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OCR for page 9
1
Introduction
Sound is an important tool used by ocean scientists to study the topography
of the seafloor and its substructure; the direction and speed of ocean currents; and
the size, shape, and number of organisms in the ocean. Four fundamental proper-
ties of sound transmission are important to understand as background for this
report:
1. The transmission distance of sound in seawater is determined by a combi-
nation of geometric spreading loss and an absorptive loss proportional to the
sound frequency (see Box 1.1~. Thus, attenuation of sound increases as its
~ .
frequency increases.
2. The speed of sound is proportional to the temperature of the seawater
through which it passes.
3. The sound intensity decreases with distance from the sound source. Gen-
erally, the decrease in sound intensity ranges between 1/r (r = distance from the
source) and 1/r2 (spherical spreading), depending on characteristics of the sound
source location and transmission paths, although sound intensity can decrease
even more under certain conditions. Thus, a sound level may be as much as
60 dB lower than that of the source level at 1 km from the source (see Figure 1.1~.
Because of the wave properties of sound and propagation conditions, waves from
different sources or refracted and reflected waves from a single source can con-
verge and either add to or cancel each other, so that simple geometric models of
spreading do not always predict actual sound fields in the ocean. This is espe-
cially true in shallow water.
9
OCR for page 10
10
MARINE MAMMALS AND LOW-FREQUENCY SOUND
4. The strength of sound is measured on a logarithmic scale, 10 logic I/Iref
(I = intensity] ); therefore, 180 dB is 10 times less intense than 190 dB, and 170
dB is 100 times less intense than 190 dB.
Because of property 1, only low-frequency sounds are useful for studying
large-scale processes over long distances, such as the structure of the ocean over
scales of hundreds to thousands of kilometers. For example, sound has been used
to study circulation patterns in the ocean using tomographic techniques analogous
to the CAT [computerized axial tomography] scan technology used in medicine
(Munk and Wunsch, 1979; Munk et al., 1995~. Likewise, sound is used in
geophysical studies to characterize the subsurface structure of the seafloor. The
decrease in sound transmission distance with increasing frequency also has
implications for marine mammal communication because only low-frequency
vocalizations can travel long distances. Because of property 2, sound speed can
be used to infer the average temperature of the water volume through which the
sound waves have passed. Scientists are using the relationship of the speed of
sound and water temperature to infer whether global warming is occurring. The
Acoustic Thermometry of Ocean Climate (ATOC) experiment is monitoring the
1Intensity is considered the fundamental quantity of sound, but it is seldom measured. Instead,
pressure is normally measured. The two are related by I = p2/pocO, where p is the time-averaged
pressure, pO is the density of the medium, and cO is the sound speed in the medium. An acoustic
wave whose pressure is 1 pPa has an intensity of 0.64 * 10-22 watts/cm2. For transient signals, it is
more meaningful to refer to the energy flux density (E) of the acoustic wave. The energy flux
density is the time integral of the instantaneous intensity.
OCR for page 11
INTRODUCTION
Sca Surface
/ /
1,000 m
\~/ 135 did
2~)~}(} m
1 2~) `1D
I (31) m
1 55 d]
Source Level
~ 95 dB
\
\
\
-
-
/
FIGURE 1.1 Calculated received levels at various distances from the ATOC source,
based on spherical spreading, assuming no reflections from the sea surface or bottom.
Received levels are affected by conditions in the ocean, where the source is deployed (on
the bottom or in the water column) and thus reflection from the sea surface and the
seafloor, and directionality of the source. Spherical spreading is a proper assumption at
these distances until sound waves reach a boundary (see Urick, 1983~.
travel time of sound between sources off the coasts of Hawaii and California to
receivers around the Pacific Ocean (see Figure 1.2) for a variety of purposes (see
section below on "The ATOC Concepts.
Ambient noise levels vary both from place to place in the ocean and over
time at each location. The relative frequency bands also vary, due in part to the
nonrandom distribution of vocal animals and human-generated noise. Measure-
ments and predictions of ambient noise in the ocean were made by Knudsen et al.
(1948) and Ross (1976~. Natural ambient noise levels increase as frequency
decreases and are related to the sea state. Ross (1976) reported that the ambient
noise in areas of heavy shipping activity could range between 85-95 dB (1 Hz
bandwidth), peaking at a frequency of about 100 Hz. High sea states can produce
similar levels of ambient noise.
