Over my 18 years of waterborne search and rescue experience, different techniques have come and gone and technology has improved drastically.
With all the new and exciting tools available to assist in your efforts that promise faster response, quicker searches, minimal bottom dive times, and most importantly closure for the grieving families, it’s hard to decide what to use. The most important thing is shedding light on the darkness. Anyone who has gone near an inland lake or river can tell you they cannot see more than a few feet even on a good day thanks to floating sediment and plant particles. Lifting this veil of murky darkness and inability to see your hand in front of your face is imperative. There are many products to help you to see what’s down there.
Let’s talk a little about sonar. Just about everyone has heard the term, but most people really don’t know what it is, how it works, or if they need one. The first myth is thinking all sonars are the same and do the same thing. Sonars differ greatly in their abilities and most have very specific uses. Unfortunately you won’t be able to find just one to do everything and any salesman that tells you otherwise is probably there to just make a commission. By the end of this article you should have a clear understanding of what is out there and which tools are best for your application. Most of the time you will need a combination of different types of sonars to conduct a comprehensive search of any waterway. You need to make sure you’re not using a hammer when you need a screwdriver.
What Is Sonar
and How Does it Work?
The term “SONAR” started as an acronym for Sound Navigation And Ranging. It’s a system that uses sound instead of light to see. The process is pretty simple. It starts with the “transducer” which is the sound making part of the sonar. It sends out a sound wave then listens for that sound wave to hit an object and return. By calculating the amount of time it takes for the sound wave to leave the transducer, bounce off an object, and then return you get the distance to the target. The sonar also calculates how “much” of the sound returns and that can tell you the density and reflectivity of the target. The object and the water absorb some of the sound. Both of these calculations combined are what give you the image. The distance calculation lays the image out on the screen and the density/reflectivity calculation gives you the different color tones of the image. The harder or more reflective the target, the brighter the return since less sound is absorbed and more returns to the transducer. You can distinguish different objects such as wood, concrete, rubber, metal, flesh, and bone easily because they all have different density and reflective properties.
Of course everyone wants the best image possible and that is based on resolution. The higher the frequency the better the resolution you will have. However, there are drawbacks. Higher frequencies cannot travel as far in the water as lower frequencies. Thus there is a trade off. So you use lower frequencies to look farther in the water giving you greater range, and higher frequencies at shorter distances to give you greater resolution. Many sonars will have a dual frequency option, one low and one high giving you the ability to search wide areas then zoom in for identification.
Types of Sonar
Now let’s talk about different types of sonars. There are four main types of sonars used in SAR cases, all with different uses. The first is the back bone of underwater searches and will be used most of the time — the Towed Side Scan Sonar. If you can only get one type, these are the most versatile and provide the most information, allowing you to map out and save your targets with unbelievable and near photorealistic images and precise locations. It utilizes a device called a Towfish.
The Towfish is a missile shaped system that has the transducers mounted on the sides and is connected to the vessel with a high strength tow cable that also serves as a communication tether. You lower the Towfish to the correct height above the sea floor based on your range. To get perfect images the Towfish should be 10 percent above the sea floor of the range you want to see. So if you want to see 100 feet of range then your Towfish will be 10 feet off the bottom. The range covers only one side, so if you select 100 feet of range you will visually see 200 feet of total distance called a swath. The total swath is 100 feet of range to the Port and 100 feet of range to the Starboard. The Towfish can be used from very shallow to extremely deep, up to 300 meters or more. The shallow side is were you have to be careful. The Towfish can be used as shallow as two feet, but you have to be careful not to snag it on submerged trees or hidden rocks. I typically try to keep the system above four to five feet of depth. However you can stay in deeper water and scan into shallower water up to the shore line.
The next type of Side Scan Sonar are hull mounted systems. They are considered the little brother to actual side scans. These are typically much cheaper than towed systems and claim to get the same imaging. These systems are great for shallow water where you don’t want to risk getting your Towfish snagged or beat up on rocks. They are good to about 20 feet of depth, anything beyond that will make your target too small to see on the screen. I would use hull mounted systems on water depths from two feet up to about 10 feet. The reason it won’t work in deeper water despite claims by certain manufactures is because of the 10 percent rule. Remember the sonar wants to be 10 percent off the bottom based on your range and the transducer are affixed to your hull. In five feet of water you can have a 50 foot range or 100 feet of total swath, but at 20 feet deep you have to have your range set at 200 feet which is a 400 foot total swath. That sounds good until you realize you’re looking at a 400 foot area on a little screen. Try picking out a six foot victim on a 400 foot swath on a 15 inch computer screen. The target will be the size of a pin head. The other problem with hull mounted systems is the frequencies. They don’t usually come in higher frequencies so you won’t get the amount of detail you would get from a towed system. However if you have shallow areas of water where you don’t want to put in a towed system, hull mounted side scans are a good option.
Lets move on to a different type of sonar called Multi-Beam. Multi-beam sonars are mainly used by divers or attached to ROV’s. The imaging is very similar to what you would expect to see from a video camera or using a flash light. Like all sonar you will be able to see through the muck and sediment, but you won’t get an overall image. It’s more like using a flashlight in a dark room, it will only illuminate the area where it’s pointed. Normally you will use a side scan to locate the target from the boat then send a diver down with a hand held multi-beam sonar to physically locate the target for recovery. This eliminates the need for long bottom times and large grid searches.
Lastly you have Flash Imagers which are a type of Multi-Beam Sonar. These are typically mounted to a stopped vessel on a pole and will give you a real time 360 degree view of what’s under the boat. They are very helpful in guiding divers to the target if you have voice communication. The operator on the vessel can see a 360 degree image of what is below; as the diver descends he will be seen in the viewing image. The operator can then direct the diver to the target.
A comprehensive modern SAR case usually involves two to three kinds of sonar and sometimes all four. The search will begin with a Side Scan Sonar for an area search to locate the target. A vessel with a towed system will conduct a parallel area search of deeper water (five - ten feet and above) while a hull mounted system will search next to shore or in areas less than ten feet deep. A towed system can be used for both, but a hull mounted system can only be used for the shallows. Once an anomaly is located the search vessel will drop a Multi-Beam Flash Imager (commonly called a 360) above the site to monitor the area in real time. Divers will be sent in using Multi-Beam hand held systems so they can see where they are going while being directed by the operator of the 360.
The use of sonars will dramatically reduce the number of man hours used on a search and drastically improve your ability to locate a target. It cuts down on the dangers of diving in unknown waters and waters with known dangers. They will cut your search time from weeks to days and possibly down to a few hours. Most sonars will pay for themselves within the first year or two based on cutting out the overtime expenditures commonly associated with searches. With hundreds of uses outside of victim recovery, you will find that sonars will become an integral tool that you can’t work without. They are used all over the world for things like evidence recovery, homeland security, drug interdiction, catastrophe surveys, damage surveys, or any other reason you may need to see what’s under the water. It’s all about having the right tool in your tool box.