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Federal Register / Vol. 86, No. 157 / Wednesday, August 18, 2021 / Notices
Ensonified Area Here, we describe operational and environmental parameters of the activity that will feed into identifying the area ensonified above the acoustic thresholds, which include source levels and acoustic propagation modeling.
The acoustic propagation modeling methodologies are described in greater detail in Appendix A of UAGIs IHA
application. The survey will primarily acquire data using the 2-airgun array with a total discharge volume of 1,040
in3 and an approximately 15-second shot interval. During approximately 12
percent of the planned survey tracklines, the 6-airgun, 3,120 in3 array will be used with a 60-second shot interval. All tracklines will be surveyed with a maximum tow depth of 9 m. The modeling assumed an airgun firing pressure of 2,540 psi. Propagation modeling for UAGIs application follows the approach used by the LamontDoherty Earth Observatory LDEO for other, similar IHA applications. LDEO
uses ray tracing for the direct wave traveling from the array to the receiver and its associated source ghost reflection at the air-water interface in the vicinity of the array, in a constantvelocity half-space infinite
homogeneous ocean layer, unbounded by a seafloor. To validate the model results, LDEO measured propagation of pulses from a 36-airgun array at a tow depth of 6 m in the Gulf of Mexico, for deep water 1,600 m, intermediate water depth on the slope 6001,100
m, and shallow water 50 m Tolstoy et al., 2009; Diebold et al., 2010.
LDEO collected a MCS data set from R/V Marcus G. Langseth with the same 36-airgun array referenced above on an 8 km streamer in 2012 on the shelf of the Cascadia Margin off of Washington in water up to 200 m deep that allowed Crone et al. 2014 to analyze the hydrophone streamer >1,100 individual shots. These empirical data were then analyzed to determine in situ sound levels for shallow and upper intermediate water depths. These data suggest that modeled radii were 23
times larger than the measured radii in shallow water. Similarly, data collected by Crone et al. 2017 during a survey off New Jersey in 2014 and 2015
confirmed that in situ measurements collected by R/V Langseth hydrophone streamer were 23 times smaller than the predicted radii.
LDEO model results are used to determine the assumed radial distance
to the 160-dB rms threshold for these arrays in deep water >1,000 m down to a maximum water depth of 2,000 m see Table 4. Water depths in the project area may be up to 4,000 m, but marine mammals in the region are generally not anticipated to dive below 2,000 m Costa and Williams, 1999. The radii for intermediate water depths 1001,000 m are derived from the deep-water ones by applying a correction factor multiplication of 1.5.
No survey effort will occur in water depths <100 m.
The area expected to be ensonified was determined by entering the planned survey lines into a GIS and then buffering the lines by the applicable 160-dB distance see Appendix B in IHA
application. The resulting ensonified areas were then increased by 25 percent to allow for any necessary additional operations, such as re-surveying segments where data quality was insufficient. This approach assumes that no marine mammals would move away or toward the trackline in response to increasing sound levels before the levels reach the threshold as R/V Sikuliaq approaches.

TABLE 4PREDICTED RADIAL DISTANCES TO ISOPLETHS CORRESPONDING TO LEVEL B HARASSMENT THRESHOLD
Tow depth m
Source and volume
Water depth m
6 airgun array; 3,120 in3

9

2 airgun array; 1,040 in3

9

>1,000
1001,000
>1,000
1001,000

Level B
harassment zone m 1 4,640
3 6,960
1 1,604
2 2,406

1 Distance
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2 Based
based on LDEO model results.
on LDEO model results with 1.5x correction factor applied.

Predicted distances to Level A
harassment isopleths, which vary based on marine mammal hearing groups, were calculated based on LDEO
modeling performed using the NUCLEUS source modeling software program and the NMFS User Spreadsheet, described below. The acoustic thresholds for impulsive sounds e.g., airguns contained in the Technical Guidance were presented as dual metric acoustic thresholds using both the cumulative sound exposure level SELcum and peak sound pressure metrics NMFS 2018. As dual metrics, NMFS considers onset of PTS Level A
harassment to have occurred when either one of the two metrics is exceeded i.e., metric resulting in the largest isopleth. The SELcum metric considers both level and duration of exposure, as well as auditory weighting
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functions by marine mammal hearing group. In recognition of the fact that the requirement to calculate Level A
harassment ensonified areas could be more technically challenging to predict due to the duration component and the use of weighting functions in the new SELcum thresholds, NMFS developed an optional User Spreadsheet that includes tools to help predict a simple isopleth that can be used in conjunction with marine mammal density or occurrence to facilitate the estimation of take numbers.
The values for SELcum and peak Sound Pressure Level SPL were derived from calculating the modified far-field signature. The farfield signature is often used as a theoretical representation of the source level. To compute the farfield signature, the source level is estimated at a large
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distance below the array e.g., 9 km, and this level is back projected mathematically to a notional distance of 1 m from the arrays geometrical center.
However, when the source is an array of multiple airguns separated in space, the source level from the theoretical farfield signature is not necessarily the best measurement of the source level that is physically achieved at the source Tolstoy et al., 2009. Near the source at short ranges, distances <1 km, the pulses of sound pressure from each individual airgun in the source array do not stack constructively, as they do for the theoretical farfield signature. The pulses from the different airguns spread out in time such that the source levels observed or modeled are the result of the summation of pulses from a few airguns, not the full array Tolstoy et al., 2009. At larger distances, away from
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