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Measuring Ocean Depths

Measuring Ocean Depth

Introduction: (Initial Observation)

Oceans seem to evoke an emotional response in most adults and youngsters. All kinds of people are drawn to seashores around the world.
These bodies of water have influenced, for many, their way of life, their thinking, their art, their science.

The ocean represents the last frontier on earth. It is promising, challenging and mysterious. The applications of its many promising benefits may be realized only with continued research and study on a national and international level. Where better to start than with our science project!

In this project we will study the ocean depth and how it can be measured.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

Adult supervision and help is required in this project.

Information Gathering:

Find out about oceans and how they affect our lives. Read books, magazines or ask professionals who might know in order to learn about the methods that can be used to measure the depth of ocean.
Try to find out what are some of the things people use from the ocean? What do you think a deep-sea diver might see in the ocean? Why do people need to study the ocean? Why are the oceans important to us and to all life?

Keep track of where you got your information from.

Following is a sample of the information that you may find:

There is no other planet in the entire solar system like Earth. It holds nearly all the liquid water in our solar system! It is this water that allowed life to come into being and spread throughout our planet. These unique oceans exist because the planet’s surface temperature is in the range in which water remains liquid. The temperature range, 0° C to 100° C, occurs rarely in our universe; matter tends to be frozen solids or hot gases.

By looking at a map or globe, one can see that all the oceans are interconnected, and are divided by the continents. The northern parts of the Atlantic and Pacific Oceans which go around the North Pole, form the Arctic Ocean. The southern parts of the Atlantic, Pacific and the Indian Oceans extend around the Antarctic Continent and form the Antarctic Ocean. These five great oceans cover approximately 70% of the surface of the earth.

Many scientists believe that the oceans holds the key to how the earth was formed. One important factor is related to erosion, and the other to the difference in the crust of the earth under land to under the ocean.

Throughout the years, land areas on earth have been changed and worn away by the forces of erosion. If the ocean floor is not as affected by erosion from the wind and water, then it may be closer to its original state. This then, will allow studies to confirm or correct our knowledge of how the earth was formed.

The crust of the earth is thinner under the floor of the ocean than under land areas. It is thought that it would be easier to reach the layer beneath the crust, the mantle, through the ocean floor, though as of this time it is still impossible. This will allow scientists to learn more than they now know about the earth and its origin.

Measuring the Depth of the Ocean

The depth of the ocean is usually measured three ways: 1) Using weight and rope, 2) using acoustic echo-sounders on ships, or 3) using data from satellite altimeters.

Weight and rope:

Early sailors needed to know ocean depths, particularly along coastlines and near harbors, so that their ships would not run aground. Ocean depths were measured with a weight on a line, and depth measurements were limited to shallow water by the lengths of line carried on board and the enthusiasm of the captain for making soundings. Not until the 1870s, during the famous Challenger Expedition, were deep soundings routinely made.

But the technique was the same: laboriously lowering a weight on a rope and trying to figure out when it hit bottom. This technique was arduous in deep water, and the results were seldom accurate for two reasons: 1) Strong currents might pull the rope and weight to the side, and 2) the length of rope needed to reach the deep seafloor was so heavy, it was difficult to tell when the weight hit the bottom.

In about the year 1870 people found they could measure the ocean depth by using echoes. Sound waves were sent into the ocean from a ship. The sound waves hit the ocean bottom and bounced back to the ship, where people kept track of the number of seconds it took for the sound to bounce back. This helped to determine how deep the ocean was at that point. Measuring the ocean depth in this way is called sounding.

Echo Sounders:

Most maps of the ocean are based on measurements made by echo sounders. The instrument transmits a burst of sound and listens for the echo from the sea floor. The time interval between transmission of the pulse and reception of the echo, when multiplied by the velocity of sound, gives twice the depth of the ocean (Figure 1).The first transatlantic echo soundings were made by the U.S. Navy Destroyer Stewart in 1922. This was quickly followed by the first systematic survey of a ocean basin, made by the German research and survey ship Meteor during its expedition to the South Atlantic from 1925 to 1927. Since that time, oceanographic and naval ships have operated echo sounders almost continuously while at sea.

