A sonar which stands for Sound Navigation and Ranging is a technique used for determining the depth and also locating underwater objects such as reefs, submarines etc.
In this method, to find the depth of ocean, a strong ultrasonic wave is sent from the ship towards the bottom of the ocean.
This ultrasonic is received back after it is reflected from the bottom of sea.
The time interval t for travel of sound waves from the ' source to the receiver after reflection is noted.
Then the depth d of ocean is given by the velocity of D = V x t/ 2, where V is the velocity of sound.