2024-24555. Takes of Marine Mammals Incidental to Specified Activities; Taking Marine Mammals Incidental to Pile Driving Training Exercises at Naval Base Ventura County, Port Hueneme

Table 1—Summary of Pile Details and Estimated Production Rates for Pile Installation and Removal During Each Training Exercise for the Initial and Proposed IHAs

Pile size/type/shape	Authorization	Number of sheets/piles	Vibratory installation/ removal duration per pile	Potential impact strikes per pile, if needed	Production rate (piles/day)				Days of installation	Days of removal	Buffer days 1	Total days per exercise
					Installation		Removal
					Vibratory hammer	Impact hammer	Vibratory hammer	Impact hammer
24-in Steel Sheet	Initial IHA Proposed IHA	15 30	10/20 minutes 10/30 minutes	NA NA	3 30	3 NA	3 30	NA NA	5 1	5 1	0 12	10 14
12-in Timber Pile	Initial IHA Proposed IHA	10 12	20/30 minutes 20/30 minutes	1,800 1,800	2 12	2 6	2 12	NA NA	5 2	5 1	0 7	10 10
14-in H-Beam Pile	Initial IHA Proposed IHA	4 6	20/30 minutes 20/30 minutes	1,800 1,800	2 6	2 2	2 6	NA NA	2 3	2 1	0 6	4 10
Totals for the Initial IHA			7.17 hours/12 hours					NA	12	12	0	24
Totals for the Proposed IHA			11 hours/24 hours					NA	6	3	25	34
1  Buffer days are included in the proposed IHA to allow for unanticipated variation in production rates and to account for any training that is slowed or delayed due to the need to meet specific training or mitigation requirements; buffer days were not considered in the initial IHA.

( print page 84538)

The anticipated duration of vibratory removal of steel sheet piles has also changed from the initial IHA. The initial IHA estimated that this activity would take 20 minutes, whereas the Navy now estimates that this activity could take 30 minutes (table 1). This increase in duration, in addition to the increased number of piles to be installed and removed each training exercise, increases both the daily duration and the total duration of active pile driving anticipated to occur. The initial IHA considered that vibratory installation and removal of all piles could take 7.17 hours and 12 hours, respectively, during each training exercise; whereas the current request anticipates that vibratory installation and removal of all piles could take 11 hours and 24 hours, respectively, during each training exercise (table 1). These changes represent a 100-percent increase in the total estimated amount of time required each training exercise for vibratory installation and removal of piles. The number of strikes estimated to be required per pile during impact hammer installation is the same as that estimated in the initial IHA; however, given more piles are anticipated to be installed, it is also anticipated that the total amount of time for impact pile driving would also increase.

This change in the total number of piles and the increased daily durations of pile durations would result in slightly larger Level A harassment zones and shutdown zones due to increased durations of pile driving activities. In addition, the additional days of estimated activity would result in a higher amount of take by Level B harassment to be proposed to be authorized compared to the initial IHA. No other changes have been made to the planned activities. The details of these requested changes are provided in table 1 and in the Estimated Take of Marine Mammals and Proposed Mitigation sections of this notice. The proposed IHA, if authorized, would be effective for 1 year from the date of issuance.

Description of Marine Mammals

A description of the marine mammals in the area of the activities for which authorization of take is proposed here, including information on abundance, status, distribution, and hearing, may be found in the Federal Register notice of the proposed IHA (88 FR 15956, March 15, 2023) for the initial authorization. NMFS has reviewed the monitoring data from the initial IHA, 2023 draft Stock Assessment Reports, information on relevant Unusual Mortality Events, and other scientific literature, and determined there is no new information that affects which species or stocks have the potential to be affected or the pertinent information in the Description of the Marine Mammals in the Area of Specified Activities contained in the supporting documents for the initial IHA.

