Choosing a sonar system used to be relatively easy. For the first few decades, the models offered for Search & Rescue didn’t differ significantly. Klein, Edgetech, Marine Sonic Technology then, later, Imagenex, Tritech and JW Fishers, all offered systems which had different features and different software, but all were towed systems.
During the past decade, both hardware and software developments have changed the game.
Autonomous Underwater Vehicles (AUV’s) and remote-control surface vessels such as Sonar EMILY have been introduced and many Search and Rescue (SAR) organizations have found hull-mounted systems, originally developed as fish finders, to be effective, economical alternatives under certain conditions.
Side Scan Sonar/Side Imaging® Sonar1 – Hull Mounted vs Towed
With the advent of inexpensive hull-mounted sonar systems such as the Humminbird® Side Imaging® sonar and professional software, users want to know if this unit will be as effective as a towed side scan sonar system. The answer is absolutely, positively, maybe!
The hull-mounted and towed sonars are two different tools. Both create high-resolution bottom imagery but their operational parameters, required experience level and cost are very different.
Which factors most influence the deployment choice of side scan sonar system?
Most purchasers focus strictly on cost but cost alone does not take into account all the characteristics important to deployment of the most optimal side scan system. Factors such as water depth, frequency of use, data processing, etc. all play a role in choosing which type of sonar system is best for your mission. The matrix below outlines the most important factors which present the advantages and disadvantages of different sonar system deployments.
Let’s go through them detail.
Why is depth so important? It has to do with the distance between the sonar transducer and the bottom. Side scan’s effectiveness depends on the sonar pulse moving across the bottom rather than straight down or perpendicular to the imaged surface like an echosounder or depth finder. This acute grazing angle produces shadows that highlight the texture of sediments and targets on the seafloor. The angle also influences resolution, so the more perpendicular returns tend to compress the imagery. The general rule of thumb was that the sonar height above the bottom or altitude should be about 15% of the set range. As the water depths increase, towed systems can deploy more cable, allowing the transducers to maintain an optimal height above the bottom.
This distance above the bottom, also known as towfish altitude is critical for good data collection. I have operated towed side scan sonars in as little as 3 feet and over 10,000 feet of water. Given enough cable and a large enough support vessel, or now a deepwater AUV, depth is not an issue for a sonar system in the most simplistic sense.
Fixed, hull-mounted systems work great in shallow water. However, as depth increases, the grazing angle changes, reducing shadow lengths and resolution. A sonar pulse from a hull-mounted sonar working in 100 feet of water must travel 100 feet before it reaches the bottom but also look further to see out to the side. In order to see imagery in 100 feet of water a typical sonar slant range setting for a hull-mounted system would be 150 to 200 feet.
The maximum effective depth for hull-mounted sonars depends on a number of factors such as the sonar frequency, size of the target, its reflectivity, bottom type, bottom clutter, etc. but for smaller inexpensive systems, it is typically under 125 ft for large targets and less than 65 feet for small targets such as drowning victims. That is not to say every deep-water search for a small target will be unsuccessful. For example, the Colorado Parks Service recently located a drowning victim in 120 feet of water with an inexpensive hull-mounted system in perfect conditions. Flat calm water with a flat uncluttered bottom. The drowning victim was the only contact, standing out from the natural environment with no visible appendages or other human features.
In summary, hull-mounted systems have depth limitations. Still, towed systems can work in greater water depths without losing resolution as long as the operator has sufficient cable with a towed system or an AUV to maintain the proper towfish altitude.
Sonar EMILY and other surface robots face the same limitations as hull-mounted systems. However, autonomous underwater vehicles (AUV) and Unmanned underwater vehicles (UUV) can theoretically operate at any depth for which the robots have been designed.
One critical component of side scan sonar image analysis is acoustic shadows. Shadows draw our eye to targets, indicate holes or mounds, provide target details and, last but not least, allow us to estimate the height of objects. One can use a flashlight to simulate the sonar and how shadows are created by different targets and bottom features. Place an object on a table with the flashlight illuminating from slightly above and to the side. Notice the shadow length. Now move the flashlight higher and watch the shadow length shorten.
The higher the sonar relative to the bottom the shorter and less pronounced are the shadows. Hull-mounted systems produce less of a shadow in deeper water than towed systems, keeping the transducer relatively close to the bottom.
Side scan sonar requires steady forward motion to create quality imagery. When the sea state kicks up, data quality is reduced. Hull-mounted sonars and surface robots suffer more than towed systems because the tow cable decouples some of the boat motion from the towfish. AUVs/UUVs are independent of surface motion except when working in shallow water.
Ease of Deployment/Operation
Hull mounted systems are by far the easiest to deploy and operate. The sonar is attached to the hull and, as long as the boat driver does not run aground, operationally, you’re good.
Towed systems require the deployment and recovery of cable. The boat driver must steer straight lines and not turn too sharply, which causes the towfish to dive towards the bottom. Once mastered and with practice, these operations become second nature.
AUVs/UUVs are very complex to program, operate and deploy effectively. These systems need to be monitored, and the team must be ready to recover a vehicle that aborts its mission or becomes entangled in an obstacle. While anyone can chuck an AUV/UUV into the water, if it is not programmed or monitored properly, more than one vehicle owner has come home with significantly less equipment than he or she started with.
Boat Driver Skill Level
As previously explained, towed systems require a skilled and practiced team to be successful. The boat driver will make or break your search. He has to maintain specific speeds, steer straight lines and turn without dropping the towfish into the bottom. Having taught many folks to tow sonar systems, I can say that boat driving is the toughest skill to teach and learn. More than once, I have grabbed the throttle or wheel to avert disaster. It did not make me very popular, but that customer would not lose a towfish on my watch.
