In open seas, the sound pulse from the Doppler transducer may not reach the bottom, but get totally internally reflected from a layer of water in between. This is known as the echo from the ‘Water Track”.
When the sound is bounced off a water layer, called a water track, speed indicated is the ‘Speed through Water”. or from a layer of water and the echo is at a higher frequency.
The frequency of the echo from the water track will follow the same Doppler principles as the echo from the bottom track. However, the speed measured from the ‘water Track’ will not be ‘Speed over the Ground’, but it will be ‘Speed through Water’.