The speed of sound in water varies with the temperature and the salinity, and all of these will differ with depth. Oceanographers and those involved in climate studies use temperature and salinity measurements to help establish and track individual bodies of water as they move through the oceans, whilst Hydrographic surveyors require accurate, timely measurements of the speed of sound to ensure that they are getting the best performance and most accurate readings whether using SBES, MBES, USBL or LBL.
USBL systems typically track one or more acoustic transponders mounted on AUV or ROV from a surface vessel. LBL systems may be used during the deployment of subsea pipelines or structures such as wellheads, as well as precise measurement during metrology operations.
Knowledge of the speed of sound is vital as the distances calculated (and hence the positions generated) are based on the time of flight of an acoustic pulse. Another complicating factor is that an acoustic wave can refract when passing through regions with different speeds of sound, thus causing apparently greater distances to be reported – this effect can sometimes be beneficial as it may be possible to extend the conventional working range of a system such as LBL. If the speed of sound can be measured, then its effects can be modelled and compensated for.
SBES and MBES (used on both surface vessels and both AUV and ROV) rely on good speed of sound measurements to be able to confidently calculate the depth for a particular point on the seabed. MBES systems benefit from having a continuous speed of sound measurement close to the transducer face as well as from periodic vertical profiles, and this combination can help reduce the impact of refraction effects, especially on the outer beams.