Marine Mammal Hearing

Hearing is the most important sensory modality for marine mammals underwater, and exposure to anthropogenic sound can have deleterious effects. To appropriately assess the potential effects of exposure to sound, it is necessary to understand the frequency ranges marine mammals are able to hear. Not all marine mammal species have equal hearing capabilities ( e.g., Richardson et al., 1995; Wartzok and Ketten, 1999; Au and Hastings, 2008). To reflect this, Southall et al. (2007, 2019) recommended that marine mammals be divided into hearing groups based on directly measured (behavioral or auditory evoked potential techniques) or estimated hearing ranges (behavioral response data, anatomical modeling, etc.). Note that no direct measurements of hearing ability have been successfully completed for mysticetes ( i.e., low-frequency cetaceans). Subsequently, NMFS (2018, 2024) described generalized hearing ranges for these marine mammal hearing groups. Generalized hearing ranges were chosen based on the approximately 65-decibel (dB) threshold from the normalized composite audiograms, with the exception for lower limits for low-frequency cetaceans where the lower bound was deemed to be biologically implausible and the lower bound from Southall et al. (2007) retained.

On May 3, 2024, NMFS published and solicited public comment on its draft Updated Technical Guidance (89 FR 36762), which includes updated hearing ranges and names for the marine mammal hearing groups and is intended to replace the 2018 Technical Guidance once finalized. The public comment period ended on June 17th, 2024. Because NMFS may finalize the Guidance prior to taking a final agency action on this proposed IHA, we considered both the 2018 and 2024 Technical Guidance in our effects and estimated take analysis below. Marine mammal hearing groups and their associated hearing ranges from NMFS (2018) and NMFS (2024) are provided in tables 2 and 3. In the draft Updated Technical Guidance, mid-frequency cetaceans have been re-classified as high-frequency cetaceans, and high-frequency cetaceans have been updated to very-high-frequency (VHF) cetaceans. Additionally, the draft Updated Technical Guidance includes in-air data for phocid (PA) and otariid (OA) pinnipeds.

Table 2—Marine Mammal Hearing Groups

[NMFS, 2018]

Hearing group	Generalized hearing range *
Low-frequency (LF) cetaceans (baleen whales)	7 Hz to 35 kHz.
Mid-frequency (MF) cetaceans (dolphins, toothed whales, beaked whales, bottlenose whales)	150 Hz to 160 kHz.
High-frequency (HF) cetaceans (true porpoises, Kogia, river dolphins, Cephalorhynchid, Lagenorhynchus cruciger & L. australis)	275 Hz to 160 kHz.
Phocid pinnipeds (PW) (underwater) (true seals)	50 Hz to 86 kHz.
Otariid pinnipeds (OW) (underwater) (sea lions and fur seals)	60 Hz to 39 kHz.
* Represents the generalized hearing range for the entire group as a composite ( i.e., all species within the group), where individual species' hearing ranges are typically not as broad. Generalized hearing range chosen based on ~65-dB threshold from normalized composite audiogram, with the exception for lower limits for LF cetaceans (Southall et al., 2007) and PW pinniped (approximation).

( print page 84539)

Table 3—Marine Mammal Hearing Groups

[NMFS, 2024]

Hearing group	Generalized hearing range *
Underwater:
Low-frequency (LF) cetaceans (baleen whales)	7 Hz to 36 kHz.
High-frequency (HF) cetaceans (dolphins, toothed whales, beaked whales, bottlenose whales)	150 Hz to 160 kHz.
Very High-frequency (VHF) cetaceans (true porpoises, Kogia, river dolphins, Cephalorhynchid, Lagenorhynchus cruciger & L. australis)	200 Hz to 165 kHz.
Phocid pinnipeds (PW) (underwater) (true seals)	40 Hz to 90 kHz.
Otariid pinnipeds (OW) (underwater) (sea lions and fur seals)	60 Hz to 68 kHz.
In-Air:
Phocid pinnipeds (PA) (true seals)	42 Hz to 52 kHz.
Otariid pinnipeds (OA) (sea lions and fur seals)	90 Hz to 40 kHz.
* Represents the generalized hearing range for the entire group as a composite ( i.e., all species within the group), where individual species' hearing ranges may not be as broad. Generalized hearing range chosen based on ~65-dB threshold from composite audiogram, previous analysis in NMFS 2018, and/or data from Southall et al., 2007; Southall et al., 2019. Additionally, animals are able to detect very loud sounds above and below that “generalized” hearing range

Potential Effects of Specified Activities on Marine Mammals and Their Habitat

A description of the potential effects of the specified activity on marine mammals and their habitat for the activities for which an authorization of incidental take is proposed here may be found in the Federal Register notice of the proposed IHA (88 FR 15956, March 15, 2023) for the initial authorization. NMFS has reviewed the monitoring data from the initial IHA, recent draft Stock Assessment Reports, information on relevant Unusual Mortality Events, and other scientific literature, and determined that there is no new information that affects our initial analysis of impacts on marine mammals and their habitat.

