How is sound used to measure temperature
in the ocean?
The speed of sound in water depends on the water properties
of temperature, salinity and pressure (directly related to the depth). A typical speed
of sound in water near the ocean surface is about 1520 meters per second. That is
more than 4 times faster than the speed of sound in air. The speed of sound in water
increases with increasing water temperature, increasing salinity and increasing
depth. Most of the change in sound speed in the surface ocean is due to changes in
temperature. This is because the effect of salinity on sound speed is small and
salinity changes in the open ocean are small. Near shore and in estuaries, where the
salinity varies greatly, salinity can have a more significant effect on the speed of
sound in water. As the depth increases, the pressure of the water has the largest
effect on the speed of sound.
The approximate change in the speed of sound with a change in each property:
Temperature 1°C = 4.0 m/s
Salinity 1PSU = 1.4 m/s
Depth (pressure) 1km = 17 m/s
Note: Changes in the speed of sound for a given property are not linear.
Under most conditions the speed of sound in water is simple to understand. Sound
will travel faster in warmer water and slower in colder water. To measure the
temperature of the water, a sound pulse is sent out from an underwater sound
source and heard by a hydrophone in the water some distance away (up to
thousands of kilometers). The time the sound takes to go from the source of the
sound to the listening device (a hydrophone) is measured. From the travel time, the
speed of sound between the source and the hydrophone can be calculated. If the
salinity and depth where the sound traveled are known, the temperature of the water
can be calculated. Two specific methods of measuring the temperature of the ocean
with sound are explained below.
Acoustic Tomography
Schematic of an ocean acoustic tomography experiment. Black dots (S) are sound sources; open dots (R)
are receivers (hydrophones). The dark area represents an ocean eddy with a different average sound speed than
the surrounding ocean water.
Acoustic Tomography uses precise measurements of acoustic travel times to draw
ocean temperature maps, showing ocean temperatures just as weather maps show
temperatures in the atmosphere. Data from many crossing acoustic paths are used
to generate these maps of ocean temperatures. In the figure above, for example,
four acoustic sources (S) are shown transmitting to five acoustic receivers (R), giving
20 acoustic paths through a region roughly 300 kilometers on a side. (This geometry
was actually used in an experiment conducted in 1981.) Suppose that the shaded
region is warmer than its surroundings. Sound that travels through the warm region
will travel slightly faster than sound that does not, because sound speed increases
with increasing temperature, as described above. The travel times of sound pulses
traveling through the warm region will therefore be slightly shorter than they would
have been if the warm region was not there. By combining all of the different travel
times it is possible to draw a map showing the warm and cold regions through which
the sound has traveled. This is important because the ocean has “weather” just as
the atmosphere does. Warm and cold “eddies” that are the oceanic equivalent of
atmospheric storms move around, grow, and weaken. There are subsurface oceanic
cold and warm fronts just as there are cold and warm fronts in the atmosphere.
These eddies and subsurface fronts have important effects on marine life, with
marine animals that prefer warm water tending to remain in warm eddies, for
example.
The basic principles used in Acoustic Tomography are closely related to those used
in CAT (Computed Axial Tomography) scans in medicine. In a CAT scan,
the absorption of X-rays are used to map a “slice” through the human body. (“Tomo”
is derived from the Greek word for cut or slice.) In Acoustic Tomography, the travel
time of sound waves are used to map temperatures in a “slice” of the ocean.
Even when it is not feasible to have enough sources and receivers to make detailed
temperature maps, acoustic travel times can be used to obtain the average
temperatures along the paths which the sound traveled. This is sometimes called
“acoustic thermometry.” The ATOC (Acoustic Thermometry of Ocean Climate)
project measured average temperatures in the North Pacific Ocean, for example,
along a number of paths. Acoustic sources off central California and north of Kauai
(Hawaii) transmitted to U. S. Navy receivers, giving a sparse network of acoustic
paths. They observed large-scale seasonal changes in ocean temperature.
Transmissions continuing for many years could be used to measure large-scale
climate change in the ocean.
Inverted Echosounders
Inverted echo sounder. Photo courtesy of Dr. Randy Watts, URI/GSO
Inverted Echosounders (IES) measure the temperature of the water column at a
single point. The IES is attached to the ocean bottom. It emits a sound pulse aimed
toward the surface of the ocean. The sound pulse will reflect off the surface of the
ocean and return to the bottom. The IES listens for the return of the sound pulse
from the ocean surface. The travel time of the sound is used to calculate the speed
of sound through the water. The temperature profile is calculated from the speed of
sound through the water. The IES must be calibrated with a measurement of the
water column properties. Sometimes a pressure sensor is used with the IES to make
the calibration.
Inverted echosounders are often used to monitor a particular region of the ocean.
They are often placed in groups (or arrays) to cover a wider area.
Animation of an inverted echo sounder.
Table of the speed of sound calculated under different ocean conditions.
Calculated using the simplified formula from Collins, 1981.
Temperature (°C) Salinity (S) Depth (km) Speed of Sound (m/s)
0 0 0 1402
0 35 0 1449
5 35 0 1470
5 35 0 1470
10 35 0 1490
5 35 0 1470
20 35 0 1521
30 35 0 1545
5 35 0 1470
20 5 0 1488
5 35 0 1470
20 10 0 1493
20 20 0 1505
20 35 0 1521
5 35 1 1487
5 35 2 1503
5 35 3 1521
5 35 4 1539
Note: Changes in the speed of sound for a given property are not linear.
Additional Resources
• Able Sea Chicks Blog.
• Acoustic Thermometry of Ocean Climate (ATOC)
• ATOC Project Homepage.
• Spindel, R.C., and P.F. Worcester 1990, “Ocean acoustic tomography.”
Scientific American, 263, 94-99 (October, 1990)
• National Academy of Sciences, Sounding Out the Oceans Secrets.
• URI-GSO IES Group.
