Furuno CSH-23/24 SONAR User Manual


 
2-4
How to discriminate fish echoes from the seabed
The figure below illustrates how two fish schools a and b are
displayed on the screen using three different tilt angles.
Case 1: Tilt angle 30 to 40 degrees. Fish school is obscured by
the seabed.
Case 2: Tilt angle 10 to 20 degrees. Fish school is located above
the seabed (midwater).
Case 3: Tilt angle 0 to 10 degrees. Fish school is located close to
the seabed.
Case 1
Case 2
Case 3
Fish school a
b
Case 3
Case 1 Case 2
Fish school aFish school a
Seabed
Seabed
}
Sea bed
Figure 2-1 Fish echo and tilt angle
Points to consider
Normally, a vertically distributed fish school is a better sonar
target than the seabed, because it reflects the transmitted pulse
back toward the transducer.
In case 3, both fish schools a and b are presented. Gener-
ally speaking, however, midwater fish schools tend to be larger
than bottom fish schools and they are often displayed near the
seabed on the display.
It is difficult to detect bottom fish when they are not distrib-
uted vertically.