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

Report No. 6945
BRN Systems and Technologies Corporation
The greatest loss occurs at high frequencies and in the region near shore
where the depth is reduced to 20 m. In this case a TL of 22 dB is observed
for both the 315 Hz and 1 kHz bands, with a value of about 18 dB at 100 Hz.
Since the propagation at 100 Hz is apparently not influenced very much by
either the upward slope or by the change in SVP conditions over the 10 km
range examined in the modeling procedure, it it likely that a significant
amount of low frequency acoustic energy is reaching the near shore area by
bottom refracted transmission in the water-saturated sediments. As shown by
the model results for the very shallow Case 1 geometry, it is necessary for
marine mammals to be very near shore to gain significant shielding from loud
low frequency offshore sources.
4.3 Air-To-Water Transmission
Of the several papers available in the literature concerning transmission
of sound from air into water, most do not consider the effect of shallow water
conditions. Urick (1972) presents a discussion of the effect and reports data
showing the difference in the underwater signature of an aircraft overflight
for deep and shallow conditions. No analysis is presented which would permit
estimation of the effective TL underwater for shallow water multipath
transmission conditions. Young (1973) presents an analysis which, while
directed at deep water applications, derives an equivalent underwater source
for an aircraft overflight which can be used for direct path underwater
received level estimates. Unfortunately, for the aircraft - marine mammal
encounter geometry relevent to this study, the usual sound transmission
involves both direct and bottom reflected paths. Because of this, it was
necessary to develop an analytical model to help predict the total acoustic
exposure level for marine mammals in shallow water near the path of an
aircraft overflight (Malme and Smith 1988).
The model, which was developed for both this study and the related LGL
study of pinniped response to aircraft noise (Johnson et al. 1988), provides
for calculation of the acoustic energy at an underwater receiver contributed
by both the direct sound field and a depth-averaged reverberant sound field.
The direct sound field is produced by sound transmitted into the water along a
direct refracted path from the airborne source to the underwater receiver.
The reverberant sound field is produced by sound reflecting from the bottom
and surface as.it travels outward from the region directly under the
aircraft. An analysis developed by P.W. Smith, Jr. based on an earlier study
of shallow water sound propagation (Smith 1974) is used to predict the
horizontally propagating sound field produced by the reflected sound energy.
Figure 4.8 shows the geometry and parameters used in developing the
air-water transmission model. As depicted in the figure, sound from an
elevated source in air is refracted upon transmission into water because of
the difference in sound speeds in the two media. A virtual source location is
formed which is the apparent location of the source for the sound path in
water. Because of the large difference in sound speeds between air and water
(a ratio of about 0.23) the direct sound path is totally reflected for grazing
angles less than 77 degrees. For smaller grazing angles sound reaches an
underwater observation point only by scattering from wave crests on the