Estimated Take of Marine Mammals

A detailed description of the methods and inputs used to estimate take for the specified activity are found in the Federal Register notices of the proposed and final IHAs for the initial authorization (88 FR 15956, March 15, 2023 and 88 FR 28517, May 4, 2023, respectively). Specifically, the source levels and marine mammal occurrence data applicable to this authorization remain unchanged from the previously issued IHA, as do the thresholds and methodology for estimating take by Level B harassment.

The Navy, however, anticipates that the number of piles that could be installed or removed per day would increase compared to the initial IHA so that personnel can meet new training requirements during the proposed IHA authorization period (see table 1). In addition, the Navy anticipates that the vibratory removal of 24-in steel sheets could take 30 minutes, whereas the initial IHA considered this activity could take 20 minutes. As described in the Federal Register notices of the proposed and final IHAs for the initial authorization (88 FR 15956, March 15, 2023, and 88 FR 28517, May 4, 2023, respectively) for the initial authorization, the ensonified area associated with Level A harassment accounts for both source level of the specified activity and duration of that activity. Increasing the number of piles to be installed/removed per day, as well as the anticipated duration of vibratory removal of steel sheet piles, increases the total estimated daily duration of the activity, and thus, the extent of the Level A harassment zones. Note; the estimated distances to the Level B harassment isopleths would not increase because activity duration is not considered in their calculation.

Further, to ensure we have considered an adequate estimate of take by Level A harassment and in order to support consideration of the best available science, we have conducted basic comparative calculations using both the existing Technical Guidance (NMFS, 2018) and the draft Updated Technical Guidance (NMFS, 2024) for the purposes of understanding the number of potential takes by Level A harassment. The thresholds used to assess Level A harassment from each Guidance are provided in tables 4 and 5, respectively. The references, analysis, and methodology used in the development of these thresholds are described in NMFS' 2018 Technical Guidance and NMFS' 2024 draft Updated Technical Guidance, respectively, both of which may be accessed at: https://www.fisheries.noaa.gov/​national/​marine-mammal-protection/​marine-mammal-acoustic-technical-guidance.

The Navy's proposed activity includes the use of impulsive (impact pile driving) and non-impulsive (vibratory driving) sources.

Table 4—NMFS' 2018 Thresholds Identifying the Onset of Permanent Threshold Shift (PTS)

Hearing group	PTS onset acoustic thresholds 1 (received level)
	Impulsive	Non-impulsive
Low-Frequency (LF) Cetaceans	Cell 1: Lpk,flat : 219 dB; LE,LF,24h : 183 dB	Cell 2: LE,LF,24h : 199 dB.
Mid-Frequency (MF) Cetaceans	Cell 3: Lpk,flat : 230 dB; LE,MF,24h : 185 dB	Cell 4: LE,MF,24h : 198 dB.
High-Frequency (HF) Cetaceans	Cell 5: Lpk,flat : 202 dB; LE,HF,24h : 155 dB	Cell 6: LE,HF,24h : 173 dB.
Phocid Pinnipeds (PW) (Underwater)	Cell 7: Lpk,flat : 218 dB; LE,PW,24h : 185 dB	Cell 8: LE,PW,24h : 201 dB.
Otariid Pinnipeds (OW) (Underwater)	Cell 9: Lpk,flat : 232 dB; LE,OW,24h : 203 dB	Cell 10: LE,OW,24h : 219 dB.
1  Dual metric acoustic thresholds for impulsive sounds: Use whichever results in the largest isopleth for calculating PTS onset. If a non-impulsive sound has the potential of exceeding the peak sound pressure level thresholds associated with impulsive sounds, these thresholds should also be considered. ( print page 84540)
Note: Peak sound pressure ( Lpk ) has a reference value of 1 µPa, and cumulative sound exposure level ( LE ) has a reference value of 1µPa2 s. In this table, thresholds are abbreviated to reflect American National Standards Institute standards (ANSI, 2013). However, peak sound pressure is defined by ANSI as incorporating frequency weighting, which is not the intent for NMFS' 2018 Technical Guidance. Hence, the subscript “flat” is being included to indicate peak sound pressure should be flat weighted or unweighted within the generalized hearing range. The subscript associated with cumulative sound exposure level thresholds indicates the designated marine mammal auditory weighting function (LF, MF, and HF cetaceans, and PW and OW pinnipeds) and that the recommended accumulation period is 24 hours. The cumulative sound exposure level thresholds could be exceeded in a multitude of ways ( i.e., varying exposure levels and durations, duty cycle). When possible, it is valuable for action proponents to indicate the conditions under which these acoustic thresholds will be exceeded.

Table 5—NMFS' 2024 Thresholds Identifying the Onset of Auditory Injury (AUD INJ)

Hearing group	AUD INJ acoustic thresholds 1 (received level)
	Impulsive	Non-impulsive
Underwater:
Low-Frequency (LF) Cetaceans	Cell 1: Lp, 0-pk,flat : 222 dB; LE, p, LF,24h : 183 dB	Cell 2: LE, p, LF,24h : 197 dB.
High-Frequency (HF) Cetaceans	Cell 3: Lp, 0-pk,flat : 230 dB; LE, p, HF,24h : 193 dB	Cell 4: LE, p, HF,24h : 201 dB.
Very High-Frequency (VHF) Cetaceans	Cell 5: Lp, 0-pk,flat : 202 dB; LE, p, VHF,24h : 159 dB	Cell 6: LE , p, VHF,24h : 181 dB.
Phocid Pinnipeds (PW) (Underwater)	Cell 7: Lp, 0-pk,flat : 223 dB; LE, p, PW,24h: 183 dB	Cell 8: LE, p, PW,24h: 195 dB.
Otariid Pinnipeds (OW) (Underwater)	Cell 9: Lp, 0-pk,flat : 230 dB; LE, p, OW,24h : 185 dB	Cell 10: LE, p, OW,24h : 199 dB.
In-Air:
Phocid Pinnipeds (PA) (In-Air)	Cell 11: Lp, 0-pk.flat : 162 dB; LE, p, PA,24h : 140 dB	Cell 12: LE, p, PA,24h : 154 dB.
Otariid Pinnipeds (OA) (In-Air)	Cell 13: Lp, 0-pk,flat : 177 dB; LE, p, OA,24h : 163 dB	Cell 14: LE, p, OA,24h : 177 dB.
1  Dual metric acoustic thresholds for impulsive sounds: Use whichever results in the largest isopleth for calculating AUD INJ onset. If a non-impulsive sound has the potential of exceeding the peak sound pressure level thresholds associated with impulsive sounds, these thresholds should also be considered.
Note: Peak sound pressure ( Lpk ) has a reference value of 1 µPa, and cumulative sound exposure level ( LE ) has a reference value of 1µPa2 s. In this table, thresholds are abbreviated to reflect American National Standards Institute standards (ANSI, 2013). However, peak sound pressure is defined by ANSI as incorporating frequency weighting, which is not the intent for NMFS' 2018 Technical Guidance. Hence, the subscript “flat” is being included to indicate peak sound pressure should be flat weighted or unweighted within the generalized hearing range. The subscript associated with cumulative sound exposure level thresholds indicates the designated marine mammal auditory weighting function (LF, MF, and HF cetaceans, and PW and OW pinnipeds) and that the recommended accumulation period is 24 hours. The cumulative sound exposure level thresholds could be exceeded in a multitude of ways ( i.e., varying exposure levels and durations, duty cycle). When possible, it is valuable for action proponents to indicate the conditions under which these acoustic thresholds will be exceeded.

NMFS developed optional User Spreadsheet tools to accompany both the 2018 Technical guidance and the 2024 draft Technical Guidance that can be used to relatively simply predict an isopleth distance for use in conjunction with marine mammal density or occurrence to help predict potential takes. We note that because of some of the assumptions included in the methods underlying these optional tools, we anticipate that the resulting isopleth estimates are typically going to be overestimates of some degree, which may result in an overestimate of potential take by Level A harassment. However, these optional tools offers the best way to estimate isopleth distances when more sophisticated modeling methods are not available or practical. For stationary sources, such as vibratory and impact pile driving, the optional User Spreadsheet tools predict the distance at which, if a marine mammal remained at that distance for the duration of the activity, it will be expected to incur PTS. Inputs used in the optional User Spreadsheet tools for the proposed IHA based on the Navy's request are reported in table 6. The isopleths calculated for the proposed IHA based on NMFS' 2018 Technical guidance and NMFS' 2024 draft Technical guidance, as well as those considered in the initial IHA (based on NMFS' 2018 Technical guidance), are reported in table 7.

