What is Echo Sounder | Working Principle | Sonar vs Echo Sounder

An Echo Sounder, also known as a Sonar Depth Finder or Sounder. It is a device used to measure the depth of water by transmitting sound pulses into the water and measuring the time it takes for the sound waves to bounce off the seabed or any objects below the surface and return to the device. It operates on the principle of echolocation like how bats navigate. This article details What is Echo Sounder, its Working Principles, Types, Uses, Differences with Sonar, Errors, Accuracy Checks, Components.

What is Echo Sounder?

Echo sounding is used to detect the depth of water, involves transmitting acoustic waves into water and recording the time interval between the emission and return of a pulse. It is used for ranging other targets like fish schools. Echo-sounding is a faster method of measuring depth than the older technique of lowering a sound line until it reached bottom.

It is used by the operators on the watch to get accurate information. And also, used to measure the depth of the water and mainly the under-keel clearance. It transmits sounds from the bottom of the ship to the sea bed. Transmitter, transducer, receiver, and display unit are the main components. Moreover, the transducer can be either electro-strictive or magneto-strictive.

Working of Echo Sounder

Magneto strictive transducers use iron, cobalt and nickel. Magnetostriction is effective up to a few hundred-kilohertz frequencies. For electrostriction, it uses various types of lead materials and works on compressive or tensional forces. These forces will create a change of length or dimension in the material.

The unit of display can be a paper recorder, video type or digital info display.

The formula used by the echo sounder is – distance = speed x time / 2.

The transmitter creates an electrical pulse which later passes through the transducer. The transducer located at the bottom of the ship converts it in to sound waves. These sound waves may either hit the sea bed or anything in between.

As soon as the transducer receives back the sound waves, it is converted back to current and reflected on a digital or video display.

Types of Echo Sounder

There are two types of Echo Sounder-

Single-Beam Echo Sounders (SBES)

These sounders were developed 80 years ago. They were used instrumentally for basic oceanic discoveries and standardization. Single-beam echo sounders are also known as fathometer or depth sounder. They work on a single sound pulse in a single narrow beam and mainly they can measure one point per echo.

In recent days, single beam echo sounders are used for depth measurements, sub-bottom measurements and seabed imaging.

Multi-Beam Echo Sounders (MBES)

These multi-beam echo sounders releases a multidirectional beam to get information in the affected area. The returned sound waves by transducers provides clear information about the depth of the water, the shape of the river, lake and other underwater features.

Use of Echo Sounder

The operator checks his watch if the equipment is working properly and has all the set up before using the echo sounder. Initially, the operator has to be familiar with the mode of options of the echo sounder in order to avoid confusion and the grounding will be prevented if possible. The navigation mode will be the normal mode. In navigation mode, the echo sounder measures the depth of the water from the transducer position to the seabed. Sometimes, it is known as the under keel clearance.

The next available mode is draft below the surface (DBS). This is the depth of water which includes the draft of the vessel. To enter the ship’s draft, there is a draft button also. The history and log data mode allows the officers to see ship’s previous readings along with the data recorded in the echo sounder.

How to Check Accuracy of Echo Sounder?

The Echo Sounder must be set properly before it gives an accurate reading.  The initial setting can be done in two stages –The moment of transmission, the echo sounder must be set to the depth of transducers below the waterline. And also, the speed of the echo sounder stylus is aligned with the sound velocity in seawater.

The bar check is the most widely used technique by industry experts to check echo sounder. This is insisted for shallow waters up to depths of about 300 meters, at which a good clear echo can be received from the bar. However, it is very difficult to check the accuracy of echo sounders in the shallow waters. The weather must be good enough with good wind, smooth and calm area and tidal streams are loose.

The transducer is very close to the seabed and hence it is difficult to know when the echo is actually received. The form of the seabed, strong current and drift can greatly affect the sound waves whereas the readings can be doubtful.

Components of Echo Sounder

Echo Sounder has the following basic components –

Transducer

It is used to generate the sound vibrations and it also receives the reflected sound vibration.

Pulse Generator

For the transmitting transducer, this pulse generator produces electrical oscillations.

Amplifier

The amplifier amplifies the weak electrical oscillations which is generated by the receiving transducer on reception of the reflected sound vibration.

Recorder

It is used to measure and indicate depth.

Principles Used – Working of Echo Sounder

There are two principles used–

Ranging

In echo sounder, the stylus is mounted on a circular belt driven by means of a stylus motor which moves at a certain speed and transmission takes place when the stylus passes the zero marks. A magnet fixed on the stylus belt triggers the transmitter to transmit a pulse every rotation of belt when stylus is at zero mark on the paper scale. The transmission on the acoustic waves from the transducer is combined with the stylus at the zero mark. To overcome certain issues, some of echo sounding machines work on phasing technique.

Phasing

The speed of the stylus motor remains constant during phasing. The transmission point is advanced, instead of changing the speed of the stylus. Different sensors are placed around the stylus belt. When the magnet passes the sensors, generates the pulse and in turns activates the transmitter. To note an important point is when using phasing technique, we must always start sounding at lowest range and check for echoes. If needed adjust the gain control and then switch over to higher range.

Errors of Echo Sounder

Below mentioned are some of the errors –

• Velocity of propagation in water
• Stylus speed error
• Pythagoras error
• Multiple echoes
• The thermal and density layers
• Zero-line adjustment error
• Cross Noise
• Aeration

Difference between Sonar and Echo Sounder

Both are quite similar. However, they have few differences –

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