JRC JFE-680 - Appendix: Noise and Seabed Characteristics; Understanding Noise Types in Sonar Display

Actual Pictures
Zero line
Seabed
DISP TRANS
DISP SURF
DISP KEEL
figure A
Seabed
Zero line
Third reflection of bottom
Second reflection of bottom
Seabed
In case of a shallow seabed or when increasing the amplifier sensitivity, two seabed lines
may be recorded. This results from a multi-reflection of ultra-sonic wave between the
seabed and hull bottom or surface of sea, in such manner: An emitted ultrasonic wave once
reflected at the seabed returns toward the transducer or surface or sea but reflected at the
hull bottom or surface of sea and again reflected at the seabed toward the transducer.
Such multiple recording of the seabed may appear due to change of bottom quality. A
double or triple reflection may be sometimes recorded.
In any case, a first reflection recording from the zero line represents a real seabed return. A
first, second and third reflection lines of seabed arrange with approximately equal spacing on
the recording.
In addition, the shade of the reflection lines fades little by little away from the fast line on the
recording. From these conditions, they can be easily identified as a multi reflection.
Appendix 68