Simrad ES60
4
857-160970 / Rev.H
Wave propagation
The velocity of sound wave propagation in the sea varies
slightly with temperature, salinity and pressure. The velocity
varies between 1440 and 1520 m/s in shallow sea water, while a
velocity around 1480 m/s can be expected at 1000 m depth. In
shallow fresh water the velocity is approximately 1430 m/s.
A good average value to be used in the Environment dialogue
box is 1470 m/s.
Figure 1 The
wave propagation
principles
The ES60 transmits high energy sound wave pulses into the sea.
A flat bottom reflects the transmitted wave as if it were a mirror.
The propagating energy is spread over a larger and larger area as
it travels down to the bottom and up again. The energy is spread
over a four times larger area every time the travel distance
doubles.
A large school of fish reflects sound waves similarly. This type
of spreading is referred to as square-law or 20 log TVG (Time
Varying Gain) spreading.
The situation is slightly different when observing the echoes
from individual fish. The transmitted wave undergoes
square-law spreading when travelling from the surface and
down to the fish. The swim bladder of the fish scatters a small
fraction of the arriving energy in all directions. Travelling from
the fish and back towards the surface the scattered wave
undergoes another square-law spreading. The combined effect is
referred to as quad -law or 40 log TVG spreading.
In the echo sounder’s Echogram dialogue box 20 log TVG
spreading is called School Gain and Bottom Gain, while 40 log
TVG spreading is called Fish Gain.
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