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

Report No. 6945
BBN Systems and Technologies Corporation
specific site. This distance is sufficient to provide for inclusion of siteand
frequency-specific propagation effects for depths less than 50 m and is
representative of distances for which behavioral influences have been observed
in several species exposed to moderate source levels (Sec. 2.4).
The effective density of a given source type within a specific 1 km 2
region is determined by observation or by the use of statistical probability
based on knowledge of source concentration locations and travel patterns for
moving sources. The effective source density is used to determine the probability
of encounter (Pe) for specific source - marine mammal situations. This
can be applied to moving sources as well as fixed sources by recognizing that
the probability of a marine mammal encounter'ng a source (or vice-versa) is
prqportional to the number of sources per km 3 and to the number of mammals per
km . It is also proportional to the speed of travel of both source and
receiver if they are moving. This is a result of the model requiring an
estimate of Pe over a 2 hour reference period rather than just an instantaneous
value. This probability calculation requires estimates of both the
number of sources and the number of mammals in a given area, as well as speed
of travel information. In developing the physical acoustics portion of the
model we have assumed that a subject mammal is present, so that this portion
of the joint probability estimate is unity. Thus, for the present, we need to
consider only the probability of this mammal encountering a source.
If a specific source type may be found with equal probability anywhere
within a defined area, then the probability of encountering this source within
a 1 km 2 zone surrounding a randomly selected receiver location is 1/A where A
is the total area defined in the modeling procedure. If there are N sources
in the total area, then Pe = N/A which is equal to the source density. This
procedure is applicable to both fixed and moving sources since, for a fixed
source, the receiver may be located anywhere in the model area with equal
probability unless specific sites are being modeled. In this case, fixed
sources in the area would have a Pe = 1.
If the source types being considered are not uniformly distributed
because of geographic or operational constraints, then appropriate probability
functions must be used to specify Pe in terms of receiver location. These
specialized probability functions can be estimated by considering the areas of
concentration associated with specific source types within a larger total
region involved in a general analysis. This procedure is used in applying the
SNC Model to specific OCS planning areas where source concentrations such as
fishing areas, coastal shipping lanes, and airports are located. In these
cases, the area used in estimating the Pe value for a given source type is
determined by the size of the region(s) where these sources are located most
of the time.
In the special case of airports, the region of highest sound concentration
is located off the ends of runways. When the flight pattern from a
runway is located over water, aircraft sound enters the water along a narrow
track under the flight pattern and is propagated horizontally to a degree
determined by the bottom conditions and water depth, as shown previously in
Section 4.3. At some distance from the airport the sound in the water
produced by larger aircraft usually drops below ambient levels as the aircraft