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Federal Register / Vol. 86, No. 157 / Wednesday, August 18, 2021 / Notices 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 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.
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 modeled in 1 Hz bands were 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 broadband spectrum, resulting in broadband weighted source levels by hearing group that could be directly incorporated within the User Spreadsheet i.e., to override the Spreadsheets more simple weighting factor adjustment. Using the User Spreadsheets safe distance methodology for mobile sources described by Sivle et al., 2014 with the hearing group-specific weighted source levels, and inputs assuming spherical spreading propagation and source velocities and shot intervals specific to
the planned survey, potential radial distances to auditory injury zones were then calculated for SELcum thresholds.
For full detail of the modeling methodology used for estimating distance to Level A harassment peak pressure and cumulative SEL criteria, please see Appendix A of UAGIs application.
Inputs to the User Spreadsheets in the form of estimated source levels are shown in Appendix A of UAGIs application. User Spreadsheets used by UAGI to estimate distances to Level A
harassment isopleths for the airgun arrays are also provided in Appendix A
of the application. Outputs from the User Spreadsheets in the form of estimated distances to Level A
harassment isopleths for the survey are shown in Table 5. As described above, NMFS considers onset of PTS Level A
harassment to have occurred when either one of the dual metrics SELcum and Peak SPLflat is exceeded i.e., metric resulting in the largest isopleth.

TABLE 5MODELED RADIAL DISTANCES m TO ISOPLETHS CORRESPONDING TO LEVEL A HARASSMENT THRESHOLDS
Source volume
Level A harassment zone m
Threshold LF cetaceans
6-airgun array 3,120 in3

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2-airgun array 1,040 in3

Note that because of some of the assumptions included in the methods used e.g., stationary receiver with no vertical or horizontal movement in response to the acoustic source, isopleths produced may be overestimates to some degree, which will ultimately result in some degree of overestimation of Level A harassment.
However, these tools offer the best way to predict appropriate isopleths when more sophisticated modeling methods are not available. NMFS continues to develop ways to quantitatively refine these tools and will qualitatively address the output where appropriate.
For mobile sources, such as this seismic survey, the User Spreadsheet predicts the closest distance at which a stationary animal would not incur PTS
if the sound source traveled by the animal in a straight line at a constant speed.
Auditory injury is unlikely to occur for mid-frequency and low-frequency cetaceans given very small modeled zones of injury for those species all estimated zones less than 10 m for mid-

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frequency cetaceans, up to a maximum of 51 m for low-frequency cetaceans and 34 m for phocid pinnipeds, in context of distributed source dynamics.
Similarly, for high-frequency cetaceans, the maximum modeled injury zone for the low-energy array 88 percent of survey effort is 73 m and auditory injury would be unlikely to occur during use of that array. The source level of the array is a theoretical definition assuming a point source and measurement in the far-field of the source MacGillivray, 2006. As described by Caldwell and Dragoset 2000, an array is not a point source, but one that spans a small area. In the far-field, individual elements in arrays will effectively work as one source because individual pressure peaks will have coalesced into one relatively broad pulse. The array can then be considered a point source. For distances within the near-field, i.e., approximately 23
times the array dimensions, pressure peaks from individual elements do not arrive simultaneously because the observation point is not equidistant
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MF cetaceans 0
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HF cetaceans 0
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from each element. The effect is destructive interference of the outputs of each element, so that peak pressures in the near-field will be significantly lower than the output of the largest individual element. Here, the estimated Level A harassment isopleth distances would in all cases other than for highfrequency cetaceans be expected to be within the near-field of the array where the definition of source level breaks down. Therefore, actual locations within this distance of the array center where the sound level exceeds relevant harassment criteria would not necessarily exist.
In consideration of the received sound levels in the near-field as described above, we expect the potential for Level A harassment of lowand midfrequency cetaceans and phocid pinnipeds to be de minimis, even before the likely moderating effects of aversion and/or other compensatory behaviors e.g., Nachtigall et al., 2018 are considered. A similar conclusion may be drawn for high-frequency cetaceans relative to use of the low-energy airgun
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