12.2.1 Salinity
Generally, salinity ranges from 32 – 38 parts per thousand (ppt) in ocean water. A change in salinity
will create density changes, which affect the velocity of sound. As a general rule, a change in salinity
of only 1 ppt can cause a sound velocity change of 1.4m/sec. There are many influences on the
salinity concentration in sea water.
1. Evaporation
2. Precipitation
3. Fresh water influx from rivers
4. Tidal effects (salt wedges)
12.2.2 Temperature
Temperature is the major influence on sound velocity in water. A 1°C change is equal to
approximately a 4m/sec change in velocity. Once the upper layer is passed, the temperature
normally decreases until pressure becomes the more dominating influence on the velocity of sound,
which is approximately at 1000 metres. The normal influences on the temperature component of
sound velocity include:
1. Solar heating
2. Night time cooling
3. Rain / run off
4. Upwelling
12.2.3 Refraction Errors
Refraction errors occur due to the wrong sound velocity profile being applied to the data. The error
increases away from nadir and, as such, is more apparent in the outer beams. The visual effect is
that the swath will curl up (smile) or curl down (frown). The actual representation is that the
soundings are either too shallow or too deep.
Figure 139: Refraction Error indication
At an angle of 45
°
in 10 meters of water, a ±10 meters per second velocity error will result in a depth
error on the order of ± 4.6 cm.
• Convex (smiley face) = Sound velocity profile used higher than real profile
• Concave (frown face) = Sound velocity profile used lower than real profile
Page 143 of 210
Version 5.0 Rev r002
Date 05-08-2014
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