A fathometer is used to measure:

Concept

An echo sounder determines water depth by transmitting a sound pulse toward the bed and recording the echo’s round-trip time. If v is the speed of sound in water and t is the elapsed round-trip time, the depth is d = vt/2 because the pulse travels down and back.

Application

A fathometer is a type of echo sounder. It uses this travel-time principle to indicate the depth of water beneath a vessel, so the measured quantity among the choices is ocean depth.

Contrast

• Earthquakes are monitored by recording ground motion with seismographs.

• Rainfall is measured with a rain gauge.

• Sound intensity is measured with an acoustic intensity probe or meter.

Cross-check

The name fathometer comes from fathom, a traditional unit of water depth, which is consistent with the instrument’s marine depth-measuring role.

Therefore, a fathometer measures ocean depth.