Furuno NAVNET TZ2 TZTL12F Operator's Manual page 244

Bottom echoes
The strongest echoes are from the bottom, and are normally shown in reddish-brown
or red. The colors and the width change with bottom material, depth, sea condition,
installation, frequency, pulse length and sensitivity.
Difference in depth
Second
bottom echo
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
particles can give a long tail on the low frequency picture.
Difference in sensitivity
Rough bottom
Mud & sand
Rock base
7. FISH FINDER (SOUNDER)
7-25