JRC JFE-700 Instruction Manual page 88

In case of flat seabed, thin second return of seabed may sometimes appear, which is slightly
below the actual seabed.
In either case, the dim or thin echoes are false and produced by side lobes of ultrasonic
beam from the transducer. Any false echo is thinner than and parallel to a real echo.
The echo of a seabed with abrupt slope is recorded as a lone difficult to see and less
discriminative, since it tends to accompany with a false echo due to the side lobe and the
inherent property of directivity.
In particular, a seabed with abrupt slope and heavily rugged surface provided an echo very
difficult to display on the recording.
10. Appendix
10-4