Table 6—NMFS User Spreadsheet Inputs

	Vibratory pile driving			Impact pile driving
	16-Inch timber piles	14-Inch steel H beam	24-Inch steel sheet	16-Inch timber piles	14-Inch steel H beam
Spreadsheet Tab Used	A.1) Non-Impul, Stat, Cont	A.1) Non-Impul, Stat, Cont	A.1) Non-Impul, Stat, Cont	E.1) Impact pile driving	E.1) Impact pile driving.
Source Level (SPL)	162 dB RMS	147 dB RMS	159 dB RMS	160 dB SEL	170 dB SEL.
Transmission Loss Coefficient	15	15	15	15	15.
Weighting Factor Adjustment (kHz)	2.5	2.5	2.5	2	2.
Time to install/remove single pile (minutes)	30	30	30
Number of strikes per pile				1,800	1,800.
Piles to install/remove per day	12	6	30	6	2.
Distance of sound pressure level measurement (meters)	10	10	11	10	10.

( print page 84541)

Table 7—Calculated Distances and Areas to the Estimated Level A (Based on NMFS' 2018 Technical Guidance and NMFS' Proposed 2024 Update to the 2018 Technical Guidance) and Level B Harassment Thresholds by Pile Type and Pile Driving Method for the Initial and Proposed IHAs

Activity	Pile description	Authorization period	Piles per day	Level A harassment distances (m) (2018 Technical Guidance/2024 Proposed Update to the 2018 Technical Guidance) 1		Level A harassment areas (km2 ) for all hearing groups 2	Level B harassment distance (m) all hearing groups	Level B harassment areas (km2 ) for all hearing groups 2
				PW	OW
Vibratory Installation/Removal	16-inch Timber Piles	Initial IHA Proposed IHA	3 12	4.8 16.0/45.7	0.3 1.1/15.4	<0.1 <0.1	3  6,310 3  6,310	<0.3 <0.3
	14-inch Steel H Beam	Initial IHA Proposed IHA	2 6	0.5 1.0/2.9	0 0.1/1.0	<0.1 <0.1	631 631	<0.3 <0.3
	24-inch Steel Sheet	Initial IHA Proposed IHA	3 30	3.4 4  20.4/4  58.4	0.2 4  1.4/4  19.7	<0.1 <0.1	3  4,379 3  4,379	<0.3 <0.3
Impact Installation	16-inch Timber Piles	Initial IHA Proposed IHA	3 6	36.8 76.5/126.5	2.7 5.6/47.1	<0.1 <0.1	47 47	<0.1 <0.1
	14-inch Steel H-Beam	Initial IHA Proposed IHA	2 2	170.6 170.6/282.2	12.4 12.4/105.2	<0.1 <0.1	216 216	<0.1 <0.1
1  The initial IHA only calculated distances to the Level A harassment thresholds based on the 2018 Technical Guidance, whereas the proposed IHA considers calculated distances to the Level A harassment thresholds based on both the 2018 Technical Guidance and the 2024 Proposed Update to the 2018 Technical Guidance.
2  Harassment areas have been truncated where appropriate to account for land masses.
3  The maximum harassment distances are approximately 790 m for Wharf 4 South, 795 m for Wharf 4 East, and 655 m for Wharf D due to the presence of land masses in the project area, which truncate sound transmission.
4  The Navy made a typographical error made in their calculation of the distance to the Level A harassment thresholds. Specifically they used a sound pressure level measurement distance of 10 m when the correct measurement distance is 11 m. Therefore, the distances provided here are slightly larger than the distances provided in their IHA application.

The stocks taken, methods of take, and types of take remain unchanged from the initial authorization. Here, we estimate the proposed amount of taking by Level B harassment for both species using the same method and rates of daily occurrence for California sea lions (342 individuals) and harbor seals (21 individuals) used in the initial IHA, applied across the proposed 136 days of activity, resulting in 46,512 and 2,856 instances of take by Level B harassment, respectively (table 8). While the estimated distances to the Level A thresholds and the shutdown zones are larger here than in the initial IHA (see table 7 and the Proposed Mitigation section of this notice), and in some cases larger than the estimated distances to the Level B thresholds (see distances for impact driving in table 7), they are still relatively small ( i.e., less than 171 m or 283 m based on NMFS' 2018 Technical Guidance and NMFS' 2024 draft Technical Guidance, respectively) and it is unlikely that an individual would remain in these zones long enough to incur PTS. Further, we anticipate that the Navy will be able to effectively shut down operations as necessary to avoid any take by Level A harassment. Therefore, take by Level A harassment is not proposed to be authorized. During the initial authorization period, the Navy detected 40 California sea lions and 4 harbor seals within the estimated Level B harassment zone.

