Deep Trekker has released the following guide that explains how sonars help with underwater inspections and surveys, as well as information such as:
- What sonar is
- How it is used
- How it works
- Applications of sonar systems
- Different types of sonar
Sonar systems have many applications. This guide explains all you need to know about sonar systems, including using sonar for tunnel inspections, search and recovery missions, and more.
The video above is an example of how sonar can help with underwater target identification and navigating toward it. Sonar is often the favorite tool in the toolkit for those that work underwater. Utilizing sonar is more of an art than science in many cases, so it can be frustrating for those initially exposed to it. It is a powerful option to have though as it can provide position information, context for the environment around you, and imaging capabilities in even the murkiest water.
There are different types of sensors that utilize acoustic technology:
- Echosounders / Altimeters
- Mechanical Scanning (CHIRP, Sector Scanning)
- Doppler Velocity Logs
- Ultra Short Baseline Positioning
- Side Scan Sonars
- 2D Imaging Sonars
- 3D Imaging Sonars
These technologies all serve different purposes. This article focuses on how sonars help with underwater inspections and surveys.
Sonars help with:
- Navigation in Murky Water or Large, Open Spaces
- Identification of Key Targets
- Helping measure features such as sediment levels or defect size
- Situational awareness
Underwater applications for sonar include:
- Bathymetric Surveys
- Pipeline Inspections
- Explosive Ordnance Disposal & Mine Countermeasures
- Search and Recovery
- Marine Salvage
- Infrastructure Inspections
- Offshore Wind Installation Support Surveys
- Ocean Science, Discovery, & Academic Research
Since GPS does not work underwater and getting good quality imagery with just a camera can be a challenge, especially in murky water, sonar is an excellent technology for anyone with underwater work to understand and utilize.
WHAT IS SONAR AND HOW IS IT USED?
SONAR (short for sound navigation and ranging) is a method by which sound waves are used to locate/map out objects and landscapes in the environment. In essence, a cluster of sound waves directs towards any environment. While some waves will bounce off the objects, others will reflect towards the emitter.
For instance, if you are to insert a tube into the open sea and hold the other end up to your ear, you will definitely be a sight to see. However, you will be able to hear the sounds of the animals down below and the groaning of the ships far away.
It’s often touted that Leonardo Da Vinci was the first to discover this phenomenon, but this cannot be proven true. The method behind SONAR wasn’t invited recently by humans, it has been in use as a natural function of life by whales, bats, and other animals for millions of years.
With the understanding of the time passed and the speed of sound before the wave returns, the receiver can assess the distance of the object from the emitter. Even though SONAR works in the open air, it’s most effective underwater. This has to do with the fact that sound travels further in water.
To the potential of SONAR, whales can discern movement and object shape the size of rocks from over 60 feet away. In fact, whales rely on SONAR more than they do on their kin, forage, or sight.
HOW DOES SONAR WORK?
The components of an active sonar system will consist of a display, transducer, transmitter, and receiver. Active sonar works by transmitting an impulse directly from the transmitter and transmuting it into a wave of sound with the transducer. When the wave hits an object, the sound will rebound.
The echo then returns to the transducer, which transmutes the sound into an electrical impulse amplified by the receiver. This data is then sent to the display. Sometimes, the transducer helps detect sound waves as well as transmit them.
How does one detect the sound source and calculate the distance of the object from the origin? Using a multitude of hydrophone sensors, SONAR systems record the intensity of the sound and phase when each ping hits a sensor.
Phase is the timing delay that occurs when receiving the sound wave. The sensor that records the greatest amplitude and has the least phase is the one nearest to the point of reflection.
Another important factor in SONAR performance is the environment of use. The performance of SONAR systems is variable based on the environment of the ocean, which is unpredictable at times. Regular ocean studies are critical for acoustic propagation models to ensure accurate estimation of ranges.
For example, one of the challenges in SONAR is scattering. This phenomenon occurs from small objects in the body of water from the depth of the bottom to the height of the surface. Much like light scatters from light in the fog, the same is similarly applied to this water interference.
TYPES OF SONAR SYSTEMS
At some point, humans have developed SONAR with superior resolutions and range. The simplest SONAR systems consist of our ears and voice box. It’s the same system we use when we scream when atop a mountain and hear our echo.
However, SONAR developed by the military can travel thousands of miles. The sweeping range allows the system to cover 80% of the ocean beds from sound waves that use a mere 4 vantage points.
Despite the efficacy of light and the superior velocity of RADAR, only SONAR helps create seafloor maps, develop nautical charts, predict hazards, and discover shipwrecks.
