Side Scan Sonar for Search and Recovery

By Gus Marchetti, Salvage master. Reviewed by Dana Whitcombe, technical reviewer.

Marine Salvage & Recovery | Underwater Inspection I remember a 200-foot tug that went down off Cape Hatteras in 2011. We searched for three days with a towed pinger. Nothing. Then we switched to an EdgeTech 4200 side scan. Two hours later, we had a target, sitting upright in 60 meters of water. Side scan sonar is the difference between poking around blind and actually seeing the bottom. It's an acoustic picture that lets us find a 20-foot hull or a single container from a hundred meters away. I've run this gear for 22 years, and it's still the first tool I rig when something goes in the water. If you're looking for something small — a body or a piece of evidence — side scan is often the only way. But it's not magic. You have to choose the right system, plan the lines, and read the returns like a pro. Here's how I do it.

Choosing the Right System

Frequency is the first decision. Low frequency, say 100 to 300 kHz, covers ground fast but gives you a fuzzy picture. High frequency, 600 kHz and up, gives sharp detail but a narrow swath. I keep two fish on my boat: a 300 kHz for searching wide and a 1200 kHz for close re-acquisition. If I'm looking for a whole wreck, I run the 300. For a body or a small piece of debris, I switch to the 1200 and fly lower. The towfish itself matters. Old Klein and EdgeTech models are tanks. Newer digital ones like the Marine Sonic Mk3 give better imagery. The tradeoff is cost. A used analog fish runs a few thousand dollars; a new digital one runs north of fifty grand. For a one-off search, rent—call it $500 to $1,500 a day. Don't overthink it. The best side scan for search and recovery is the one you can operate at 3 a.m. On a pitching deck.

Planning the Grid

Seeing the bottom is the easy part. The hard part is covering every square meter, because if you miss a line you miss the target. I use Hypack or SonarWiz to lay out parallel lines. Set the spacing to about 80% of your swath width. That gives you 20% overlap. Your altitude matters as much as your lines. The rule of thumb is to fly the fish at 10 to 15 percent of the range. So if you're scanning 100 meters per side, keep the fish 10 to 15 meters above the seafloor. Run the boat at 3 to 5 knots. Target size sets the range: a tire needs 50 meters, a shipping container 150, a tug 200. It all depends.

Reading the Returns

Side scan returns are acoustic shadows, not photos. A hard target gives a bright return and a long shadow. Soft bottom gives a smooth, dark tone. The shadow's shape and length tell you how tall the target is and which way it faces. When I see a strong return, I don't get excited. I mark it and move on. A lot of returns are tires or boulders. Does the shadow show corners? That's a container. I watch the same return on two lines. If it shows up on both, it's real. If only on one, it's noise.

Running the Search Well

Rookie mistakes: running too fast. 3 knots feels slow, but your image depends on it. And people don't adjust for current. Your towfish moves with the water. I set my lines into the current so the fish stays on track. Watch the altimeter. A fish too low will hit bottom or tangle. You'll feel the cable slack, and then you're spending an hour untangling a $30,000 towfish from wreckage. I've done it. Side scan is part of a bigger kit. It won't show buried cable — use a magnetometer. Once you've got a target, send an ROV or a diver. Side scan gets you on the wreck; it doesn't replace eyes.

Frequently Asked Questions

How deep can side scan sonar work? Depends on the towfish. Low-frequency systems can see to 2,000 meters. High-frequency fish are usually limited to 100–200 meters because you need to fly low. Most search and recovery happens in under 100 meters. What's the smallest object you can find? A basketball-size object at short range. With a 1200 kHz system flying 5 meters off the bottom, you can pick up a beer can in good conditions. But you have to be close. Do I need special training? You can learn the software in a day, but interpreting returns takes weeks. Work with an experienced operator for your first search. A bad survey is worse than no survey. Can a diver find things faster? For large targets, no. A diver might cover 50 square meters per hour; side scan can cover thousands. But divers are essential for confirmation and recovery.