Section outline

  • 　One way of describing what is going on in the ocean to humans using ultrasonic waves, which are invisible to the eye and inaudible to the ear, is called an echogram. See the figure below.

    　



    The horizontal direction indicates time and the vertical direction indicates depth (distance from the ship). If there is an object in the water, the sound will bounce back. The distance to the object can be determined by measuring the time between the sound and its return and multiplying it by the speed of sound. This is the same principle as the "Yamabiko". What is the speed of sound in water and in air, respectively? Also, by measuring the loudness of the returned sound, you can find out the size of the other party. When one fish is detected, it appears as a "^" on the echogram. When a large number of fish are detected, the "^" overlaps to form a school. Recently, some echograms have been developed that display the 3D position and size of fish in the sea, as shown in this echogram.