As a matter of fact, the SONAR patent had experienced sanctions after the events of the Titanic. The purpose of the SONAR was and is to identify objects under the surface that might prove challenging to ships. Furthermore, the World Wars brought serious advancements that pushed forward the importance of warfare submarines and underwater surveillance. From this advancement, two main types of sonar emerged; active and passive.
Active sonar relies on a projector and a receiver to determine range, bearing and relative motion of the target. The acoustic projector generates a sound wave that spreads outward and is then reflected back by a target object. The receiver then picks up and analyzes this reflected signal.
One example of active SONAR are submarine vessels. Submarines transmit acoustic energy and discover objects in the vicinity via the time delay between the acquisition of the echo.
Besides being able to detect the presence of an object, the rise of advanced modern tools allows us to determine the size, orientation, and shape of an object with great detail.
Deep Trekker ROVs utilize active sonar which sends out a sound wave at a particular frequency and then listens for how long it takes for that sound wave to return after bouncing off objects in the water and the seafloor. Multibeam imaging sonar uses multiple beams of sound to paint an image of what’s in front of the ROV.
In surveillance scenarios, the systems used are passive SONAR. In this case, the method does not need a local transmitter because it listens to waves transmitted by other devices.
This means the device gathers sounds made by sea life, ships, and other depth surfaces. However, the machines in a passive system cannot determine the location of the sound source without the assistance of listening devices. They must work together to determine the transmitter location without having their presence known (in a military setting).
Compressed High-Intensity Radar Pulse or CHIRP sonar tool often used for bottom-tracking and fish-finding. Instead of pinging a single frequency like traditional 2D sonar, Chirping devices transmit a sweeping range of frequencies. With each pulse, the transducer starts vibrating at a low frequency, which is then modulated upward to a high frequency over the duration of the pulse.
APPLICATIONS OF SONAR SYSTEMS
SONAR technology is not exclusively used by the military, so let’s take a look where else it’s used.
Bathymetry is the study of the “beds” or “floors” of water bodies, including the ocean, rivers, streams, and lakes. In this circumstance, SONAR is subject to use to determine the depth of ocean floors, allowing for safe navigation of the waters. This is how nautical charts have depth readings on them.
Pipeline inspections can be difficult in murky water with high-frequency SONAR. Gas and oil companies use SONAR to detect damage, spans, and rock dump integrity.
Explosive Ordinance Disposal / Mine Countermeasures
Furthermore, SONAR is also used to detect explosive danger underwater. As the seafloors become exploited, it’s important to identify cables and pipes in them. Finding unexploded bombs, mines, and torpedoes is important for overall safety and wellbeing. Finally SONAR is also fitted into submarines and ships to allow for underwater communication with other entities. It is common to see a towed or boat mounted side scan sonar used to scan large areas in patterns for anomalies, and then use a ROV with imaging sonar to reidentify the target and navigate toward it to confirm if it is the explosive.
Search and Recovery (Public Safety/Law Enforcement)
By Navy photo by Photographer’s Mate Airman Tina Lamb. [Public domain], via Wikimedia Commons
Sonar technology, it is often used during Search and Rescue missions to make it easier and more efficient to locate evidence or victims of boating accidents or potential drownings in unclear waters. SONAR is also regularly used in search and rescue. Side-scanning systems help locate bodies and guide the divers to the site of the recovery. ROVs with imaging sonars offer a safe alternative to verify and recover the victim instead of divers.
Marine Salvage Efforts
Sonar technology also has the ability to help locate underwater objects in deep salvage operations where murky waters may hinder maximum camera visibility. Identifying a shipwreck or other large structure from up to 200m (656’) away isn’t a stretch for some ROV mounted imaging sonars.
Offshore Wind Installation Support Surveys
It’s also used for offshore wind turbines. Powerful SONAR systems are used for the assurance of safe installation of wind turbines. These sites must be subject to survey with great accuracy to ensure the foundation of the turbine is secure in the water.
Large structures such as bridge pilings, dock piers, quay walls, and dams can be difficult to perform underwater inspections on due to the sheer size relative to a set of eyes through a camera on a ROV or a diver. Sonar helps provide a broad view of the structure, allowing the inspector to have a better understanding of the structure, to identify major defects as well as measure changes over time by comparing to previous sonar surveys.
Ocean Science, Discovery and Academic Research
Finally, the addition of sonar technology is invaluable to underwater discovery and research by helping academics and researchers to monitor aquatic life or environmental conditions below the surface.
