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Ocean Depth

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Author: Janice VanCleave

Mapping a Profile of the Ocean Floor

The first scientific attempt to measure the depth of the ocean was made during an expedition by the British ship H.M.S. Challenger between 1872 and 1876. The measuring method on the Challenger was called sounding and involved lowering weighted ropes to the ocean bottom. Today investigators use echo-sounding sonar and other methods to determine ocean depths.

In this project, you will model the use of sonar to determine ocean depths. You will also determine how to use echo soundings to graph a profile of the ocean floor and learn how distances between echo soundings affect accuracies of profiles.

Getting Started

Purpose: To model the use of sonar to determine ocean depth.

Materials

  • Tennis ball
  • Helper
  • Stopwatch
  • Calculator

Procedure

Mapping a Profile of the Ocean Floor

Example:

0.56 sec + 1.02 sec + 0.59 sec +1.0 sec = 3.17 seconds

D = 1/2t × S

The speed of sound in ocean water is about 5,000 feet per second (1,500 meters per second), which can be written 5,000 ft/sec (1,500 m/sec).

Example:

D = 1/2 × 0.79 sec × 5,000 ft/sec (1,500 m/sec)

    = 1,975 feet (592.5 m)

  1. Hold your arms against the side of your body, then bend your elbows at a 90° angle so that your hands are held straight out in front of you.
  2. Hold the ball in one hand.
  3. Standing still and keeping your elbows against your sides, practice bouncing the ball several times until you can throw it with the least amount of force necessary to cause it to return to your other hand (see Figure 44.1).
  4. Ask a helper to measure the time it takes for the ball to leave one hand and return to the other. When your helper says "go" and starts the stopwatch, throw the ball. When you catch the ball, say "stop" and have your helper stop the watch and record the time that has passed as the echo time.
  5. Repeat step 4 three more times and average the results by adding together the four recorded echo times and dividing the sum by 4.
    •                   3.17 sec ÷ 4 = 0.7925 = 0.79 seconds
  6. The depth of the ocean floor (D) is one-half the average echo time (t) multiplied by the speed of sound in water (S). Use the average echo time to model how the depth in water could be determined by the following formula:

Results

The time for the ball to bounce and return will vary depending on the height of the person throwing it. For the example, the average time was 0.79 seconds. Using this as a model of the sonar echo time in the ocean, the ocean depth was calculated to be 1,975 feet (592.5 m).

Why?

Mapping the ocean floor requires methods different from those used on land. In the past, the depth was measured by a method called sounding. Sounding has nothing to do with sound. To take a sounding, knots were tied in a rope at intervals of 1 fathom (6 feet, or 1.8 m). A weight was tied at one end of the rope and dropped over the side of the ship. The number of knots that went over the side before the weight struck bottom was counted. The number of knots equaled the depth in fathoms.

A modern method of measuring ocean depth is echo sounding, which is a method of sending out sound from a transmitter and measuring the echo time (time it takes sound leaving a transmitter to travel to an object, be reflected, and return to a receiver). Echo sounding is often called sonar. The term sonar is an acronym derived from SOund Navigation And Ranging. Sonar is the method or the device used to determine ocean depth or distance by calculating the echo time of sound.

In this experiment, the bouncing ball represents sound bouncing off the ocean floor. The time for the round trip—the echo time—was measured and used to calculate the one-way distance—the depth.

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