Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technologies Corporation
be used with the TL spectra in Fig. 4.1 to estimate received levels near the
ground. Examples of this procedure are presented in Sec. 5.3.
4.2 Underwater Sound Transmission
In unbounded deep water sound transmission characteristics are determined
by geometric spreading loss and molecular absorption of the sound energy in
the same manner as in atmospheric transmission. Molecular absorption losses
are much smaller underwater, however, and are not significant for frequencies
less than 5 kHz and ranges less than 5 km. Sound transmission in shallow
water is.influenced by reflection losses from the bottom and surface, refraction
from sound speed gradients, reflection and refraction from subbottom
layers, and scattering from rough surfaces. All these effects must be
considered along with geometric spreading loss to obtain estimates of the
received level at some distance from a source.
The large variability in temperature and salinity characteristics of
Alaskan coastal waters has a significant influence on sound propagation. Two
representative sound speed profiles are shown in Fig. 4.2. The strong surface
layer condition occurs in many areas during July - September when solar heating
is high. The higher temperature region near the surface is associated
with a lower salinity layer produced by runoff from rivers which floats on top
of the denser ocean water. While the sound speed in fresh water is slower
than that in ocean water, the temperature difference near the surface more
than compensates for the effect of the lower salinity. Since sound travels
faster in warm water than cold, the net effect is a downward refraction of
horizontally traveling sound rays. This produces more bottom reflections per
kilometer and higher transmission loss than would be the case if the high
sound speed surface layer did not exist.
During the period of November - May when the surface is generally colder
than the water at depth, the sound speed profile tends toward the neutral
condition shown in Fig. 4.2. Under these conditions sound is not refracted
downward and the influence of the bottom on the transmission loss is reduced.
In ice-covered areas, the colder region near the surface produces upward
refraction so that the ice layer roughness often becomes a more significant
influence in sound transmission loss than the bottom properties (Milne 1967).
Several analysis techniques and computer-based mod-els have been developed
to aid in the prediction of acoustic transmission loss characteristics (Miles
et al. 1987; Malme, Smith and Miles 1986). These procedures use measured
sound speed profiles, bottom-loss parameters, and surface loss parameters in
addition to spreading loss calculations to obtain their results. Several
models have been developed for Navy applications such as the Generic Sonar
Model (Weinberg 1985). Most of these are intended primarily for application
to deep water areas. However, a recently developed model which is based on a
procedure for solving the parabolic wave equation (Lee and Botseas 19821, can
be applied to shallow water transmission. Moreover, it has provision for
range-dependent parameters such a a sloping, non-uniform bottom, and rangevarying
sound speed profiles. This l'Implicit Finite-Difference (IFD) Computer
Modeln developed at the Naval Underwater Systems Center was used to compute