In conclusion, the combined evidence of our work and these two other suprathreshold investigations strongly suggest that the current underwater sound pressure level exposure limits are invalid and err on the unsafe side by significant amounts.
http://www.dtic.mil/dtic/tr/fulltext/u2/a220935.pdf
Our data suggest that LFS exposures up to 145 dB re I pPa at frequencies between 100 and 500 Hz will have minimal impact on the recreational diver.
As a conservative measure, the consensus decision of scientists involved in the LFS program was that the maximal SPL intensity for the guidance should be set at 145 dB re 1 uPa (20).
(LFS= low-frequency underwater sound)
http://archive.rubicon-foundation.org/xmlui/bitstream/handle/123456789/2368/11732884.pdf?sequence=1
Human guidelines were established based on psychological aversion testing. There was only a two percent aversion reaction subjectively judged as "very severe" by divers at a level of 148 dB. NSMRL therefore determined that scaling back the intensity by 3 dB (a 3 dB reduction equals a 50 percent reduction in signal strength) would provide a suitable margin of safety against psychological aversion for divers. Hence, NSMRL set the RL criterion for recreational and commercial divers at 145 dB. This criterion was endorsed by the Department of the Navy, Bureau of Medicine and Surgery (BUMED) on 18 October 1999.
The Navy's adoption of the 145-dB guidance for operation of low frequency underwater sound sources in the presence of divers is considered a conservative, protective decision. During operation of the SURTASS LFA sonar, the distance from the source to where the RL is 145 dB (the 145-dB sound field) varies from site to site due to the high variability in underwater sound propagation characteristics and deployment protocols. The most reliable method for ensuring that the criterion of 145-dB maximum RL is maintained at known recreational and commercial dive sites involves the application of validated underwater acoustic models of sound propagation using site-specific environmental parameters. Results provide an estimation of sound pressure level (SPL) as a function of range and depth for each specific site.
The Navy's consistency determination further states:
The proposed SURTASS LFA sonar employment (Alternative 1, Restricted Operation) would be employed with geographic operational restrictions. Sound levels generated by the operation of the sonar would not be allowed to exceed 180 dB within 22 km (12 nm) of the coast. In addition, sound fields generated by the SURTASS LFA sonar under the Restricted Operation Alternative would not be allowed to exceed 145 dB in the vicinity of known dive sites. This is generally defined as from the shoreline out to the 40-m (130 ft) depth contour, but it is recognized that there are other sites that may be outside of this boundary. The latter would be identified using information obtained from the worldwide Divers Alert Network (DAN) and other available literature.
http://www.coastal.ca.gov/cd/CD-113-00.pdf
Temporary auditory-threshold shifts induced by exposures to continuous tones in water.
Paul F. Smith (Naval Submarine Medical Research Laboratory, Groton, CT 06349)
Four bare-headed divers were exposed for 25 min to continuous tones in water at frequencies of either 700, 1400,o r 5600 Hz at average sound pressure levels( SPL) between1 43.1 and 165.1d B above 20 uPa. These SPL are comparable to those produced by some underwater hand-held tools. These conditions yielded individual temporary auditory-threshold shifts (TTS) between2 3 and 55 dB 2 rain post-exposure. Recovery required up to 50 h or more depending on exposure conditions. Non auditory effects including middle-ear sensations and a reddened ear drum were also observed. Four additional divers were exposed successively for 10-min periods to pure tones of 5600, 1400, and 700 Hz (in that order) in air at 100 dB and in water at various SPL between 125 and 150 dB. Divers incurred moderate TTS from all exposures. For equivalent intensities in air and in water, 700-Hz exposures produced equivalent TTS. Less TTS was induced in water at 1400 Hz; more TTS in water at 5600 Hz. The results suggest that, within the predictive accuracy of the TTS technique, several hand-held tools now in use by military and civilian divers are extremely hazardous to hearing.
http://archive.rubicon-foundation.o...ndle/123456789/8484/NSMRL_1122.pdf?sequence=1
Mitigation Measures
In Canada regulators require operators of any high-powered acoustic source, be it an airgun array, active sonar or demolition charge, to obtain a permit to operate and to take appropriate steps to reduce the risk to the marine environment. The following mitigation steps are required and have been adopted by seismic operators around the world as a code of practise:
1. Taking care not to operate a high powered acoustic source near an area where threatened species are known to congregate.
