if a sea otter is sighted within the EZ at any time.

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If sea otters are observed during a ramp-up effort or prior to startup, a PSO
must record the observation and monitor the animals position until it moves out of visual range. Seismic work may commence if, after a full and gradual effort to ramp up the underwater sound level, the sea otter is outside of the EZ and does not show signs of visible distress for example, vocalizing, repeatedly spy-hopping, or fleeing.
15. The following actions must be taken in response to sea otters in mitigation zones:
Seismic work will be shut down completely if a sea otter is observed within the 500-m 0.3-mi EZ for the full array or the 100-m 328-ft EZ for the 40cui array.
When sea otters are observed in visible distress for example, vocalizing, repeatedly spy-hopping, or fleeing, seismic work must be immediately shut down or powered down to reduce noise exposure.
The shutdown procedure will be accomplished within several seconds of the determination that a sea otter is in the applicable EZ or as soon as practicable considering worker safety and equipment integrity.
Following a shutdown, seismic work will not resume until the sea otter has cleared the EZ. The animal will be considered to have cleared the EZ if it is visually observed to have left the EZ
or has not been seen within the EZ for 30 minutes or longer.

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Any shutdown due to sea otters sighted within the EZ must be followed by a 30-minute all-clear period and then a standard full ramp-up.
Any shutdown for other reasons resulting in the cessation of seismic work for a period greater than 30
minutes must also be followed by full ramp-up procedures.
16. Operators may reduce power to seismic equipment as an alternative to a shutdown to prevent a sea otter from entering the EZ. A power-down procedure involves reducing the volume of underwater sound generated. Vessel speed or course may be altered to achieve the same task.
Whenever a sea otter is detected outside the EZ and, based on its position and motion relative to the seismic work, appears likely to enter the EZ but has not yet done so, the operator may power down to reduce high-level noise exposure.
When a sea otter is detected in the SZ, an operator may choose to power down when practicable to reduce Level B take, but is not required to do so.
During a power-down, the number of airguns in use will be reduced to a single mitigation airgun airgun of small volume such as the 655-cm3 40-in3
gun, such that the EZ is reduced, making the sea otters unlikely to enter the EZ.
After a power-down, noisegenerating work will not resume until the sea otter has cleared the EZ for the full airgun array. The animal will be
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