Bottom sounding sonar that directs and active sonar pulse down to the ocean bottom and measures the time for the pulse to reflect back and hence the distance to the bottom. The pulse is a high frequency short duration pulse that is considered secure because it attenuates quickly and is hard to detect.

depth finder for determining depth of water or a submerged object by means of ultrasound waves

A device to measure the depth of water, by timing the travel of a sound wave from the ship to the ocean bottom and return.