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Federal Register / Vol. 86, No. 106 / Friday, June 4, 2021 / Notices
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 information specific to the planned survey i.e., the 2.2 m/s source velocity and worst-case 23-s shot interval, potential radial distances to auditory injury zones were then calculated for SELcum thresholds.
Inputs to the User Spreadsheets in the form of estimated source levels are shown in Appendix A of LDEOs application. User Spreadsheets used by LDEO 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
Level A harassment zone m
Source volume
Threshold
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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 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 relevant peak isopleth distances would in all cases be expected to be within the nearfield 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 the relevant peak SPL
thresholds would not necessarily exist.
In general, Caldwell and Dragoset 2000
suggest that the near-field for airgun arrays is considered to extend out to approximately 250 m.
In order to provide quantitative support for this theoretical argument, we calculated expected maximum distances at which the near-field would transition to the far-field Table 5. For a specific array one can estimate the distance at which the near-field transitions to the far-field by:

Lz D=-

4;i, with the condition that D >> l, and where D is the distance, L is the longest dimension of the array, and l is the wavelength of the signal Lurton, 2002.
Given that l can be defined by:

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Phocids
Otariids
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where f is the frequency of the sound signal and v is the speed of the sound in the medium of interest, one can rewrite the equation for D as:

D - Lz 4v and calculate D directly given a particular frequency and known speed of sound here assumed to be 1,500
meters per second in water, although this varies with environmental conditions.
To determine the closest distance to the arrays at which the source level predictions in Table 5 are valid i.e., maximum extent of the near-field, we calculated D based on an assumed frequency of 1 kHz. A frequency of 1
kHz is commonly used in near-field/farfield calculations for airgun arrays Zykov and Carr, 2014; MacGillivray, 2006; NSF and USGS, 2011, and based on representative airgun spectrum data and field measurements of an airgun array used on the Langseth, nearly all greater than 95 percent of the energy from airgun arrays is below 1 kHz Tolstoy et al., 2009. Thus, using 1 kHz as the upper cut-off for calculating the maximum extent of the near-field should reasonably represent the nearfield extent in field conditions.
If the largest distance to the peak sound pressure level threshold was equal to or less than the longest dimension of the array i.e., under the array, or within the near-field, then received levels that meet or exceed the threshold in most cases are not expected to occur. This is because within the near-field and within the dimensions of the array, the source levels specified in Appendix A of LDEOs application are
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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, and NMFS continues to develop ways to quantitatively refine these tools and will qualitatively address the output where appropriate.
For mobile sources, such as the proposed 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 cetaceans, otariid pinnipeds, and phocid pinnipeds given very small modeled zones of injury for those species all estimated zones less than 15 m for mid-frequency cetaceans and otariid pinnipeds, up to a maximum of 44 m for phocid pinnipeds, in context of distributed source dynamics.
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
SELcum
Peak

HF
cetaceans
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