The depth of the ocean at a given location is commonly measured by which specialised oceanographic instrument?

Introduction / Context:
Measuring the depth of oceans and seas is essential for navigation, laying submarine cables, studying marine life, and understanding the structure of the ocean floor. Special instruments and methods are used to determine how far the seabed is below the water surface. This question asks you to identify the instrument commonly used for depth measurement in oceanography.

Given Data / Assumptions:

• We are interested in measuring vertical depth from the sea surface to the seabed.
• The instruments listed include anemometer, areometer, seismometer, and fathometer.
• Each of these instruments measures a different physical quantity in practice.
• We assume standard textbook definitions for these devices.

Concept / Approach:
A fathometer is a type of echo sounding device used to measure water depth. It sends sound pulses downward and measures the time taken for the echo to return from the seabed. Using the speed of sound in water, the instrument calculates the depth. An anemometer is used to measure wind speed, an areometer measures the relative density of liquids, and a seismometer records vibrations inside the Earth due to earthquakes. Only the fathometer is directly designed for ocean depth measurement.

Step-by-Step Solution:
Step 1: Recall that ocean depth is commonly determined by echo sounding, where sound waves travel to the bottom and reflect back. Step 2: The device that performs this function by generating and detecting sound pulses in water is called a fathometer. Step 3: Use the relation depth = (speed of sound in water * time taken for echo to return) / 2 to understand how the measurement works. Step 4: Recognise that an anemometer is a meteorological instrument used on land or at sea to measure wind speed, not depth. Step 5: Identify areometer as a device for measuring specific gravity of liquids, and seismometer as an instrument for recording seismic waves, neither of which is used for depth measurement.

Verification / Alternative check:
Marine charts and nautical maps often mention soundings taken by echo sounders or fathometers. Ships and submarines are equipped with depth sounding systems based on this principle. In contrast, wind reports from ships give readings in nautical knots measured by anemometers, and earthquake observatories use seismometers to record ground motion. This separation of roles among instruments confirms that the fathometer is associated with ocean depth.

Why Other Options Are Wrong:
Anemometer: Measures wind speed and sometimes wind direction, not water depth. Areometer: Used to measure relative density of liquids, such as in battery acid testing, not the depth of oceans. Seismometer: Records seismic vibrations inside the Earth due to earthquakes or explosions, not ocean depth.

Common Pitfalls:
Learners may mix up instrument names because many end with the suffix meter. A good strategy is to remember the root words: anemo relates to wind, hydro or phono often relate to water or sound, and seismo connects to earthquakes. The term fathometer comes from fathom, a unit of depth used in marine contexts, which hints clearly at its purpose.

Final Answer:
The depth of the ocean is commonly measured by a fathometer.