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Federal Register / Vol. 86, No. 106 / Friday, June 4, 2021 / Notices 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 constant-velocity half-space infinite homogeneous ocean layer, unbounded by a seafloor. To validate the model results, LDEO measured propagation of pulses from the 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 Langseth array towed at 9 m depth 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 data >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 the R/V
Langseth hydrophone streamer were 2
3 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.
Water depths in the project area may be
up to 2,800 m, but marine mammals in the region are generally not anticipated to dive below 2,000 m e.g., Costa and Williams, 1999. LDEO typically derives estimated distances for intermediate water depths by applying a correction factor of 1.5 to the model results for deep water. In this case, the estimated radial distance for intermediate 1001,000 m and shallow <100 m water depths is taken from Crone et al. 2014, as these empirical data were collected in the same region as this proposed survey. A correction factor of 1.15 was applied to account for differences in array tow depth.
The estimated distances to the Level B harassment isopleths for the array are shown in Table 4.

TABLE 4PREDICTED RADIAL DISTANCES TO ISOPLETHS CORRESPONDING TO LEVEL B HARASSMENT THRESHOLD
Tow depth m
Source and volume
36 airgun array; 6,600 in3

12

Water depth m >1000
1001000 <100

Level B
harassment zone m 1 6,733
2 9,468
2 12,650

1 Distance 2 Based
based on LDEO model results.
on empirical data from Crone et al. 2014 with scaling.

Predicted distances to Level A
harassment isopleths, which vary based on marine mammal hearing groups, were calculated based on modeling performed by LDEO 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 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 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.

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The values for SELcum and peak SPL
for the Langseth airgun arrays 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 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 the source array center, sound pressure of all the airguns in the array stack coherently, but not within one time sample, resulting in smaller source levels a few dB than the source level
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derived from the farfield signature.
Because the farfield signature does not take into account the large array effect near the source and is calculated as a point source, the modified farfield signature is a more appropriate measure of the sound source level for distributed sound sources, such as airgun arrays. L
DEO used the acoustic modeling methodology as used for estimating Level B harassment distances with a small grid step of 1 m in both the inline and depth directions. The propagation modeling takes into account all airgun interactions at short distances from the source, including interactions between subarrays, which are modeled using the NUCLEUS software to estimate the notional signature and MATLAB
software to calculate the pressure signal at each mesh point of a grid.
In order to more realistically incorporate the Technical Guidances weighting functions over the seismic arrays full acoustic band, unweighted spectrum data for the Langseths airgun array modeled in 1 Hz bands was used to make adjustments dB to the unweighted spectrum levels, by frequency, according to the weighting functions for each relevant marine mammal hearing group. These adjusted/
weighted spectrum levels were then converted to pressures mPa in order to integrate them over the entire
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