Furuno GP-1670F Operator's Manual page 99

Bottom contour
The tail from a hard bottom is longer than the tail from a soft bottom, because the hard
bottom reflects more of the ultrasonic pulse. An echo from shallow water gives a stron-
ger reflection than the echo received from deep water. A longer bottom tail appears
on slopes, because of the difference in travel time at both edges of the beam angle.
For the rough bottom, echoes are reflected on many different planes, which displays
echoes in many layers, giving a 3D effect.
Soft bottom
Hard bottom
Nature of a bottom
The nature of a bottom is known from the intensity
and length of the bottom tail. To find the nature of
a bottom, use a long pulse length and normal
gain. For the hard and rough bottom, the bottom
echo is reddish-brown with a long tail. For the mud
or sand bottom, the bottom echo has less red and
with a short tail. A bottom with a lot of small parti-
cles can give a long tail on the low frequency pic-
ture.
Fish quantity
The size and density of a school of fish are indi-
cators of the quantity of fish.
7. FISH FINDER OPERATIONS
Rough bottom
Mud & sand
Large school Large school
Size and density of school of fish
Rock base
Small school Small school
7-23