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

Report No. 6945
BBN Systems and Technologies Corporation
Time Ratio - The fraction of time that a source is within 5 dB of its
maximum output level (see definitions in Sec. 3.1.2).
Measured/Estimated - "Measuredtt if the original reference included either
source level data or transmission loss information to derive it,
"Estimated" if the original reference included only received level and
range information.
Reference - See the Sources Cited Section for the complete reference
listing.
3.3.1 Industrial noise sources
This section includes representative source information from the
petroleum industry and other types of Alaskan coastal industries as shown in
Table 3.2. The table is arranged in decreasing order of source level in the
dominant bandwidth. Information on the temporal characteristics of the
sources is also included but this column is primarily based on estimates.
Unfortunately many data references do not include information on the time
pattern of sources.
The loudest industrial sources can be seen to be the seismic survey
airgun array and the vibroseis system used for on-ice seismic exploration.
The levels reported are peak 1/3 octave levels for the airgun array and
average 1/3 octave, as converted from narrow-band data, for the vibroseis.
The vibroseis data were measured by a hydrophone in water under the ice at a
position to the side of the array (Cummings et al. 1981). Both sources
deliver short bursts of energy. The loudest of the sources that produce much
longer high level sound sequences is the icebreaker which is used in both
petroleum and transportation industries. The high level sound from icebreaker
operation is produced by propeller cavitation as the vessel pushes against the
ice with very little forward motion. The underwater sound of breaking ice is
not a significant factor in the sound output of the icebreaker.
The source level data shown in Table 3.2 for the icebreaker was obtained
for operation of the Canadian icebreaking supply vessel ROBERT LEMEUR at the
Corona drill site in the Alaskan Beaufort Sea. This vessel has a shaft
horsepower rating of 9,600 BHP. The U.S. Polar Class icebreakers, which have
a rated maximum horsepower of 60,000 BHP, and many of the other Canadian
icebreakers are larger and are expected to have higher radiated noise
levels. While no data were found for the Polar Class icebreakers operating in
heavy ice, their predicted source levels are about 8 dB higher than that of
the LEMEUR, on a horsepower scaling basis. A detailed analysis of icebreaker
noise is given in Appendix B as an example of statistical procedures used for
describing a time-varying source level spectrum.
Industrial source temporal characteristics
Figure 3.11 shows some of the results of a probability density analysis
by Greeneridge Sciences of a continuous series of 1/3 octave pressure level
spectra. This series was obtained from a 14 min. segment of radiated noise
from the ROBERT LEMEUR operating in heavy ice at the Corona Site in the