OPERATING PRINCIPLES OF SONAR
Scanning SONAR systems are often defined by their fan-shaped sound beam with a narrow horizontal and wide vertical beam for recognizing the cross-section in environmental acoustic.
While SONAR can be relatively straightforward to use, it is truly a complex science. The specifics of acoustics are valuable to understand in order to fully grasp the concept of sonar.
By putting together the sound speed in the water with the time in which the reflection was received, the SONAR system calculates the distance that the sound traveled.
The question for calculating the acoustic distance is:
Distance = known sound speed in water x (calculated sound delay upon return / 2)
From this, we can understand that the difference in sound speed can drastically affect the accuracy of distance to a target. Usually, the sound speed in bodies of saltwater is about 1500 m/s. However, this number varied based on the operating depth of the system, water temperature, and salinity.
Sound speed calculators can help in getting better approximations in variable operating environments. Even though scanning SONAR systems do not have the capacity to accurately calculate sound speed, the values change in the display for the right environment.
SONAR targets with concrete material densities like rocks, metals, gas are quite different from water, as they will have powerful reflective echoes. Echoes from sand, mud, silt, and plants are not as powerful because their density is similar to water or they absorb sound energy.
The echo strength displays on the device as a bright indicator with vivid color. In typical palettes, bright colors in underwater sonar images represent strong echoes and dark colors represent weak echoes.
Sound Beam Patterns
When visualizing how scanning SONAR works in terms of sound, they are often referred to as a flashlight in a dark room. Only the illuminated area by the light is subjection to vision for the user, the rest will remain dark.
When compared to light beams, acoustic beams from SONAR have a fixed height and width, which is the beam pattern. It’s this acoustic beam that “illuminates” the targeted water locale with sound energy instead of light energy.
Scanning SONAR systems usually have a narrow horizontal and wide vertical beam, which results in a narrow path for the energy to transmit from the transducer. To portray the image of objects in the environment, the head of the transducer inside the SONAR will rotate with a stepper motor and then move the head into an arc to generate slices of the object on the display.
Returning to the example of the flashlight, if you take pictures of the area as the light sweeps across the room, you will have slices of the room lit up. As the user, you will not see the entire room lit up, but putting together the slices will let you see the total area lit up.
Object Visibility, Slant Range, Arrival Angle
Objects within the beam patterns of the SONAR system illuminate acoustically and their reflections will be echoed to the SONAR for interpretation.
Any objects outside of the pattern, below, outside, or above do not occur on the display of the SONAR viewer. Scanning SONAR systems are not able to determine the difference between objects that have the same slant range (vertical arrival angle).
For example, if two objects are at the same vertical range in front of the SONAR above each other, the SONAR will show the objects as a single object even though it’s from a collection of their echoes.
For individuals searching the bottom of the body of water and understanding that signal strength at larger distances decreases, they can mount their SONAR systems at specific angles. This will allow greater clear images of the seabed to be subject to portrayal on the display.
If the system angles down and has a low altitude at a steep, only a narrow total area will present itself on the display. With increasing altitude, the SONAR system will illuminate a wider plain of the sea bottom.
When an operator searches for objects on the sea bottom, the optimal results from SONAR will come from optimizing the altitude above the bottom with the system angled down.
This will provide the greatest imaging range for the bottom by the signal strength of the SONAR system.
Both mechanically and side-scanning SONAR systems can get about 70% of the seabed covered if the system has at least a 10-degree angle and a 10% operating range altitude.
For instance, at a range of 10 meters, the SONAR system should have 10% of that as an altitude. That’s 1 meter above the sea bottom. At 20 meters, the altitude should be at least 2 meters.
This is known as the 10% rule. There are other “rules” in solar usage, but this is the most concrete of them all.
Acoustic Shadow, Distance, and Altitude
Once again returning to the analogy of a flashlight, it applies to when a SONAR locates an object to determine the shape, orientation, and height. These objects will have an acoustic shadow illuminated much like it would if there was a visible light being shined on them.
If the SONAR system has a steep down angle and a large altitude, the acoustic shadows will be short. Short shadows can be hard to see, making it hard to assess the object. If the sonar altitude is close to the bottom and the down angle is shallow, shadows cast by the objects will belong.
Acoustic shadows cast by far away objects are narrow due to sonar beam angular geometry. The width of the shadow will increase as the SONAR system moves closer to the target. With a wider shadow, it can be hard to see other objects in the shadow, because there is no acoustic energy directed at them.
