ture, salinity, and pressure. The water column 
consists of multiple density layers rather than a 
steadily increasing density with depth. A direct 
method to measure density is needed. 
(2.) Acoustic Properties Sound is the principal 
means of communication and detection in the sea. 
For locating objects, positions, and terrain features 
or for probing the nature of sediments, sound is 
essential. Unfortunately, temperature changes with 
depth bend sound waves, and the changes vary 
with time and space. 
For these reasons, water temperature is the 
ocean property most widely measured. The Navy 
alone makes more than 5,000 temperature sound- 
ings per month. Such data are essential for the 
Naval Weather Service to derive daily maps of 
near-surface sonar propagation conditions. 
Sound from a source in the ocean’s near-surface 
region follows many diverse paths generally classi- 
fied as: 
—Surface duct. Sound travelling in the near-surface 
region. 
—Bottom bounce. Sound reflected off the ocean 
floor. 
—Convergence zone. Refracted sound travelling 
along a deep path and sometimes reinforced with 
energy from the bottom. 
The development of operations in the latter 
two categories requires geophysical data. New 
technology for measuring temperature, salinity, 
and pressure includes: 
—Salinity-temperature-depth system (Figure 23) 
which records water salinity and temperature at 
various depths. 
—Expendable salinity-temperature-depth system. 
—Airborne radiation thermometer which measures 
sea surface temperature by infrared radiation, 
enabling an aircraft or a satellite to quickly amass 
data over a large area. 
—Buoy temperature sensor cables. 
—Expendable bathythermographs for measuring 
temperatures at various depths by surface vessels 
and aircraft. 
333-091 O-69—9 
Figure 23. STD sensor for measuring water sa- 
linity, temperature, and depth being lowered 
from USC&GSS Oceanographer. (ESSA photo) 
(3.) Electromagnetic Properties Seawater rapidly 
absorbs almost every wave length in the electro- 
magnetic spectrum. Radio waves (except ex- 
tremely long high-energy waves) are attenuated 
immediately in water. Infrared waves are absorbed 
by water molecules. Ultraviolet, X-ray, and gamma 
rays are absorbed by electrons or atomic nuclei. 
However, water is relatively transparent to visible 
and near-ultraviolet light. 
(4.) Salinity Salinity is defined in terms of 
dissolved solid material in seawater. The salts 
of sodium and chlorine are the most important, 
accounting for approximately 85 per cent. Of 
the various constituents, only calcium is pres- 
ent in a state of saturation; seawater is far 
from saturated with the others. Seawater’s ability 
to dissolve large amounts of solids and gases 
without reacting chemically with them is one of its 
most important properties. 
Salinity varies in different ocean areas; how- 
ever, approximately 90 per cent of seawater falls 
within 34 to 35 parts per thousand. New dis- 
coveries have been made in hot spots where 
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