2. Being aware of the source level and operating frequency of an acoustic system, as well as the size of possible zones of influence around that system (
e.g., ranges where animals are likely to be harassed or injured).
3. Employing soft-starts,
i.e., a gradual ramp up of the sound level, to scare off any marine life before operating at full power.
4. Watching for marine mammals and fish in the immediate vicinity of an acoustic system, prior to operation, and shutting down when animals are spotted within predetermined zones of influence.
Thus, when operated with care, airgun arrays, should not pose a threat to marine life.
http://www.dieselduck.info/library/06 bc oil/2003 BC Offshore - Seismic_vs_Sonar.pdf
Background
Guidance Note DMAC 012 was issued in 1979 after consideration of the knowledge concerning the effects of seismic operations on divers in the water at that time. Over the last 30 years DMAC has discussed this guidance on a number of occasions and attempted to gain further knowledge from reports of diver/seismic operation interaction. Some of the few available reports have added to our knowledge.
1 Seismic airgun activity results in the transmission of acoustic waves through the water which the diver experiences as a noise analogous to a piling hammer. Multiple reflections of this acoustic wave from the sea surface, seabed and other structures may result in this sounding like a low frequency rumble.
2 The intensity of the sound experienced by the diver is principally dependent on the power of the seismic airgun array and the distance between the diver and the seismic airgun, but other factors may have important effects. These factors include the water depth at which the seismic activity takes place, the presence of thermoclines (layering due to changes in temperature), the depth of the diver versus the depth of the thermocline, bottom conditions, salinity and the sea state.
3 Not all seismic surveys are the same (e.g. ocean bottom cable surveys (OBC), streamer(s), vertical seismic profile surveys (VSP), site surveys, etc.) and there are differences in the types and purpose of source arrays used around the world, e.g. airguns, boomers, sparkers, etc.
4 The multiple factors involved make it difficult to determine a safe or tolerable distance, particularly in shallow water, without performing communication exercises between seismic and diving operations.
5 The duration of a divers exposure may limit tolerance.
Guidance
1 Where diving and seismic activity will occur within a distance of 10 kilometres, a joint risk assessment should be conducted, between the operators involved and the seismic and diving contractors in advance of any simultaneous operations.
2 Where possible, plans should be made to avoid overlapping seismic and diving activities. Where this is not possible, the activities should be prioritised and a simultaneous operations (SIMOPS) plan developed.
3 The parties should perform a communication exercise or test at the start of simultaneous operations to determine the acceptable safe distance for the local conditions. Starting at a distance of 10 kilometres, the seismic source array will be gradually ramped up, and the seismic vessel gradually moved closer to the diving operation, with constant communication between the diving supervisor and the seismic party manager. (Note: seismic source ramp ups are now the industry standard in all situations.)
4 The minimum safe distance, as determined from the testing outlined above, should not be compromised by either party.
5 There should be regular contact (at least daily) between the seismic vessel and diving vessel so that both are aware of each others work program for the day.
http://www.dmac-diving.org/guidance/DMAC12.pdf
10 kilometers = 6.21371192 miles
The impacts of exploration activities on sensitive diving and under-water recreational activities would therefore be primarily restricted to the effects of increased underwater noise as a result of the seismic and bathymetric surveys. As with marine mammals it is expected that three types of injury to humans can result from exposure to high underwater sound levels:
Shifts of hearing threshold repeated or continual exposure to high level sound results in a gradual deterioration of hearing through permanent threshold shifts (1) (PTS) or temporary threshold shifts (TTS).
Tissue damage tissue damage usually arises from the near instantaneous increase in pressure, which forms shock waves of explosive pulses. As rise times are not rapid in non-explosive seismic sources (such as those that will be generated during the seismic surveys), tissue damage from such sources is likely to be negligible.
