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

Report No. '6945
BBN Systems and Technologies Corporation
of sensing infrasound pressure fluctuations, turbulent flow may contribute to
the ambient noise sensed by the mammal. Figure 3.2 provides three curves for
turbulence pressure fluctuations due to tidal and oceanic currents ranging
from a low of 40 cm/s (0.8 kt) to a high of 400 cm/s (8 kt), based on an
analysis presented by Wenz (1962). He shows that turbulence pressure (6)
varies as:
where p is the density of the ocean water and is the r.m.s. velocity of
fluid within a turbulence cell. This turbulence velocity is about equal to 5%
of the mean flow velocity (U) of the ocean current. Hence the turbulence
pressure levels are proportional to the square of the turbulence velocity.
The frequency of the pressure fluctuations is directly proportional to the
mean flow velocity.
Tidal currents along the Alaskan coast can be extreme at narrow entrances
to tidal bays. For instance, at Inian Pass at the entrance to Icy Strait in
Southeast Alaska, and in Glacier Bay at Sitakaday Narrows, 7 knot (360 cm/s)
tidal currents are common. The U.S. Department of Commerce Nautical Chart No.
17300 states that currents in Inian Pass may reach 8-10 knots (400-500 cm/s).
Reed and Schumacher (1986) show that ocean currents driven by long term
prevailing winds and geostrophic flow in the Alaska Coastal Current have
prevailing rates during most of the year of 40-50 cm/s particularly along the
Alaska Peninsula and in the Shumagin Island area. In the fall, the Alaska
Coastal Current causes currents of greater than 100 cm/s in several areas of
the Aleutian Islands. Pearson et al. (1981) report prevailing surface
currents of 40-60 cm/s in the North Aleutian Basin and Norton Basin areas.
Tidal currents in constricted areas of these regions can also reach the high
rates seen in Southeast Alaska.
Thus, the three turbulence curves related to oceanic and tidal currents
(Fig. 3.2) provide an indication of the very low frequency envelope or range
of "sound" pressure levels which can be experienced along some parts of the
Alaskan coastline.
3.2.5 Seismic noise (earthquakes and volcanic activity)
Since the southern coastal and continental shelf regions of Alaska
represent one of the most seismically-active regions on earth, particularly
from the Cook Inlet area west along the Alaska Peninsula and Aleutian I'sland
chain, it is important here to consider underwater sound signals due to
earthquakes as well as the seismicity of Alaska. The MMS lease sale areas
which have the most potential of experiencing earthquakes and short-term
underwater sounds due to them are those located in the Gulf of Alaska and
Bering Sea regions.
Figure 3.2 provides representative underwater sound spectra associated
with two earthquakes: a Magnitude 4.75 earthquake occurring at Cape
Mendocino, California--890 km from the measurement system (Milne, 1959) ; and a