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Echo Sounder – How Deep is the Sea?

written by: Ricky • edited by: Lamar Stonecypher • updated: 2/11/2012

If you are peacefully enjoying your holidays on a cruise liner (or sailing on any other ship for that matter), the last thing you would want is to find yourself stuck in shallow water. How do you think we can measure the depth of sea with the help modern electronic equipments?

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    The ship is really huge in size and you know from Archimedes principle that to float in water it needs to displace a volume of water equal to its weight. This means that a substantial portion of the ship is immersed inside the water so there should be sufficient depth of water lest the ship runs aground. A ship which has run grounded can be source of immense damage in terms of marine pollution and damage to the cargo and crew as well.

    I cannot say for sure about the days of the yore when primitive people travelled on logs but nowadays electronics has come to the rescue of the sailors and read how deep sea facts such as the depth of the ocean floor is measured in the current times

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    How to Measure Depth?

    There can be several methods to measure depth such as inserting a sounding tape or a measuring stick but these are only sufficient for measuring shallow water depth and not applicable at sea. Moreover the ship is usually moving and there needs to be a method to continuously monitor depth. This is accomplished by the use of an instrument known as the Echo Sounder which is described below.

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    What is the Echo Sounder?

    As the name implies this instrument measures the depth based on the principle of reflection of an echo of waves transmitted by a wave generator and the time is noted for the wave to return, Knowing the velocity of the wave and the time to return back, it is possible to calculate the distance traversed by the wave using the simple mathematical formula of

    s = v * t


    • s is distance
    • v is velocity
    • t is time taken

    Since the wave has to travel up and down the actual depth is nearly half the distance so if d is the depth

    d = s/2 = v*t/2

    This principle is demonstrated in the diagram below which shows the arrangement and how it works.

    Echo Sounding Arrangement  

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    How does an Echo Sounder look like?

    It is all very well to know the theory but how does the instrument look in reality. Well you can simply take a look the picture below and see the echo sounder actually used on board marine vessels. The picture shows the transmitter receiving unit and the display unit and the various control buttons on the panel. It is not necessary to understand the function of each of these controls but this image should give you a broad idea about the actual instrument and its look and feel.


    There is another instrument known as the echo graph it is different from the echo sounder in the sense that instead of telling us the instantaneous depth at any particular moment, it tells the depth continuously and plots it on a graph. Hence this also gives the ocean floor map along the route of the ship.

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