A sonar working at low frequencies, commonly a chirp sweep from a few kilohertz to around 20 kHz, or a lower-frequency boomer or sparker, whose energy passes into soft sediment and reflects off layer boundaries. The resulting profile shows sediment thickness, rockhead, buried pipelines and old channel fills. Hydrographers use it on dredging, cable route and engineering jobs rather than for charting depths.
Sub-bottom shows two metres of soft infill over hard clay, so the dredger can take it with the cutter.
Also heard as
- SBP
- chirp
- pinger
- sub-bottom
Related terms
Singlebeam echo sounder
An echo sounder that measures one depth directly beneath the vessel on each ping, using a single vertical beam.
Fluff
A soft, fluid mud layer over the firm seabed that gives different depths at different sonar frequencies.
Backscatter
The strength of sound returned from the seabed, used to map bottom type alongside depth.
Towfish
A streamlined body towed on a cable behind the vessel that carries side-scan or other sensors close to the seabed.