How to Read a Sonar Image If You Are Not a Surveyor
By Cheryl Ansel, DOT bridge program engineer, NHI-qualified team leader. Reviewed by Amanda McCallister, editor.
Underwater Inspection | Cathodic Protection & Anodes October 6, 2014. The Saluda River bridge in Piedmont, South Carolina. I was standing in the boat with a young engineer named Marcus. He was proud of the side-scan sonar contact he had just marked. It was a dark blob at the base of pier 7, with a long black streak behind it. He said it was a crack. It wasn't. It was the acoustic shadow of a timber debris pile. The shadow made the pile look seven feet tall instead of two. That single mistake taught me what non-surveyors almost always get wrong when they look at sonar. You don't need to be a surveyor to read a side-scan sonar image. I'm a bridge program engineer, not a hydrographer. But I've signed off on roughly 400 underwater inspections across 15 states. I had to learn how sonar tells the truth and how it lies. It's a matter of understanding three things: how the image is made, how to read shadows, and how to ignore artifacts. Here is the mental toolkit I use every time I look at a sonar contact. If you're an engineer or a maintenance person who gets handed a pile of sonar screenshots, this is for you.
Sonar is a flashlight, not a camera
Side-scan sonar sends pulses of sound from a towfish or hull-mounted transducer. It records the strength of returning echoes and their travel time. As the boat moves, it builds the image line by line. Bright pixels mean a strong return. Dark pixels mean a weak return or no return. So far, so simple. The catch is that brightness depends on angle. A smooth, flat bottom can act like a mirror. If the sound glances off at the wrong angle, the bottom may appear dark even though it's hard concrete. A soft, fine mud facing the beam can return a stronger echo than you'd expect. So never assume dark means 'soft' or light means 'hard.' You are looking at reflectivity, not color.
Shadows are your ruler
The most powerful tool in side-scan interpretation is the acoustic shadow. When a sound wave hits an object, the area behind it gets no sound. That absence traces a dark line on the image. The shadow length tells you the object's height. If you know the height of the sonar fish above the bottom and its slant range to the object, you can set up a simple triangle: shadow length divided by the distance to the shadow's end equals object height divided by fish height. The software does it for you. But the eyeball skill is what matters. A long, thin shadow means a vertical pile or a member. A short, wide shadow means a low chunk of concrete or a log. And the shadow points away from the transducer. If your shadow points right, the fish is on the left. Always check that before you call in a dive team.
Scale: the image lies without it
The fastest rookie error is measuring an object by pixels. Sonar images do not have a constant scale. Across track, the scale depends on the range setting. Along track, it depends on the boat speed and pulse rate. If the boat speeds up, the image stretches like a rubber band. There is also slant-range distortion. A flat object near the edge of the swath appears farther from the centerline than it actually is, because the sound travels on a slant path. Good software corrects this. But a printed screenshot may not have that correction applied. Before you measure anything, ask two questions: What range was set? What was the boat speed? If you don't know, treat every size estimate as a guess.
Artifacts that look real but aren't
Sonar is full of ghosts. The first time you see a school of fish, you may think you've found a lost bridge fender. Fish appear as bright dots or small clusters. They cast odd shadows because they move. Bubbles from boat wakes or bottom gas show up as bright smears. Side-lobe interference can generate a false return across the whole swath from a very hard object. You also get second-bounce echoes: a strong signal that bounces between the bottom and the water's surface, then returns to the fish. That makes a mirror image at twice the range. Here is the rule I teach every inspector: real objects sit on the bottom and cast a clean, steady shadow. Fish, bubbles, and noise do not. When you see a target, look for its shadow. If you don't see a shadow, you are probably seeing an artifact.
A field checklist before you sign off
When someone hands me a sonar report, I run through five checks. One, is the image georeferenced? Do I have a navigation track and heading? Without location, the interpretation isn't worth the paper it's printed on. Two, is the range scale visible? Look for the distance markers on the swath. Three, did the boat pass any known reference objects? Run over a known pile or rock early in the survey. That calibrates your eye for the rest of the run. Four, compare shadow lengths to the bridge plans. If a pile should be 12 feet long and the shadow suggests 3 feet of protrusion, that's a data point. Record it, but don't automatically write 'crack' or 'breakout.' Five, remember what sonar cannot do. It cannot see cracks, spalls, or concrete discoloration. It can only see gross geometry and texture. If you need surface condition, you still need a diver or camera. I sign off with a statement like: Sonar indicates no gross geometric change at Piers 2-4; diver inspection required for surface condition. That is the truth.
Frequently Asked Questions
Can I measure water depth from a side-scan image? Only roughly. Side-scan is designed for imagery, not accurate bathymetry. The slant range to the bottom can give you a coarse depth estimate, but for bridge scour we typically use a separate multibeam echosounder or a depth sounder. Don't plan a repair based on a side-scan depth computation. Why does a sonar contact show a bright dot and a long shadow, but divers find nothing? It's often a fish swimming above the bottom. A fish near the bottom can cast a shadow on the bottom, but it moves between pings. That makes a bright dot with a shadow that doesn't line up. A suspended log or tire can also fool you. The fix is to rerun the contact. If the object doesn't appear consistently on two different passes, treat it as a transient. Do I need a surveyor's license to interpret sonar? No, but you need fundamental training. The National Highway Institute offers underwater bridge inspection courses that cover sonar interpretation. I also recommend a half day where you put a diver in the water and run sonar over the same feature. Once you see what the sonar looks like when you know exactly what's below, your eye learns fast.