NMFS Biological Opinion on U.S. Navy training ... - Govsupport.us

FINAL PROGRAMMATIC BIOLOGICAL OPINION ON U.S. NAVY ACTIVITIES IN THE HAWAII RANGE COMPLEX 2008-2013
beams covering 120˚ azimuthal sectors that can be swept from side to side during transits (D’Spain et al. 2006).
Waveforms of both sonar systems are frequency modulated with continuous waves (D’Spain et al. 2006).
Sound Propagation
Near an ocean’s surface (roughly the uppermost 150 feet), the sound field will be normally dominated by sound
generated by wave action, rain, and other surface activity; that would mask most anthropogenic sounds. Below the
surface area of this mixed layer, depth (pressure) dominates the sound speed profile and the sound’s speed increases
with depth. Below the mixed layer, sea temperatures drop rapidly in an area referred to as the thermocline. In this
region, temperature dominates the sound speed profile and speed decreases with depth. Finally, beneath the thermocline,
the temperature becomes fairly uniform and increasing pressure causes the sound speed profile to increase
with depth.
Acoustic waveguides, which include surface ducts as well as the SOFAR (sonar fixing and ranging) channel and deep
sound channel of deep waters, focus sound from sources within the waveguide to long ranges. Surface ducts are
acoustic waveguides that occur in the uppermost part of the water column when water near the surface is mixed by
convection by surface wave activity generated by atmospheric winds. This mixing forms a surface layer with nearly
constant temperatures so that sound speeds in the layer increase with depth. If sufficient energy is subsequently
reflected downward from the surface, the sound can become “trapped” by a series of repeated upward refractions and
downward reflections to create surface ducts or “surface channels”. Surface ducts commonly form in the winter
because the surface is cooled relative to deeper water; as a result, surface ducts are predictable for certain locations
at specific times of the year.
Sound trapped in a surface duct can travel for relatively long distances with its maximum range of propagation
dependent on the specifics of the sound speed profile, the frequency of the sound, and the reflective characteristics of
the surface. As a general rule, surface duct propagation will increase as the temperature becomes more uniform and
depth of the layer increases. For example, a sound’s transmission is improved when windy conditions create a wellmixed
surface layer or in high-latitude midwinter conditions where the mixed layer extends to several hundred feet
deep.
5.1.5 Explosions (including pressure waves and sound field)
The U.S. Navy plans to continue to employ several kinds of explosive ordnance in the Hawai’i Range Complex
(Table 9). Explosives detonated underwater introduce loud, impulsive, broadband sounds into the marine
environment. At its source, the acoustic energy of an explosive is, generally, much greater than that of a sonar, so
careful treatment of them is important, since they have the potential to injure. Three source parameters influence the
effect of an explosive: the net effective weight of the explosive warhead, the type of explosive material, and the
detonation depth. The net explosive weight accounts for the first two parameters. The net explosive weight of an
explosive is the weight of only the explosive material in a given round, referenced to the explosive power of TNT.
The detonation depth of an explosive is particularly important due to a propagation effect known as surface-image
interference. For sources located near the sea surface, a distinct interference pattern arises from the coherent sum of
the two paths that differ only by a single reflection from the pressure-release surface. As the source depth and/or the
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