Figure 1:Echo sounders measure depth of the ocean by transmitting pulses of sound and observing the time required to receive the echo from the bottom.

Because we know the speed of sound in water (usually 1,500 meters per second), the round-trip time can be used to calculate depth.

Satellite Altimetry:

Gaps in our knowledge of ocean depths between ship tracks have now been filled by satellite-altimeter data. Altimeters profile the shape of the sea surface, and the shape of the surface is very similar to the shape of the sea floor.

The Relationship Between Sea Level and the Ocean’s Depth:

Excess mass at the seafloor, for example the mass of a seamount, increases local gravity because the mass of the seamount is larger than the mass of water it displaces, rocks being more than three times denser than water. The excess mass increases local gravity, which attracts water toward the seamount. This changes the shape of the sea surface.

An altimeter is basically a microwave radar pulse that is sent from the orbiting satellite (space station?), bounces off the earth’s surface, and returns to the orbiting spacecraft. (see Figure 1). The travel time of this pulse (the time it takes to get to the surface of the Earth and return to the satellite) is then recorded. This is the same principle that allows an individual looking at a radar screen to determine how far an object is.

Figure 2: A satellite altimeter measures the height of the satellite above the sea surface. When this is subtracted from the height R of the satellite’s orbit, the difference is sea level relative to the center of the Earth. The shape of the surface is due to variations in gravity, and to ocean currents which produce the oceanic topography.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to find out how can we measure the ocean depth! Your experiments may be conducted in a small pound, a small lake or part of a river. Safety precautions and adult supervisions are required.

This project does not need a question or hypothesis. It is just an educational measurement, observation and reporting activity. If you need to have a question and a hypothesis, these are some sample questions you may use to form your own questions:

  • Is the center of the pound, the deepest point of the pond?
  • How does the depth of ….. river change across the river along the …. Bridge?

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

Since we are not studying the effect of something on something else we do not need to identify variables. For example if you are studying on the effect of ocean depth on water temperature, then you need to identify variables. Even you may draw a chart or graph to show how does the temperature vary in different depths.

If you are studying the depth across a river or pond, then you may define variables like this:

Independent variable (also known as manipulated variable) is the distance from the side (where you start).

Dependent variable (also known as responding variable) is the depth.

Control variable is the time. Do all measurements or observations in the same day.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

If you are planning to demonstrate and test some of the existing methods of measuring the ocean depth, you will not need to have a hypothesis. However if you come up with a new idea for a new method, that will be your hypothesis.

Sample Hypothesis:

My hypothesis is that the amount of under water light reduces in deeper waters, so we can use an under water light sensor to estimate the depth of water. One light sensor can be mounted under the ship and another on the side. As the ship moves toward the deeper areas, the readings in these two light meters will read further apart. (This is just an example for those who may want to see a sample hypothesis. We are not going to test this.)

Following are sample hypotheses for the other two questions:

  • The center of a pound is the deepest point of the pond.
  • The center of a river is the deepest point of the river.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment 1:

Since ancient times mariners had realized that a knowledge of the depth and shape of the seafloor was needed to navigate and explore the oceans. The first measurement ever made at sea was probably the measurement of depth using a rope with a weight on the end.

If you are accompanied with an adult and have access to a boat and safety jacket, you may perform this experiment in a local lake or pool. Otherwise do it in smaller scale in a water tank. To measure the depth of a pool, you can do it from the side and don’t need any floating device


  1. String, rope or nylon fishing line.
  2. Ruler
  3. Water tank or any large pot with 6 inches or more water.
  4. Weight (any heavy metal object will do it. Sellers of fishing supplies carry lead weighs that are relatively small, but heavy. A steel nut can also be used as a weight)
  5. Some stones or other heavy objects for the bottom of your tank to simulate under water mountains (elevations).
  6. Paper and pen to record your data.
  7. Food coloring (optional)

Keep your weight and rope and use it for your display.


