Measuring the sea with sound

Ships measure the depth of the sea with ultrasound, using a device called SONAR — Sound Navigation And Ranging.

A transmitter on the bottom of the ship sends pulses of ultrasound down to the sea bed. They reflect off the sea bed and come back to a detector. The time taken is measured, and from it the depth is found.

Figure to be added

Figure 4.36 — a ship sending ultrasound pulses down through the water from a transmitter, and a detector receiving the pulses reflected from the sea bed.

Example 1

Problem

The time between an ultrasound pulse leaving a ship and its reflection from the sea bed being detected is 4 s. Find the distance from the ship to the sea bed. (Take the speed of sound in water as 1440 m s^-1.)

Solution

distance the sound travels in 4 s = 1440 × 4 m

that is down and back, so the depth is half of it

distance to the sea bed = (1440 × 4) ÷ 2

= 2880 m

Don't forget to halve it

The pulse goes to the sea bed and comes back, so the measured time covers twice the depth. Forgetting to divide by 2 doubles the answer.

Besides measuring depth, ultrasound is used to search for shoals of fish and to find ships that have sunk.