Hull Mounted vs Towed Side Scan Sonar Systems: A Sonar Expert’s Perspective

Vince Capone

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.

EdgeTech 4125 SAR Model

EdgeTech 4125 SAR Model

During the past decade, both hardware and software developments have changed the game.

Riptide AUV

Riptide AUV

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.

Humminbird Search and Rescue Training

Humminbird Search and Rescue Training

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.

Depth

Why is depth so important? It has to do with the distance between the sonar transducer and the bottom. Side scan sonar 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. As the sonar

Towed Side Scan Sonar Ping

Towed Side Scan Sonar Ping

pulse travels through water the pulse energy decreases resulting in a lower signal to noise ratio. Also, the longer a sonar pulse travels the more the ping expands, reducing resolution. Shorter ranges also ping faster, putting more pings on objects being imaged and, the more pings, the better the sonar can resolve the target. Therefore, it is important to choose a system designed to operate at the right depth.

As the water depths increase, towed systems can deploy more cable, allowing the transducers to maintain an optimal

Side Scan Sonar Winch

Side Scan Sonar Winch

distance above the sea floor. This distance above the bottom is also known as the towfish altitude. I have operated towed side scan sonars in as little as 3 feet of water and in over 10,000 feet of water. Given enough cable and a large enough support vessel, in the simplest sense, depth is not an issue for a towed system.

However, as depth increases, fixed, hull mounted systems logically increase the distance between the transducer and the bottom. 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.

Hull Mounted Sonar in Deeper Water

Hull Mounted Sonar in Deeper Water

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 150 ft for large targets and less than 100 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, recently a parks service group located a drowning victim in 120 feet of water with an inexpensive hull mounted system.

In summary, hull mounted systems have depth limitations but towed systems can work in greater water depths without losing resolution as long as the operator has sufficient cable to operate the system at the proper towfish altitude.

Sonar EMILY and other surface robots face the same limitations as hull mounted systems. Autonomous underwater vehicles (AUV) and Unmanned underwater vehicles (UUV), however, can theoretically operate at any depth for which the robots have been designed.

Humminbird Equipped Sonar EMILY

Humminbird Equipped Sonar EMILY

Acoustic Shadows

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.

Hull Mounted Sonar Acoustic Shadow

Hull Mounted Sonar Acoustic Shadow

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 which keep the transducer relatively close to the bottom.

Surface Motion

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 very 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.

Deploying Towed Side Scan Sonar

Deploying Towed Side Scan Sonar

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, however, at first they can be daunting.

AUVs/UUVs are very complex to program, operate and deploy effectively. These systems need to be monitored and the team ready to recover a vehicle which aborts its mission or becomes entangled in an obstacle. While anyone can chuck an AUV/UUV into the water, if it is not programed or monitored properly, more than one vehicle owner has come home with significantly less equipment then 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 very 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 by far the toughest skill to both 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 was not going to lose a towfish on my watch.

Hull mounted systems do 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. Should you get in trouble, you can always stop, something you never do while towing. These systems are ideal for surveying around piers and obstructions where maneuverability is limited.

Obviously, robots which execute searches with no human input are easy peasy.

Potential Loss

Results of Side Scan Towfish Impacting an Uncharted Submerged Cliff

Results of Side Scan Towfish Impacting an Uncharted Submerged Cliff

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 method of recovery.

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.

Vessel Requirements

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 cable required equals 3 times the water depth.

Small Fire Rescue Boat

Small Fire Rescue Boat

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. Deep water AUVs/UUVs usually require larger boats with special launch and recovery equipment.

Launching Small UUV

Launching Small UUV

Post Mission Analysis

Traditional towed systems and AUV/UUV systems have software for reviewing sonar data. These software systems allow you to mark and measure contacts as well as determine coverage and present data for reports. There are many third-party software packages which can process sonar data as well.

Hull mounted systems have less sophisticated operating software which makes them easier to use but they have fewer functions. It is hard to show sonar coverage and measure contacts without employing professional 3rd party software which has been expensive and required 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 clunky and cumbersome to use. SAR HAWK™ is a recently introduced software system for use with Humminbird® sonars which inexpensively closes the data processing gap between hull-mounted and towed systems. First and foremost, it provides the ability to view sonar data in the command center on a high-resolution monitor at the data’s full resolution. SAR HAWK™ also provides the ability to create coverage maps superimposed on Google Earth, to locate and measure targets, and to produce reports.

Side Scan Sonar Training

Side Scan Sonar Training

Training Requirements

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 relatively the same training and skill level across all systems and is not a differentiator. There is however, a substantial difference in the skills required to operate the different sonar systems and to drive the boat.

HELIX Training

HELIX Training

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 system such as sonar EMILY are more complex but still 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 to operate. 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, in the worst case, the towfish impacts the bottom. These systems are best used by dedicated teams with frequent missions which keep skills fresh.

Side Scan Sonar OPS

Side Scan Sonar OPS

AUVs/UUVs require a very high degree of training and computer skills. These vehicles must be programed and maintained, all of which requires 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 are a skill which must be practiced. Without practice, the probability of mission success is reduced.

Cost

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.

Summary

When looking for a side scan sonar system you need to consider your environment, types of targets, available vessel, skill level, available team training time, your mission and your pocket book.

Hull mounted systems are relatively inexpensive and very effective in shallow, relatively flat water. 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 ability to cover more area quickly. However towed systems are more expensive and require more training, practice and experience to use effectively.

If you are working on large deep-water bodies, towed systems are definitely preferred. However, working inland on smaller bodies of water, enclosed harbors, canals or shallow coastal bays hull mounted systems can be a very cost-effective alternative.

Have more questions?

Visit Black Laser Learning’s sonar forum and post a question. With luck, someone else asked the same question and the answer is already there!

Need to know the definition of a sonar term? Check out Black Laser Learning’s sonar glossary.

If you need more detailed information you can email  vince.capone@blacklaserlearning.com

Notes:

  1. 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.
  2. 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 systems.
Hull Mounted Side Imaging Sonar Patch

Hull Mounted Side Imaging Sonar Patch

Towed Side Scan Sonar Patch

Towed Side Scan Sonar Patch

 

 

 

 

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