Final answer:
A Doppler log in the bottom return mode measures the velocity of a vessel relative to the seafloor, using the Doppler effect to calculate its speed based on the frequency shift between the transmitted and received acoustical signals.
Step-by-step explanation:
A Doppler log in the bottom return mode indicates the velocity of a vessel relative to the seafloor. This mode of operation employs acoustical signals which are transmitted towards the seafloor and then reflected back to the vessel. The Doppler log measures the frequency shift between the transmitted and received signals, which is caused by the movement of the vessel over the bottom. The shift in frequency, known as the Doppler effect, allows the system to calculate the speed of the vessel relative to the bottom. This is particularly useful for navigation in areas where satellite signals, like those from GPS, may not be reliable or available.