Honda HE-670 Operation Manual page 12

HOW TO DISTINGUISH THE NATURE OF BOTTOM
In case you distinguish the nature of bottom, you judge from the thickness of bottom
image and the situation of second reflection. At the hard bottom like rock, reflection of
transmitted wave becomes strong and bottom image becomes thick and the second
reflection becomes easily to appear. On the other hand, at the soft bottom like sand or
mud the reflection becomes weak and bottom image becomes thin and the second
reflection becomes hard to appear.
• Second reflection.......Ultrasonic wave transmitted from transducer is reflected from
sea bottom and then it is reflected from both sea surface and bottom finally. In short,
second reflection is image of two round-trips between sea surface and bottom.
Rock Sand or mud
7
Second reflection