A Doppler speed log indicates speed through the water in which mode?

• A. Volume reverberation mode
• B. Single-beam mode
• C. Bottom track mode
• D. Surface mode

Answer: (A)

Speed through the water is obtained by analyzing the Doppler shift of echoes from particles moving with the water. The instrument has several operating modes, and the one that uses echoes from the entire water column provides a direct measure of the water’s movement relative to the ship. This is volume reverberation mode. It doesn’t rely on the bottom or the surface, so it reflects true water speed rather than speed relative to the seabed or surface conditions. Bottom-track mode, by contrast, uses the seabed echo and gives speed over ground, not speed through water; surface mode relies on surface backscatter and isn’t a direct measure of water velocity; single-beam mode is less robust for accurately determining water speed. So the mode that indicates speed through the water is volume reverberation mode.