Hull-mounted systems require straight survey lines, but you’re never in danger of losing the sonar by turning too sharply or slowing down when you should be speeding up. You can always stop if you get in trouble, something you never do while towing. These systems are ideal for surveying adjacent piers and obstructions where maneuverability is limited.
Obviously, robots which execute searches with no human input are easy peasy as survey patterns are programmed into the software, and the robot does all the work.
AUVs/UUVs are highly susceptible to being lost without some physical connection to the surface. While experienced AUV/UUV operators rarely lose a vehicle, it is not uncommon when running search OPS to have a robot snag on fishing gear, a wreck, or even under an uncharted ledge.
For reasons described previously, towfish can be lost even with a cable to the surface. I have been on a ship where a deep-water system has hit an uncharted submerged cliff and I have friends who have snagged shipwrecks. While towed systems can be lost, more often, it is a question of damage.
Surface robots could be lost if you’re running from shore and the battery dies or the vehicle develops a problem. Most surface robot operators have a chase boat or some alternative recovery method.
Obviously, you have to work extremely hard to lose a hull-mounted system. Don’t run aground, and you’re good. Most often, any damage occurs moving on and off trailers if the transducer has not been properly mounted.
Generally, towed systems operate from slightly larger vessels unless working very shallow water. I have run towed systems from a 12-foot inflatable in calm, clear weather. However, I tend to want more boat when deploying cable and a shelter for my computer and monitors. Deeper towed search OPS require winches and longer lengths of sonar cable, all of which require bigger boats. The general rule of thumb is that the required cable equals 3 times the water depth.
Small hull-mounted side scan sonars can generally be operated from vessels less than 100 ft or from a surface robot as small as 6 feet in length.
The smaller AUVs and UUVs can be launched from shore or small inflatable boats. Deepwater AUVs/UUVs usually require larger boats with special launch and recovery equipment.
Post Mission Analysis
Traditional towed and AUV/UUV systems have software for reviewing sonar data. These software systems allow you to mark and measure contacts, determine coverage, and present data for reports. There are many third-party software packages which can process sonar data as well.
Hull-mounted systems traditionally have had less sophisticated operating software, making them easier to use but have fewer functions. It is hard to show sonar coverage and measure contacts without employing professional 3rd party software, which is expensive and requires significant training. Fire rescue personnel, sheriffs, and other public safety groups rarely have the time to learn and stay fresh with these systems. There are a few inexpensive software programs, but these are sometimes clunky and cumbersome.new options are becoming available for SAR operators.
Search operations have three skill requirements of the search team.
- Ability to operate the sonar
- Ability to understand what they are seeing on the sonar
- Ability to effectively drive search patterns with the boat
Image analysis requires the same training and skill level across all systems and is not a differentiator. However, there is a substantial difference in the skills required to operate the different sonar systems and to drive the boat.
Hull-mounted systems are the easiest to use and are best for sporadic use. They require the fewest computer skills and the least training. Operators definitely benefit from training but the training level is less than required by towed or robotic systems. Also, while a steady hand on the wheel improves the search coverage, the sonar equipment is not at risk.
Surface robotic systems, such as sonar EMILY, are more complex but relatively easy to learn. The analysis skills are the same as any other side scan sonar system. Search patterns are easily handled with autonomous search pattern capabilities.
Towed systems are more complex than hull mounted and require a fair amount of knowledge and computer skills. Coordination between boat driver and sonar operator is critical to successful search operations. If the boat driver does not respond to the sonar operator’s instructions, at a minimum, data quality is reduced, and, worst case, the towfish impacts the bottom. These systems are best used by dedicated teams with frequent missions which keep skills fresh.
AUVs/UUVs require a very high degree of training and computer skills. These vehicles must be programmed and maintained, requiring significant training. Like towed systems, these systems are best for dedicated teams who spend a lot of time working with the unit.
One final note on training: regardless of the system, training must be followed up by practice. Sonar search OPS is a skill that must be practiced. Without practice, the probability of mission success is reduced.
Hull-mounted systems are relatively inexpensive when compared to towed systems. Towed systems vary in cost, with less expensive models now available2 but still are more expensive than even the most high-end hull-mounted system. AUVs/UUVs are coming down in price but still are more expensive than all the other systems.
When looking for a side scan sonar system, you need to consider your environment, types of targets, available vessels, skill level, team training time, your mission, and your pocketbook.
Hull-mounted systems are relatively inexpensive and very effective in shallow with calm conditions. Towed systems are more versatile and effective in more environments. The more expensive towed models have better software and provide a higher quality sonar image at longer sonar ranges with the ability to cover more area quickly. However, towed systems are more expensive and require more training, practice, and experience.
If you are working on large deep-water bodies, towed systems are definitely preferred. Working inland on smaller bodies of water, enclosed harbors, canals or shallow coastal bays hull-mounted systems can be a cost-effective alternative.
- Humminbird® uses the trademarked Side Imaging® terminology for a side scan image. Throughout this document, the term side scan will refer to all side looking, bottom imaging sonars. Please also note the matrix and discussion do not include the depth sounder, echosounder, Down Imaging® or fish finder sonars present on most hull-mounted systems.
- While less expensive towed systems are available, you get what you pay for. Less expensive systems have less effective software and do not perform as well as the more expensive ones.
© 2022 Vincent Capone