Acoustically induced decompression sickness - Crum and Mao (1996) suggested that significant acoustically induced bubble formation could be expected at received levels of over 210 dB.
Much of the limited information available on the impact of underwater noise on humans is from military sources. The U.S. Navy has conducted two studies of relevance (see www/surtass-lfa-eis.com):
The Applied Research laboratory of the University of Texas carried out 437 tests on 87 divers over the period 1993 to 1995. Divers were subject to a nine 100 second 50 percent duty cycle 160 dB pulses of varying frequency above 160 Hz. The study did not induce any long term effects on major organ systems and concluded that sound pressure levels of below 160 dB would not be expected to cause physiological damage to a diver.
Studies conducted by the U.S. Office of Naval Research (ONR) and the U.S. Naval Submarine Medical Research Laboratory (NSMRL) in conjunction with a consortium of university and military laboratories developed guidance for safe exposure limits for recreational and commercial divers to low frequency sound, particularly SURTASS Low Frequency Active Sonar (LFAS). The studies concluded that the maximum intensity used during tests (received level of 157 dB) did not produce physiological evidence of damage in human subjects. A two percent very severe aversion reaction was recorded in divers at a level of 148 dB. The NSMRL therefore determined (by scaling back the intensity by 3 dB (a 50 percent reduction in signal strength) that a received level of 145 dB would provide a suitable margin of safety for divers. Consequently, in June 1999, NSMRL set interim guidance for the operation of low frequency underwater sound sources in the presence of recreational divers at 145 dB. This guidance has been endorsed by both the Navys Bureau of Medicine and Surgery and the Naval Sea System Command (British Ministry of Defence, 2004).
Richardson
et al (1995) also noted a number of vertigo and discomfort effects to human divers from underwater sounds. The underwater seismic array emissions are expected to be in the order of 220 - 250 dB re 1μPa at 1 m at source. Richardson
et al (1995) noted that in water depths of 25 to 50 m deep, airgun arrays are often audible to ranges of 50-75 km and that detection ranges can exceed 100 km with efficient propagation or in deep water. Application of such attenuation rates suggest that seismic sounds could be heard by divers for considerable distances from source. In shallow water (20 to 110m deep) basic cylindrical spreading modeling suggests that the limit for humans would be met at around 56 km from the source. However, this does not include the effect of bottom attenuation, which could affect the result by a factor of five.
Permanent Threshold Shift (PTS) refers to an increase in the threshold of hearing that is permanent, not temporary. It is an unrecoverable deafening due to physiological damage to the hearing organs that does not diminish with time. PTS may occur as a result of long-term exposures and/or extremely loud noises. Repeated exposures that cause to temporary threshold shift (TTS) can induce PTS as well.
Mitigation of Impact on Diving and Underwater Related Activities
A detailed communication plan, including a grievance procedure, with regards to the survey timing and potential impacts should be developed. This plan should also list the contact details of dive operators and spearfishing organisations in the region and these groups should be contacted before the commencement of the proposed surveys (specifically the seismic surveys).
As detailed in the UK Diving Medical Advisory Committee (DMAC): Safe Distance from Seismic Surveying Operations (DMAC 12 Rev. 1 July 2011) document the following guidance should be adhered to:
o Where diving and seismic activity will occur within a distance of 10 kilometres of each other, a joint risk assessment should be conducted, between the operators involved and the seismic and diving contractors in advance of any simultaneous operations. Where possible, plans should be made to avoid overlapping seismic and diving activities. Where this is not possible, the activities should be prioritised and a simultaneous operations (SIMOPS) plan developed.
o The parties should perform a communication exercise or test at the start of simultaneous operations to determine the acceptable safe distance for the local conditions. Starting at a distance of 10 km, the seismic source array will be gradually ramped up, and the seismic vessel gradually moved closer to the diving operation, with constant communication between the diving supervisor and the seismic party manager. (Note: seismic source ramp ups are now the industry standard)
o The minimum safe distance, as determined from the testing outlined above, should not be compromised by either party.
