Sound velocity probe

In the mid 20th century, sonar systems were developed to allow the measurement of underwater distances using the two way travel time of an acoustic pulse.

Firstly, the surveyor can use an air filled metal bar lowered below the transducer, attached at each end by a rope marked with depth values.

If the values on the rope can be assumed to be correct, then the bar is lowered at set depth intervals and observed on the echo sounder trace.

This has the advantage of being quicker than a bar check and it can be performed when there is boat motion due to swell and sea, although any draft offset of the vessel is not identified using this method.

Transducers used in sound velocity probes are typically of a high frequency (around 1 - 4 MHz) as the transmit and receive distances are close enough to mitigate any significant absorption losses.