When many objects are in the same area and covered by shadows, increasing the angle and altitude to produce short shadows will help optimize object distinction.
ECHOES IN SONAR
As demonstrated in previous parts of this article, objects are often illuminated from an angel by the SONAR system. Thus, only the surfaces and edges close to the system will be subject to display.
The object surface that is perpendicular to the system will result in the strongest echoes. Whereas, surfaces with less optimal angles will make the acoustic waves reflect from the system, delivering bad results.
All of these acoustic principles apply to large environments. For example, when viewing boat hulls and dock fingers in the water, the bright will return the features of objects in the line of sight. Areas hidden away from the echo returns will be seen as areas with no return or as shadows.
Even with the unique profile of acoustic interpretation, SONAR systems can be very valuable when mounted with an ROV. Without a SONAR system, the pilot of the ROV will have to rely on the visual relay of the objects and features via a camera. In low visibility environments, it might be hard to see with the range often being under a meter.
SONAR systems will drastically increase this range, allowing the pilot to detect objects from further away. Instead of going over the seabed to find objects, the ROV can remain stationary and scan the entire environment. The pilot can then get an understanding of the area with the man-made objects, natural surfaces, and areas to ignore.
Because ROVs have a low mass, unnecessary movement is common on both vertical and horizontal planes. For SONAR systems, the image is developed as the transducer rotates on the center-point axis. If the ROV moves due to the environment or pilot input before the image acquisition, the result might be smeared.
In these cases, it might be better to narrow the plane scan or sit the ROV at the bottom to induce a faster refresh rate for the ROV to rotate on the axis. Also, when looking at a display angle to the object, one must know the relative bearings where the object is subject to reading as a clockwise angle from 000 degrees R.
A polar scan entails any scan that covers a 360-degree continuum. A polar scan is great for acquiring environmental awareness around the ROV in conditions with low visibility. Cameras will usually have a 90-degree field of view, so they are unable to look around without a gimbal.
A sector scan refers to any scan that is less than a 360-degree continuum. The scans are useful for improving refresh rate while imaging the environment or keeping track of an object. The disadvantage of this scan is the loss of visuals behind or the sides.
OBJECT LOCATION WITH A MOUNTED SCANNING SONAR
Finding targets on the water column or the seabed is another way for the SONAR use, besides navigation. Learning how to use the SONAR system for finding objects requires practice because small objects are much harder to find.
The key to do this with an ROV is to turn slowly and maneuver allowing for new images to be subject to generation without smear. First, the ROV must be placed on the bottom or in a stable position.
Next, a polar scan is subject to initiation fully. After this, the relative bearing calculation to the object occurs. To continue, the ROV is turned in alignment with the object at a zero bearing.
Further on, the sector scan is subject to use to narrow down the image to about 90 degrees for the sake of a faster refresh rate. And finally, the contact remains with the target using SONAR as the ROV follows the target.
TUNNEL INSPECTIONS WITH AN IMAGING SONAR
2D Imaging sonars are an excellent option for many applications as discussed. Sending hundreds of beams into a 120 degree horizontal and 20 degree vertical band allows for a better image quality than a single beam, sector scanning type sonar. This is the type of sonar utilized in the channel survey video at the top as well as this tunnel inspection video here.
UNDERWATER REMOTE-OPERATED ROBOTS FOR YOU
Now that you know what SONAR systems are and what they are used for, you are that much closer to deciding if you would like to make use of SONAR for your own needs. In any case, there’s no rush for this and it’s even better if you do some more research.
There are many types of SONAR systems, integrated with a variety of tools, software, robots, and vehicles. Because SONAR can be subject to use in energy, infrastructure, defense, commercial diving, municipalities, maritime, defense, ocean science, and underwater discovery, it’s only better if you understand what you’re getting yourself into.
If you’re interested in learning more about SONAR or you would like to consult about the appropriate system for you, get in touch with us and we will happily accommodate your needs.
Deep Trekker offers remotely operated vehicles (ROVs) equipped with cameras that are widely used for tasks such as underwater inspections, video recording, maintenance and repair operations.
By adding the optional TriTech Micron or the TriTech Gemini 720is Multibeam Imaging Sonar attachment, you get the benefit of remote operated scanning that will significantly enhance your ROV’s ability to identify hidden objects that might otherwise go undetected by a camera system. These attachments allow for both side and forward scanning capabilities. TriTech International Limited is a leading global designer, manufacturer and integrator of reliable, high-precision underwater imaging equipment that is widely regarded as the standard for the industry.