  1. Make equally spaced marks on a rope. (For example you may tie equally spaced knots in the rope.)
  2. Lower the rope over the side of a ship (or boat or any wooden object on the surface of the water to simulate a boat) until the rope touches the bottom. (You will feel it, because it becomes lighter).
  3. Count the number of marks that go into the water.
  4. Multiply that number by the amount of space between each two marks.
  5. Repeat this across the water and record the results.

What is was the highest depth?

If you are studying the depth across a river, your data table may look like this:

Distance from the side Depth

Experiment 2: To measure ocean depth you can send a sound from the ship towards the sea floor. Measure how long it takes to hear the echo.

Did you ever clap your hand in an empty room and listen to the echo or sound reflections? The time that it takes for your clapping sound wave to move away from you, heat a barrier, reflect and get back to you in the form of an echo can be used to determine your distance from the sound barrier. Using sound echo to determine the distance or depth can be tested in many different environment and for many different applications. In an empty room, you can use sound echo to measure the distance from the wall or from the sealing. You can use this method to measure the depth of a well, the height of clouds, depth of a cave or a crack and the depth of a closed pipe or hose. Using this technique to determine the depth of oceans is called echo sounding.

An echo sounding is measurement of the two-way travel time of an acoustic signal between a sound source (ship-borne transducer) and a reflecting surface (the ocean bottom). To convert an echo sounding to depth, the travel-time measurement is commonly halved and multiplied by an assumed speed of sound through seawater, either 4,800 ft/sec or 1,500 m/sec. (Speed of sound in air is 340 m/sec)

m/sec = meter per second

ft/sec = feet per second

In this experiment we will attempt to measure the depth of a rubber hose or a pipe while we have access to one end and know that the other end is closed (so it can reflect the sound).

Material and equipment:

  • A computer with sound system
  • A sound recording and editing program
  • A sensitive microphone

(The microphone must be able to record sounds coming from distance with no filter)

Recommended Places for this experiment:

  • An empty large room with no carpet, no furniture and sound insulating material on the roof or walls.
  • The opening of a pipe that is closed on the other side.
  • The opining of a well.
  • Any other location the you can get a strong sound reflection.


Face the microphone to the direction that sound reflection is coming from.

Start recording, make sure that you can see sound waves on the display.

Make a sound bit by clapping your hand or bursting a small balloon.

Your recorder must record the original sound and it’s reflection.

Locate the pick of the original sound wave and the pick of the reflected sound wave in the sound recording program. Make a note of the time for each sound and calculate the time distance between two sounds in seconds (or fraction of second).

Multiply that by 340 m/sec (speed of sound in air) to calculate the roundtrip distance.

Divide this number by two to calculate the distance from sound source to the reflecting surface.

Note: Instead of computer you may use a chronometer. Chronometer is an exceptionally precise timer. Start timing when you here the first sound and stop it when you here the echo.

Materials and Equipment:

List of materials for experiment 1:

  • String, rope or nylon fishing line.
  • Ruler
  • Water tank or any large pot with 6 inches or more water.
  • Weight (any heavy metal object will do it. Sellers of fishing supplies carry lead weighs that are relatively small, but heavy. A steel nut can also be used as a weight)
  • Some stones or other heavy objects for the bottom of your tank to simulate under water mountains (elevations).
  • Paper and pen to record your data.
  • Food coloring (optional)

List of materials for experiment 2:

  • A computer with sound system
  • A sound recording and editing program
  • A sensitive microphone

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


After measuring the time it took for the sound to hit a barrier and come back, you will need to calculate the roundtrip distance by multiplying this time by the speed of sound. Half of this distance is the distance of sound source from sound reflector. Sound reflector can be the bottom of ocean, a wall, sealing or the bottom of a well.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


List of References