A sound velocity profile records how fast sound travels at each depth, usually between about 1450 and 1550 m/s in seawater. Speed rises with temperature, salinity and pressure, so river plumes, thermoclines and seasonal warming all reshape the profile. Multibeam processing ray-traces each beam through the layers; an old or unrepresentative profile bends the outer beams and leaves refraction errors. Profiles come from sound speed probes or from CTD casts.
New SVP shows a fresh layer down to 4 metres off the river, so cast every two hours until we're clear.
Sound speed shifts roughly 3 to 4 m/s per degree Celsius and about 1.3 m/s per unit of salinity, so a warm afternoon or a river plume is enough to need a new profile.
Also heard as
- sound velocity profile
- sound speed profile
- SSP
- SV profile
Related terms
Cast
One lowering of a sound speed or CTD probe through the water column to measure a profile.
Refraction
The bending of sonar beams as they cross water layers with different sound speeds, which distorts depths if uncorrected.
Surface sound speed
The speed of sound right at the sonar face, measured continuously so the multibeam can steer its beams.
Smiles and frowns
Cross-track curling of multibeam swaths, up or down at the edges, caused by sound speed errors.