Sonar Inspection of Flooded Shafts and Tunnels
By Ray Whitfield, ADCI-certified commercial dive supervisor. Reviewed by Amanda McCallister, editor.
Underwater Inspection | Commercial Diving Contractors Alabama That morning the water in the raw water intake shaft had gone from tea-colored to black overnight. We lost visual on the camera in four feet. I rigged the scanning sonar on the lift chain and sent it down blind. Twenty minutes later we had a full picture of the trash rack and the two-foot gap where the old weld had let go. If you're responsible for a flooded shaft or tunnel, you probably know the feeling. You need to know what's down there without emptying it or sending a diver into zero-vis. Sonar is the answer. It's not the only answer, but it's often the best first move. This is how I run a sonar inspection—the way I've done it on intake tunnels, pump pits, and cooling water pipes for the past 28 years. I'm not a salesman for any of these boxes, but I've pushed them to their limits. Here's the practical side: what gear to pick, how to string a survey, and what to ignore when the screen lights up with noise.
Why Skip the Diver and Use Sonar?
A lot of plants have a rule: no diver in a flooded shaft without a standby diver, a stage, and a decompression chamber within reach. And that's just for a clear, shallow shaft. Add silt and current and you might as well call it a pro dive job with a full crew. Sonar gets you a picture from topside. You don't need a chamber or a dry suit. You keep people out of harm's way and you get data you can argue about in an office, not just in your memory. Sonar also sees what a camera can't. In a flooded tunnel, visibility is often less than an inch. But sound passes through that water like it's air. A 675 kHz scanning sonar will pick up a 2-inch pipe hanger at 40 feet. A multibeam system can paint a whole wall in minutes. That's why I use it for the initial pass—to find out where to send a diver if we need one. I've been in that water. I'd rather not go in if I don't have to. