Noise in the Echosounder · Part IThe aim of the following article is to define what noise is in an echosounder and to distinguish its sources; we then propose a series of steps to mitigate or reduce noise and, finally, we discuss the tools available to deal with unwanted information on screen.We are sharing with you part one of this series of articles, written to unify concepts and improve the experience of fishing captains.The echosounder is a very high-sensitivity acoustic instrument. It emits ultrasound pulses that travel through the water and bounce off different objects in their path, producing echoes which, after returning to the surface, are received and processed to be turned into images on screen. An echosounder is designed to detect very low-intensity echoes.During this process, various physical phenomena can cause unwanted effects on the information the echosounder shows on screen. The screen can get “dirty” with noise or information we do not want.For the purposes of this article we will call noise any physical event that produces an unwanted acoustic consequence in our echosounder. Noise can not only add unwanted information, but also alter the detected echoes, leading to errors in the interpretation or measurement of the echoes.A) The origin of noise:Noise can have various sources:– Nature:

o Rain: usually causes bubbles at depths close to the surface.
o Volcanic or seismic events: can cause sound waves that travel underwater and are picked up by the transducer if they fall within its operating frequency range.
o Waves: cause turbulence and air bubbles.
o Animals: emit acoustic waves, as cetaceans do for example, which can interfere with the equipment.
o Algae and plankton: algae and plankton in high concentrations can cause an unwanted effect on the screen, obscuring the display of the desired echoes, usually fish.
o Ice: when it breaks or collides with the vessel’s hull, it causes sound waves that the equipment can often detect.

– The vessel (your own or another one nearby):o Vibration noise: can be caused by the rotating propeller, engine activity or the shaft line, transmitted to the hull and then into the water as sound waves, which are picked up by the echosounder transducer.o Flow or displacement noise: caused by the water flow displaced by the vessel’s hull, which creates air bubbles that interrupt the proper transmission of sound, causing abrupt breaks in the image.o Electrical noise: caused by electromagnetic waves emitted by various electrical devices on board. While vibration or flow noise enters the equipment as sound waves picked up by the transducer, electrical noise is induced in the cables (transducer, power, network, etc.) or directly in the equipment. Noise generated by battery chargers, converters, LED lights or various electric motors is common. Electrical noise can also be self-generated by the echosounder itself, since the equipment’s own electronics, due to thermal changes in the components, can produce noise that is induced directly into the receiver.– Interference from other acoustic instruments:Transmissions from other acoustic equipment can interfere with echo reception in our echosounder. These interferences can be caused by equipment such as:o Other echosounders
o Sonars
o Acoustic Doppler Current Profilers (ADCP)
o Doppler speed logsB) The solution to the noise problem:

When the echosounder shows noise on screen, users try to suppress it, often resorting to the gain control, clutter (color suppression tool) or the various smart post-processing filters available on modern equipment. During this process not only is the noise removed or mitigated, but the chances of detecting echoes are also reduced when those echoes are at the same level as the suppressed noise. If the noise is very low and does not significantly affect normal use of the equipment, it may be fine to suppress or filter it. Sometimes this is not enough and we must tackle the causes that generate it.The causes or sources of noise in an echosounder are usually many and varied, as we saw earlier in this article. Whenever possible, the best strategy for dealing with noise is to tackle its multiple causes rather than its consequences.To solve noise problems, we recommend following this list:1) Check that the installation of the equipment meets the parameters recommended by the manufacturer. Take into account, for example:a. Correct placement of the transducer on the hull.b. Running the transducer cable through its own conduit, isolated from electrical cables, keeping the cable shielding intact at all times.c. Correct ground connections.d. Correct choice of the equipment’s power supply.e. Placing the transmitter/receiver unit away from electrical equipment.f. A correct installation involves, where possible, synchronizing the installed equipment with the other acoustic instruments on board, in order to eliminate possible interference.If the installation is not correct, we must fix it as a first step to eliminate as many sources of noise as possible.2) If the installation is correct and the noise persists, a thorough search must be carried out to find its source. One possible strategy is a complete “black out” of the vessel, if possible outside the port, away from other vessels and completely stopped, and then switching on the different devices and machines on board one by one. This approach makes it possible to identify many of the most common sources of electrical and vibration noise.

3) If you suspect the noise may be displacement noise, it is useful to carry out a sea trial at different speeds and also to drift without propulsion. It may be necessary to repeat this test with different vessel loads, since this can change the hull’s displacement.4) If the cause of the noise has been identified and successfully eliminated, the problem is solved for the time being. However, for various reasons, the cause of the noise, even when identified, may be difficult or impossible to fix. Only in these cases is it advisable to resort to the tools the echosounder offers to mitigate or avoid noise.C) Strategies for dealing with noise:Echosounders offer various tools for dealing with unwanted information on screen:a) Gain control: with this control the echosounder user decides from what echo intensity level they want to see on screen, thus removing very low-intensity echoes they do not want to see because they associate them with noise or unwanted elements. When adjusting the equipment’s gain, it is always advisable to start from the saturation point and lower it until a clear image is achieved. However, this solution also reduces the size of the desired echoes in the on-screen image.b) Clutter control or color suppression: in parallel with, or as an alternative to, adjusting the gain level, the user can suppress colors from the palette shown on screen. Each color represents an intensity level, and by suppressing those associated with unwanted low-intensity echoes, users can focus on the ones they consider relevant based on their experience.c) Interference and noise filters: when the noise reaches levels equal to those of the desired echoes on screen, smarter strategies are needed. Post-processing filters are based on models of the usual behavior of noise.d) Adjusting the working frequency range: when filters cannot remove the noise, there is an alternative strategy: selecting a working frequency or frequency band in which our equipment is not affected by the noise present on board. For this purpose, the Seaman FMF echosounder has a noise detection tool that helps identify the frequency at which noise is affecting our equipment, so the operator can choose a free frequency or range. Of course, this requires the echosounder to have a suitable transducer.

In this first part, we have explored in depth the challenge of identifying noise in echosounders used in professional fishing, and we have detailed strategies to improve the display. In a future installment, we will analyze noise in echosounders with FM or CHIRP pulses, providing more tools to improve the accuracy and efficiency of the equipment.