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High frequency sound waves can be used to detect objects in deep water and to measure water depth.

The time between a pulse of ultrasound being sent out and detected and the speed of sound in water can be used to calculate the distance of the reflecting surface or object using:

Distance = speed of ultrasound in water x time taken.

Remember this is an echo method and so the time between a pulse of ultrasound being sent out and detected must be divided by two to work out the time to the seabed.

The process is very similar to ultrasound imaging.

For deep water, 50 kHz ultrasound is used.

This is because water absorbs high frequency ultrasound waves at a slower rate than for lower frequencies and so the signal can travel farther before becoming too weak to use.

This technique is applied in sonar systems used to measure the depth of the seabed and to find shipwrecks, submarines and shoals of fish.

SONAR stands for SOund Navigation And Ranging.

Bats and dolphins use a similar method, called echolocation, to detect their surroundings and to find food.

Example

A sonar system on a boat sends an ultrasound pulse towards the seabed.

The pulse is reflected, and it is detected 0.1 s later by the system.

Calculate the depth of water if the speed of sound in water is 1,480 m/s.

Answer

Distance = speed × time

Speed = 1,480 m/s

Time for ultrasound to travel to seabed and back again = 0.1 s

Time for ultrasound to travel to seabed = 0.1 s ÷ 2 = 0.05 s

Distance to seabed = 1,480 × 0.05 = 74 m

The depth of water is 74 m.