JRC JFE-680 -, Instruction Manual

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Seabed Quality Change
Mud
Rock
In case of a hard seabed composed of rocks etc., its return trails long, as shown in right chart.
In case of a soft seabed made of mud, seaweed, etc., they poorly reflect an ultrasonic wave
to result in thin recording of the seabed with short trail.
The seabed quality can be more sufficiently identified with use of wider beam angle and
longer pulse width.
Usually lower frequency is used.
Appendix 69