A marine researcher is conducting an experiment to measure the speed of sound in seawater. They use a sonar transducer mounted on the hull of a research vessel to emit a short pulse of high-frequency sound vertically downwards towards a flat, horizontal seabed located at a depth of 120 m, as shown in the diagram.
The transducer is connected to an oscilloscope that displays the emitted pulse and the returning echo. To find the speed of sound, the researcher starts a manual digital stopwatch when the pulse is emitted and stops it when the echo is observed on the screen.
Explain one way the researcher could improve their method to obtain a more accurate value for the speed of sound in seawater.