Some whales, seals, and fish use low-frequency sound to communicate and
to sense their environments (Tyack, 1998~. For example, baleen whales and
OCR for page 12
2
Dual
vl~ 1
j
MARINE MAMMALS AND LOW-FREQUENCY SOUND
it:
- \ 30°
Gus\\'-.\
a
°
~60°
FIGURE 1.2 ATOC sources at Pioneer Seamount and Kauai, showing transmission paths
to receivers in different parts of the Pacific Ocean.
some toothed whales are known to use and respond to low-frequency sound
emitted by other individuals of their species (McDonald et al.,1995; Edds-Walton,
1997; Ljungblad et al., 1997; Stafford et al., 1998~. Sharks and some other fish
species are able to sense and react to low-frequency sound (Myrberg et al., 1976;
Myrberg, 1990~. Therefore, it is possible that human-generated low-frequency
sound can interfere with the normal behavior of some marine animals and there is
some evidence that this occurs (Myrberg, 1978, 1980, 1990; Richardson et al.,
1995~. Serious misunderstandings of the potential effects of sound of various
intensities on marine mammals have occurred because the levels of sound inten-
sity in water and in air have not been consistently (or in some cases, correctly)
OCR for page 13
INTRODUCTION
13
referenced to the International System of Units (SI) standards2 that have been
established and in-water sound levels have been misunderstood to be comparable
directly to in-air levels, with which most people are more familiar. Air-water
comparisons are inherently misleading.
THE ATOC CONCEPT
Individuals studying the ocean or using it for scientific, commercial, or
military purposes use underwater sound as a major tool to monitor and explore
the ocean's contents and boundaries. Sound also enters the ocean as a result of
natural environmental processes, biological activity, and human activities unre-
lated to study of the ocean, such as the propulsion noise of ships (Table 1.1~.
Although all kinds of sounds are used, many applications have used sound in the
1- to 100-Hz frequency range because absorption of these sound frequencies by
seawater is minimal, variability caused by the environment is somewhat reduced,
and long-range propagation is possible, making underwater or subbottom remote
sensing feasible.
An example of the scientific use of low-frequency sound in the ocean was
the Heard Island Feasibility Test (HIFT), in which sound was transmitted from
one array of sources with a sound level of 221 dB (rms)3 and a frequency of 57 Hz
(rms) through the ocean to a number of receivers over distances of up to 16,000
km (Baggeroer and Munk, 1992~. A major goal of HIFT was to serve as a
prototype for regular observations of the speed of sound in the ocean as a direct
means of measuring the rate of ocean warming due to global climate change. The
regular observations were proposed as the long-term ATOC experiment.
The issue of global warming is of major significance to scientists,
policymakers, and citizens worldwide, yet it has been difficult to determine the
extent of atmospheric and oceanic warming based on observations of global air
and sea surface temperatures.4 The advantages of long-distance sound transmis-
sions in the ocean are that (1) low-frequency sound waves pass through and thus
sample a wide range of ocean depths between the source and the receivers,
(2) the summed effects of random variability along the transmission path due to
eddies and variations in ocean currents are minimized, and (3) longer-distance
2ANSI S1.8-8-1989 (ASA 84-1989), Revision of S1.8-1969 (R 1974), Reaffirmed by ANSI on
July 29, 1997. In the International system of units (SI), acoustic pressure is expressed in watts per
square meter, but the do notation is used more commonly at the present time.
3The amplitude of pulsed sounds is typically expressed as ``peak-to-peak,, (one cycle of the sine
wave) or ``zero-to-peak,, (one-half cycle). continuous sounds may be expressed as ``root-mean-
square (rms), which is the square root of the time average of the square of a quantity; for a periodic
quantity the average is taken over one complete cycle (Lapedes, 1974).
4The NRC recently issued a report on the measurement of atmospheric global warming (NRC,
2000).