o There should be regular contact (at least daily) between the seismic vessel and diving vessel so that both are aware of each others work program for the day.
o Should any diver in the water suddenly experience discomfort, the seismic source should be turned off immediately if requested to do so. The SIMOPS plan should include contingency arrangements for this situation.
o In adverse circumstances, e.g., shallow water or in the presence of thermoclines, the tolerable distance may be increased based on risk assessment or the actual conditions that exist.
o The health impact of exposure to noise in the underwater environment is difficult to assess. A divers exposure should be terminated if the noise level is considered to exceed acceptable noise exposure levels, interferes with diver communications, induces discomfort or places the diver at risk in any other way.
Residual Impact on Diving and Underwater Related Activities
Given suitable management, the significance of pathological impact of seismic surveys on divers would likely be negligible and the impacts on diving, including the overall perceived diving experience would result in an impact of low significance after mitigation.
http://www.erm.com/PageFiles/9613/5. Impact_Africa_EMPR_Chp_5_socio-economic v101.pdf
4.1 Discussion and Recommended Thresholds
The DMAC guidelines for a non-hooded diver exceed the threshold recommended by Parvin et al by 46 dB (see Table 4)
- a very large discrepancy. To put the difference into perspective, note that the DMAC discomfort threshold for
a hooded diver (206 dB re pPa) is 25 dB higher than the maximum permitted by the ethical protocol for Parvin's research. The DMAC guidelines appear to be at least partly based on a publication by Montague
& Strickland [12]. Using a reference pressure of 0.0002 dyn/cm 2, this article quotes a figure of 175 dB for 'oculo-gyral effects' and a 'tolerance limit' of 174 dB (quoted in their abstract, and based on their Fig. 6) for non-hooded divers. 3 Converting to modem units these are 201 dB re 1 uPa and 200 dB re 1 uPa. The threshold for oculo-gyral sensitivity is precisely the value quoted by DMAC
. The origin of the other values in Table 2 is not known.
By comparison with DMAC
, the remaining two sources seem more credible and are reasonably consistent with one another. In particular there is independent support for the threshold of 160 dB re I tPa for frequencies up to 4 kHz [13]
. The NURC thresholds are assumed to apply to alerted but unprotected divers. It is understood [3, 14] that the use of a neoprene suit and hood affords significant protection, potentially resulting in higher thresholds for a suited diver, but it is not known by how much; documentation from the UK-US research [14] is required before the thresholds can be safely increased.
The NURC thresholds do not specify whether the protection afforded by a neoprene suit and hood is assumed (see Sections 2.1 and 3.3). A conservative interpretation results in the thresholds recommended in Table 5 below. Thus, until this point can be clarified, sound pressure levels for alerted and hooded navy divers should not exceed the indicated thresholds. The risks for recreational divers may be greater, requiring lower thresholds. The recommended thresholds are intended for use in the context of Navy sonar, although they are also relevant to comparable equipment used in acoustic communications or oceanographic survey applications. They are not suitable for use with short impulsive sounds such as explosions, pile driving or air guns.
The risk of injury caused by accidental exposure of divers to high levels of underwater sound can be reduced by updating the DMAC guidelines with advice from research more recent than that of Montague & Strickland (1961)
http://www.google.com/url?url=http:...IQFjAD&usg=AFQjCNGfhHLeLt5H3bVkcFljklMlhQy-cg
NATO Undersea Research Centre Human Diver and Marine Mammal Risk Mitigation Rules and Procedures
3.2.3 Recreational Divers
Minimum Safety range 3000 m
US Navy tests using recreational divers indicated that the maximum acceptable sound pressure level for 600 -- 2500 Hz (without causing changes in heart rate or breathing frequency) is 154 dB5. No tests were performed on recreational divers at higher frequencies.
http://www.google.com/url?url=http:...UQFjAD&usg=AFQjCNE2AYj6KI5Wqn41D8e2mEn3skMroQ
