A device that reflects sound off the ocean bottom to sense water depth; accuracy affected by speed of sound in water.

• A. Core
• B. Crust
• C. Echo Sounder
• D. Density Stratified

Answer: (C)

Measuring depth in the ocean with sound relies on sending a short pulse and timing how long it takes for the echo to return from the seabed. The depth is determined by half of the product of the speed of sound in water and the round-trip travel time, so knowing or assuming the correct speed of sound is crucial for accuracy.

An echo sounder is the instrument built for this process: it emits the acoustic pulse, waits for the bottom’s echo, and converts the travel time into depth. Because the speed of sound in seawater varies with temperature, salinity, and pressure, readings depend on using an accurate speed profile; if the assumed speed is off, depth measurements will be biased.

The other terms don't describe a device used to measure depth by sound in the ocean: core and crust refer to Earth’s internal layers, and density stratified describes a property of the water column rather than a measuring instrument.