OCR for page 14
4
MARINE MAMMALS AND LOW-FREQUENCY SOUND
TABLE 1.1 List of Some Anthropogenic Sounds, Including Sources,
Frequencies, and Levels
Source
Frequency at Highest Level Source Level at Highest Level
1/3-Octave Band (Hz) 1/3-Octave Band (dB re 1 pPa @ 1 m)
5-m Zodiac inflatable boata 6,300
Bell 212 helicopterb 16
152
159
Large tanker 100 + 125 177
Icebreaker 100 183
ATOC 75 195c
Air gun array (32 guns) 50 2lod
HIFT 50 + 63 221e
Military search sonar 2,000-5,000 230+
SOURCE: Richardson et al. (1995, Table 6.9).
aSpeed and horsepower of engines were not given in Richardson et al. (1995).
bAircraft flyover source levels were computed by Malme et al. (1989) for a standard altitude of
1,000 ft (305 m). For consistency with other sound sources, these values were changed to a reference
range of 1 m by adding 50 dB.
CNumbers provided by ATOC investigators from actual transmissions, rather than from Richardson
et al. (1995).
dAnecdotal evidence suggests that airgun arrays may reach source levels of 240 dB.
eNumbers provided by Heard Island Feasibility Test (HIFT) investigators.
transmissions may allow the detection of smaller changes in temperature. The
designers of ATOC hope to conduct ATOC transmissions from Kauai for at least
5 additional years to make a quantitative assessment of the role that acoustic
thermometry can play in an integrated ocean-observing system for ocean weather
and climate in the North Pacific Ocean (P. Worcester, Scripps Institution of
Oceanography, personal communication, 1999~. Results from the initial ATOC
transmissions indicate that this technique shows promise for at least one of its
planned applications, namely, to use ocean temperature measurements from
ATOC to constrain climate models (ATOC Consortium, 1998~. ATOC Consor-
tium investigators compared sea-level estimates derived from historic averages,
ATOC-based tomography, recent direct measurements, results from a general
circulation model (GCM), and data from a satellite-based altimeter. Combina-
tions of the GCM, altimeter, and ATOC data show that the GCM alone underes-
timates the magnitude of the seasonal sea surface heat flux cycle. However,
despite the usefulness of acoustic tomography programs like ATOC and other
research uses of low-frequency sound in the ocean, concern exists that adding
more sound to the ocean could harm marine mammals, sea turtles, and other
organisms, as the following section will describe.
It is necessary to sample the ocean's temperature frequently enough to be
able to distinguish any trend of temperature increase amidst the "noise" of random
OCR for page 15
INTRODUCTION
15
variations created by temperature, tides, and mesoscale ocean structure.5 Infor-
mation gained from ATOC-like techniques cannot be replaced by measurements
from satellites because satellites can only sense features of the surface layer of
the ocean. The temperatures from the full depth of the ocean can be measured in
situ, but such measurements are limited in number and frequency because of the
cost and limited number of oceanographic ships, moorings, and drifters available,
compared to the great volume of the ocean. The planned Array for Real-time
Geostrophic Oceanography (ARGO) drifter program will provide a new and
innovative means of measuring ocean interior temperatures over large scales
(albeit not in an integrated, synoptic manner), and may provide a complement to,
and possibly a replacement for, ATOC-type acoustic measurements of water
temperature.
LOW-FREQUENCY SOUND AND MARINE VERTEBRATES
It is well known that some marine organisms produce low-frequency sounds
and/or can hear such sounds. For example, there is evidence that baleen whales
(such as finback [Balaenoptera physalus], blue [Balaenoptera musculus], and
humpback whales [Megaptera novaeangliae]) communicate using low-frequency
sound (e.g., reviewed in Edds-Walton, 1997~. Table 1.2 shows the frequency
range and dominant frequencies of the vocalizations of a sample of baleen whales,
toothed whales, and seals. The geographic extent of the use of low-frequency
sounds by baleen whales is being monitored on an experimental basis in the
Atlantic and Pacific oceans using a novel source of data the Integrated Under-
sea Surveillance System (IUSS) formerly known as the SOund SUrveillance
System (SOSUS) which was originally designed for tracking submarines. The
IUSS has allowed the tracking of individual whales in at least a few cases by
triangulating the positions of vocalizations over time (Clark, 1995; Stafford et al.,
1998; Watkins et al., 2000~. Such data are important in determining the migra-
tion behavior of individual whales and in assessing whether human influences
change these pelagic migrations. Richardson et al. (1995) present a comparison
of the audiograms of some species of marine mammals (see Figure 1.3~. Addi-
5As Peter Worcester, ATOC principal investigator, explained to the Committee in writing in 1999
"The required duty cycle is actually set by the need to avoid aliasing of rapidly changing oceano-
graphic phenomena. If high-frequency phenomena are sampled at too low a rate, they will errone-
ously appear in subsequent analyses as low-frequency variability. In general one needs to sample at
more than twice the highest frequency containing significant energy to avoid aliasing. In the case of
the ocean, mesoscale variability has timescales from a week to a few months, and so needs to be
sampled every few days. The tides are of course even higher frequency, but because their frequen-
cies are well known, they can be sampled adequately using a frequency of approximately 1 day out
of every few days. This combination of ocean phenomena led to a 2 percent duty cycle being used,
consisting of 1 day with six 20-minute transmissions at 4-hour intervals to adequately sample tidal
variability, occurring every fourth day to adequately sample ocean mesoscale variability."
OCR for page 16
16
MARINE MAMMALS AND LOW-FREQUENCY SOUND
TABLE 1.2 Frequencies Used in Communication and Echolocation by
Selected Manne Mammals
Species Frequency Range (Hz)a Dominant Frequencies (Hz)b
Selected Baleen Whales
Gray Whale
adults 20-2,000 20- 1,200
calf clicks 100-20,000 3,400-4,000
Humpback Whale 30-8,000 120-4,000
Finback Whale 14-750 20-40C
Minke Whale 40-2,000 60-140d
Southern Right Whalee 30-2,200 50-500
Bowhead Whale 20-3,500 100-400
Blue Whale
Atlanticf — 1 0-20h
Pacificg 10-390 16-24
Selected Toothed Whales
Sperm Whale (clicks) 100-30,000 2,000-16,000
White Whalei
whistles 260-20,000 2,000-5,900
clicks 40,000-120,000
Killer Whale
whistles 1,500-18,000 6,000-12,000
clicks 1,200-25,000
Bottlenose Dolphin
whistle 800-24,000 3,500- 14,500
clicks) 1,000- 150,000 30,000- 130,000
Selected Seals
California Sea Lion (in air) <1,000-<8,000 500-4,000
Harbor Seal (in air) <100-150,000+ <100-40,000
Gray Seal 100-40,000 100- 10,000
SOURCE: Modified from Richardson et al. (1995).
aThe frequency range listed is the lowest to highest frequencies listed by Richardson et al. (1995)
and more recent authors. Gaps in the ranges are not shown.
bDominant frequencies are essentially the bandwidth of sound that has the greatest energy. They
do not include all the frequencies produced, since there may be many weak harmonics.
CEdds (1988).
dEdds-Walton (2000).
eAlthough few recordings exist, the northern right whale repertoire is likely to be similar.
fPublished data are too limited to give the frequency range for this population.
"Stafford and Fox (1996).
hEdds (1982).
iW. Perrin and D.W. Rice, both NMFS experts in taxonomy, verified that individuals of the
species Delphinapterus leucas can be called white whales, belugas, or belukhas. White whale is
used throughout this report.
iRidgway and Au (1999).
OCR for page 17
INTRODUCTION
17
tional information about the effects of low-frequency sound on marine mammals
is contained in Chapters 2 and 3.
Low-frequency sounds are used by other marine vertebrates, including sharks
and bony fish (Myrberg, 1972, 1978, 1980,1990~. Sharks are attracted to sources
emitting such sounds as possible food indicators (e.g., Myrberg, 1978), and many
species of fish use low-frequency sounds for communication (e.g., Demski et al.,
1973~.
ORIGIN OF STUDY
As a result of issues raised by HIFT, the Office of Naval Research (ONR)
requested in 1992 that the National Research Council examine the state of knowl-
edge of the effects of low-frequency sounds on marine mammals and assess the
trade-offs between the benefits of underwater sound as a research tool and the
possible harmful effects on marine mammal populations of introducing addi-
tional low-frequency sound into the ocean. In 1994 the NRC issued a report, Low
Frequency Sound and Marine Mammals: Current Knowledge and Research
Needs, which concluded that (1) very little is known about the effects of low-
frequency sound on marine mammals and (2) it is difficult to establish regulatory
policy in the absence of data regarding such effects (see Appendix B for the
executive summary from that report). The report included a series of recommen-
dations about the kinds of research needed to fill the gaps in our knowledge.
