Ultrasonic Waves for Fathometer


Why ultrasonic waves is used instead of audible waves to measure depth of sea?

It is said in our book that ultrasonic waves
1. will not be interfered with other sound waves in water and
2. small sized objects can be located

I don't understand the above 2 reasons.


In detecting objects by bouncing waves off them, the smallest size detectable will be roughly the same size as the wavelength. In measuring the depth of water, the accuracy can be no better than about one wavelength. Ultrasonic waves have short wavelength so they can "see" smaller objects and more precisely measure depth. Also there is a lot of naturally occurring and man made noise in the ocean. If audio waves were used to measure ocean depth this noise might interfere with the depth measurement by being misinterpreted as an echo from the bottom.