Table 8—Amount of Authorized Take in the Initial IHA and Proposed Take as a Percentage of Stock Abundance, by Stock and Harassment Type

Species	Stock	Authorization	Proposed amount of taking			Percent of stock
			Level A	Level B	Total
California Sea Lion	U.S	Initial IHA Proposed IHA	0 0	36,960 46,512	36,960 46,512	14.30 18.05
Harbor Seal	California	Initial IHA Proposed IHA	0 0	2,016 2,856	2,016 2,856	6.51 9.22

Description of Proposed Mitigation, Monitoring and Reporting Measures

The proposed mitigation, monitoring, and reporting measures included as requirements in this authorization are identical to those included in the Federal Register notice announcing the issuance of the initial IHA (88 FR 28517, May 4, 2023), except for the size of the respective shutdown zones as discussed below, and the discussion of the least practicable adverse impact included in that document remains accurate. The following measures are proposed for this IHA:

• The Navy would conduct briefings between supervisors and trainees, the marine mammal monitoring team, and Navy staff prior to the start of all in-water pile driving activity, and when new personnel join the work, to ensure that responsibilities, communication procedures, marine mammal monitoring protocols, and operational procedures are clearly understood.
• During all in-water work other than pile driving (e.g., pile placement, boat use), in order to prevent injury from physical interaction with construction equipment, a shutdown zone of 10 m would be implemented. If a marine mammal comes within 10 m, operations would cease and vessels would reduce speed to the minimum level required to maintain steerage and safe working conditions. If human safety is at risk, the in-water activity would be allowed to continue until it is safe to stop.
• The Navy would establish shutdown zones for all for in-water pile driving activities. The purpose of a shutdown zone is generally to define an area within which shutdown of activity will occur upon sighting of a marine mammal (or in anticipation of an animal entering the defined area). Shutdown zones would vary based on the type of ( print page 84542) pile installation/removal activity, but are proposed to be larger than the calculated Level A harassment isopleths shown in table 9. The proposed shutdown zones have increased slightly from what was required in the initial IHA based on the Navy's request to increase the number of piles that may be installed and or removed each day, and in the case of 24-in steel sheets, the longer duration estimated to remove piles with a vibratory hammer (see table 1). The placement of lookouts during all pile driving activities (described in detail in below) would ensure that the entirety of all shutdown zones and Level A harassment zones are visible during pile installation and removal.

Table 9—Proposed Shutdown Zones During In-Water Pile Driving Activities for the Initial and Proposed IHAs

Activity	Pile description	Authorization period	Distance (m) 1
			PW	OW
Vibratory Installation/Removal	16-inch Timber Piles	Initial IHA Proposed IHA	15 20/50	15 >20/50
	14-inch Steel H Beam	Initial IHA Proposed IHA	15 20/20	15 20/20
	24-inch Steel Sheet	Initial IHA Proposed IHA	15 2  30/60	15 2  30/60
Impact Installation	16-inch Timber Piles	Initial IHA Proposed IHA	40 80/130	40 80/130
	14-inch Steel H-Beam	Initial IHA Proposed IHA	175 175/290	175 175/290
1  Shutdown zones for the proposed IHA consider calculated distances to the Level A harassment thresholds based on both the 2018 Technical Guidance and the 2024 Proposed Update to the 2018 Technical Guidance, whereas shutdown zones for the initial IHA only considered calculated distances to the Level A harassment thresholds based on the 2018 Technical Guidance. Here, we are proposing the Navy implement the larger distance as the shutdown zones, which consider the 2024 Proposed Update to the 2018 Technical Guidance.
2  The Navy considered a shutdown zone of 20 m due to a typographical error made in their calculation of the distance to the Level A harassment thresholds. Specifically they used a sound pressure level measurement distance of 10 m when the correct measurement distance is 11 m.