Subsequent to HIFT, the ATOC program was proposed with a mission to
make regular measurements of the travel times of low-frequency sound through-
out the Pacific Ocean (Figure 1.2~. As a result of concerns about the effects of
low-frequency sound added to the ocean by ATOC, the ATOC program con-
ducted the first several years of transmissions under a permit to test the effects of
ATOC sound sources on marine mammals through a Marine Mammal Research
Program (MMRP). The Defense Advanced Research Projects Agency requested
that the NRC update the information contained in its 1994 report based on the
MMRP and other results.6 In addition, the NRC was asked to ascertain how data
acquired since 1994 fulfill the research needs described in the 1994 report. An
interim NRC report published in 1996 provided guidance to the MMRP in the
midst of its observational studies. The director of the ONR program (Robert
Gisiner) and the principal investigators of the MMRP (Christopher Clark and
Daniel Costa) briefed the NRC's Committee to Review Results of ATOC's Ma-
rine Mammal Research Program in 1996 and 1999 and participated in subsequent
. .
Open c 1scusslons.
The Committee summarizes and comments on the results of the MMRP in
Chapter 2. Chapter 3 is devoted to updating the research priorities first identified
6The MMRP formed its own advisory board to provide independent advice to MMRP investiga-
tors regarding MMRP needs, plans, schedules, and research results.
OCR for page 18
18
Q
160-
~ 120-
m
-
o
a:, 80-
t60
_`
~ 120
m
-
o
Hi, 80
MARINE MAMMALS AND LOW-FREQUENCY SOUND
Underwater Audiograms of Hair Seals
T
;~>,:.
. .—
,. . ~
·
~ _ ~ I
..~..
HarborJM
.: ~
. - HarborJT
tic .. ,. i-]
~i. ~ . Harbor/K
tG - . +
/~ Ringed
.' / 1: - At
Harp
...~...
. Monk
. 1 1 ~ I
1 ,000,000
4 JO Coon - ''io,ooo ~oo,ooo
Frequency (Hz)
Underwater Audiograms of Eared Seals
C Sea LionlS
C Sea LioniK
N Fur Seal/hA
N Fur Seal/B
1 1 T I ~ I T '-i I ~ r- I t~ I - T I I I I T 1
1 )O 1,000 1O,OOO 10O,000 1,00O,OOO
Frequency (Hz)
FIGURE 1.3 Audiograms of representative seal and toothed whale species. Source:
Richardson et al. (1995~; used with permission from Academic Press. References for
these data are given in the source document. In most cases, the data represent measure-
ments on one or two individuals of the species.
NOTE: Complete audiograms should be U-shaped. If not, hearing should be tested at
higher or lower frequencies, as necessary. For example, the audiograms of the true seals
(shown as "hair seals" in the figure) appear to be truncated at lower frequencies.
OCR for page 19
INTRODUCTION
140-
100-
a'
m
at}
0, 60-
20-
140-
it
~ 100-
a)
m
_
o
u, Sol
19
Underwater Audiograms of Odontocetes
w.
,(
Sax
''' 1
.. _ , ,,. ~1
I -I I I I 111' -- -' - l~~-l~T r-rll' - I I I ~ 11~1.
10 100 1,000 1 O'OOO
Frequency (Hz)
.,
Am.
~-
.'
Y .
Am,
14
->c- -
Beluge
Killer Wh
Herb Porp
_
Beiji
. . . .
100,000 1,000,000
Underwater Audiograms of Odontocetes
x~
No
.~
i,
NC_
or
At"=
\ '.
" ~ - - 1- ~ 1 1 1 ~ I
1 ,000 1 0,000
Frequency (Hz)
A.,
Bat [:)ol/J
...~...
Bot Dol/L
False Kil
Risso's D
Bo~u
I - 1--1-- 1 T 1 T 1 TTT|
1 00,000 1 ,000,000
20- . . .
10 100
_ 1 1 111111-
OCR for page 20
20
MARINE MAMMALS AND LOW-FREQUENCY SOUND
in the 1994 NRC report, based on data obtained from research conducted by the
MMRP, as well as the results of other relevant research such as that sponsored by
the ONR program on marine mammals and ocean acoustics. Based on this
comparison of recent research achievements and research needs listed in the 1994
report, the present report identifies areas in which gaps in our knowledge con-
tinue to exist. Chapter 4 discusses regulatory issues, such as how permits for
acoustic and marine mammal research are issued. Chapter 5 draws together the
Committee's findings and provides recommendations based on these findings.
Representative terms from entire chapter:
baleen whales
