High frequency sound waves can be used to detect objects in deep water and to measure water depth. The time between a pulse of sound being transmitted and detected and the speed of sound in water can be used to calculate the distance of the reflecting surface or object.
This technique is applied in sonar systems used to find shipwrecks, submarines and shoals of fish. Bats and dolphins use a similar method, called echolocation, to detect their surroundings and to find food. If the time and speed are known, then the distance to the position of the object that reflects the sound can be calculated using:
distance = speed of sound in water × time taken
This is when: