Introduction to Acoustics

5.1 Ocean Acoustic Environment
The acoustic properties of the ocean, such as the paths
along which sound from a localized source travel, are
mainly dependent on the ocean sound speed structure,
which in turn is dependent on the oceanographic environment.
The combination of water column and bottom
properties leads to a set of generic sound-propagation
paths descriptive of most propagation phenomena in the
ocean.
5.1.1 Ocean Environment
Sound speed in the ocean water column is a function
of temperature, salinity and ambient pressure. Since the
ambient pressure is a function of depth, it is customary
to express the sound speed (c) inm/s as an empirical
function of temperature (T) in degrees centigrade, salinity
(S) in parts per thousand and depth (z) in meters, for
example [5.7, 11, 12]
c =1449.2 + 4.6T − 0.055T 2 + 0.00029T 3
1440
0
1
2
3
+ (1.34 − 0.01T)(S − 35) + 0.016z . (5.1)
4
Water depth (km)
1460 1480 1500 1520 Sound speed (m/s)
Polar region
profile
Surface duct
profile
Warmer surface
water profile
Mixed layer profile
Thermocline
Deep sound channel axis
Deep isothermal layer
Fig. 5.2 Generic sound-speed profiles. The profiles reflect
the tendencies that sound speed varies directly with temperature
and hydrostatic pressure. Near-surface mixing can
lead to almost isovelocity in that region. In polar waters,
the coldest region is at the surface
Underwater Acoustics 5.1 Ocean Acoustic Environment 151
Figure 5.2 shows a typical set of sound speed profiles
indicating the greatest variability near the surface.
In a warmer season (or warmer part of the day,
sometimes referred to as the afternoon effect), the
temperature increases near the surface and hence the
sound speed increases toward the sea surface. In
nonpolar regions where mixing near the surface due
to wind and wave activity is important, a mixed
layer of almost constant temperature is often created.
In this isothermal layer the sound speed increases
with depth because of the increasing ambient pressure,
the last term in (5.1). This is the surface duct
region. Below the mixed layer is the thermocline
where the temperature and hence the sound speed
decreases with depth. Below the thermocline, the temperature
is constant and the sound speed increases
because of increasing ambient pressure. Therefore,
between the deep isothermal region and the mixed
layer, there is a depth at minimum sound speed referred
to as the axis of the deep sound channel.
However, in polar regions, the water is coldest near
the surface so that the minimum sound speed is at
the surface. Figure 5.3 is a contour display of the
sound speed structure of the North and South Atlantic
with the deep sound channel axis indicated by
the heavy dashed line. Note that the deep sound channel
becomes shallower toward the poles. Aside from
sound speed effects, the ocean volume is absorptive
and causes attenuation that increases with acoustic
frequency.
Shallower water such as that in continental shelf
and slope regions is not deep enough for the depthpressure
term in (5.1) to be significant. Thus the winter
profile tends to isovelocity, simply because of mixing,
whereas the summer profile has a higher sound speed
near the surface due to heating; both are schematically
represented in Fig. 5.4.
The sound speed structure regulates the interaction
of sound with the boundaries. The ocean is bounded
above by air, which is a nearly perfect reflector; however,
the sea surface is often rough, causing sound to scatter in
directions away from the specular reflecting angle. The
ocean bottom is typically a complicated, rough, layered
structure supporting elastic waves. Its geoacoustic
properties are summarized by density, compressional
and shear speed, and attenuation profiles. The two basic
interfaces, air/sea and sea/bottom, can be thought
of as the boundaries of an acoustic waveguide whose
internal index of refraction is determined by the fun-
Part A 5.1