What is the basic principle behind how sonar works?

Sonar is based on the principle of reflection of Ultrasonic Sound waves. Ultrasonic sound waves are high frequency waves with frequency over 20Khz. The SONAR transmitter in the Ship sends the ultrasonic sound waves towards the sea bed from where they get reflected

Explanation:

a pulse of ultrasonic waves is sent into the water where it bounces off a target and comes back to the source (ultrasonic waves are pitched too high for humans to detect). The distance and location can be calculated by measuring the time it takes for the sound to return. By knowing the speed of sound in water, the distance is computed by multiplying the time it takes for the sound to return. By knowing the speed of sound in water, the distance is computed by multiplying the speed by one-half of the time travelled (for a one-way trip). This is active sonar ranging (echolocation).

Explanation: