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

Report No. 6945
BBN Systems and Technologies Corporation
Referring to Fig. 5.3, the black bars represent the SNC rating for the
dominant bandwidth of a specific source. The height of a bar is proportional
to the acoustic energy density level produced by a source type in the Chukchi
Sea Planning Area. The dark gray ba'rs represent the Reference SER rating
which is proportional to degree of matching between the hearing response
(known or assumed) of a species and the acoustic output bandwidth of a source.
The Reference SER r ting is based on the assumption that the species density
in the area is l/km 3 . It is necessary to include the actual species density
in the SER rating since, if the density is zero, there is no probability of
acoustic interaction. Therefore, an area SER rating is also determined which
includes the species density value (observed or estimated) for the area. This
is shown by the light gray bars in Fig. 5.3. If the species density is very
low, the area SER values will be significantly lower than the reference values
showing that, while a species sensitivity for a given source may be high, the
probability for an acoustic interaction in a given area may be low because of
a low probability of encounter.
In order to obtain SER values for gray whales it was necessary to derive
an estimated hearing sensitivity characteristic since no measured data are
available for any baleen whale (Sec. 2.3). This was done using a scaling
procedure based on knowledge of their vocalization frequency range and an
assumption that their maximum hearing sensitivity is comparable to that of the
smaller whales and pinnipeds for which data are available. Since the vocalization
range of gray ihales extends to below 50 Hz, this implies that their
hearing sensitivity is good in this range also. As a result they may be
potentially more influenced by low frequency industrial noise than are species
which have extended high frequency hearing sensitivity. The estimated hearing
sensitivity characteristic and its derivation are discussed in Appendix D.
This estimated gray whale hearing response characteristic was used to
enable SER values to be obtained for this important species. The hearing
sensitivity characteristic for the walrus, which is also .unknown, was assumed
to be similar to that of the California sea lion. Hearing characteristics of
the white whale have been measured and are given in Section 2.3.
The SER values from Table 5.4B which are used for the graphs in Fig. 5.3
show the ratings for source - species pairs which have a high potential for
acoustic interaction in the Chukchi area during summer conditions. As
indicated by the reference SER values, the gray whale is potentially the
species most influenced by all of the sources. The SER rating for the seismic
array - gray whale encounter is the highest in the table, followed closely by
the icebreaker and KULLUK rating for the same species.
The SER value for the gray whale - tug/barge encounter is lower than the
gray whale ratings with the other sources. While the maximum output 1/3
octave band for this type of source nearly coincides with the estimated
maximum hearing sensitivity range of the gray whale, the maximum output level
is more than 15 dB below that of the other sources. The small craft shown in
the SNC model results would also have low SER values. Thus the potential
impact of noise from commercial transport and small craft activities on gray
whales in the Chukchi Sea area during summer conditions is predicted to be
less than produced by the oil industry activities used in the modeling