Page 19 - Electronic Navigation Cyber Book

sound varies from about 1432 meters/sec. in fresh water, to about 1535
meters/sec. in the salt water of high salinity. For depth sounding
equipment design purpose a sound speed value of 1500 meters/sec. are
assumed.
For normal applications on merchant's vessels, the indication of the depth
value based on the average speed of sound (1500 m/sec.) causes a
minor error when changing from Salt Water (SW) to Fresh Water (FW).
Assuming there is a small correction adjustment for SW, the true depth
in FW is found to be about 3% less than the indicated depth. As can be
seen, this deviation is very small and thus insignificant for practical
considerations.
When a sound wave passes through an interface of two mediums,
besides suffering the loss of energy, they refract as well as reflect (critical
refraction) at the boundary where the two media meet.

The Practice of Echo Sounding

The echo sounding principles are used as follows, to measure depth. A
short sound pulse is transmitted from the ship's bottom towards the
seabed where it is reflected back towards the hull as an echo.

The time interval between transmission of the pulse and the receipt of the
echo is measured, and the depth is found from the expression:
Depth = velocity X time divided by 2

The frequency of the sound vibrations created in the water during pulsing
is usually at the upper end of, or above the audible range. A depth
sounder instrument can be obtained in three different forms based on the
transmitted frequency:
.1 using 14khz frequency (Low-end sounders)

.2 using 14 kHz to 30 kHz frequency (Normal sounders)

.3 using 30 kHz frequency (High-end sounders)
The choosing of frequency depends on the requirements of the user. The
chosen frequency is basically a compromise to avoid interference from
audible ship motion noise, and water losses through the seawater.

Generally, low frequencies are mostly affected by ship noise and high
frequencies by water losses, which are caused by absorption.
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