OEAS 604
LECTURE NOTES
SOUND AND LIGHT IN THE OCEAN
Both sound and light can be transmitted in the ocean. While sound can be
transmitted quite efficiently over long distances in the ocean, light is attenuated rapidly,
only impacting the near surface region. The physical properties of sound and light
transmission are well understood and are integral to the ways in which oceanographers
study the ocean.
Sound in the Ocean
Because sound is easily transmitted through seawater, it is used both actively and
passively in oceanography. The speed of sound (c) equation can be approximated as:
c = 1448.96 + 4.6T − 0.055T 2 + 0.0003T 3 + (1.39 − 0.012T )( S − 35) + 0.017Z
where T is temperature (degrees C), S is salinity (psu) and Z is depth (dbars). The
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average speed of sound in the ocean is approximately 1500 m/s and varies by about 4 m/s
for each degree change in temperature, about 1.4 m/s for each unit change in salinity, and
about 17 m/s for each 1000 m of depth. Given typical temperature and salinity profiles
in the ocean, the speed of sound generally decreases with depth from the surface toward
the base of the thermocline (mainly because of temperature effects) and then increases
with depth below that (mainly because of pressure effects). As a result, a region of
minimal speed of sound is usually found below the main thermocline in the ocean. This
interface creates a sound “pipeline” or “deep sound channel” within the oceans that
allows the transmission of low frequency sound over thousands of kilometers. This
sound fixing and ranging (SOFAR) channel was discovered in 1943 by an American
team led by Maurice Ewing and J. L. Worzel. Ewing and Worzel demonstrated that the
SOFAR channel was capable of transmitting the low frequency, long-wavelength sound
waves over thousands of kilometers. SOFAR channels occur because of the refraction of
sound waves that occurs when sound moves through regions of varying sound speed.
Like all waves, sound waves are refracted and bend toward regions where wave
propagation is slower. As a result, sound is focused at the vertical location coincident
with the speed of sound minima.
While SOFAR channels allow sound to transmitted long distances through the
ocean, both absorption and scattering attenuate sound energy. The attenuation of sound is
well approximated by Beers law, given as:
I = I 0 e −jR
where the intensity of sound (I) is a function of the initial intensity (I0), which
exponentially decreases as a function of range (R), depending on the attenuation
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coefficient R. The absorption coefficient increases with the square of the sound frequency
(R~f2) and as a result, high frequency sounds are attenuated much more rapidly than
lower frequency sounds. Sound is also reflected in the ocean, most notably in regions
where the acoustic impedance changes. The most obvious regions for reflection are at
the surface and bottom of the ocean. However, strong internal density gradients can also
lead to significant reflection. Reflection increases with decreasing angle of incidence and
with larger changes in acoustic impedance. As a result, sound is almost completely
reflected at the air-sea interface. In contrast, a small fraction of sound is reflected at the
seabed allowing sound to penetrate into the underlying sediments.
Light in the Ocean
Sunlight in the ocean is important for many reasons: It heats seawater, warming
the surface layers; it provides energy required by phytoplankton; it is used for navigation
by animals near the surface. Because light travels slower in water than in air, some light
is reflected at the sea surface. However, most of sunlight reaching the sea surface is
transmitted into the sea. Once light enters the ocean, it is attenuated rapidly. Attenuation
is due to absorption by pigments and scattering by molecules and particles and depends
on wavelength. Blue light is absorbed least; red light is absorbed most strongly. While
the attenuation of light is a function of wavelength, in many situations it is easier to
simply represent the total attenuation of visible light. Consistent with how we represent
the attenuation of sound, the attenuation of light also follows Beers law:
I = I0e −kz
In this case, I represents the irradiance at any given depth for a known surface irradiance
€ is now given as k and z represents the distance from the
(I ). The attenuation coefficient
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surface. In very clear open ocean water, attenuation coefficients are small and light can
penetrate to greater depth. In turbid coastal waters, light is attenuated rapidly, with high
corresponding values of attenuation coefficient. Chlorophyll pigments in phytoplankton
absorb and scatter light, increasing the attenuation (decreasing light availability). The
presence of chlorophyll also alters the color of the ocean. As a result, the intensity of
color that is reflected back from the ocean surface can provide information about the
concentration of chlorophyll in surface waters. This allows satellite-derive estimates of
biologic productivity on global scales.