The call when the sounder's tracking gate loses the true seabed return and either drops out or locks onto something else: a multiple echo that has bounced between seabed and hull, fish or kelp, a sharp thermocline, or the wake. It happens on steep slopes, over very soft mud and in aerated water. The operator widens the gates, changes power or gain, or re-picks the bottom later in processing.
Lost bottom on the drop-off again, it's grabbed the second return at double the depth.
Also heard as
- losing bottom
- lost lock
Related terms
Dropout
Missing pings or beams where the sonar failed to record a valid depth, leaving gaps in the data.
Bottom pick
The depth chosen from an echo trace as the true seabed, either by the sounder's tracker or by an operator.
Singlebeam echo sounder
An echo sounder that measures one depth directly beneath the vessel on each ping, using a single vertical beam.
Aeration
Air bubbles under the hull or across the transducer face that block sound and cause lost pings.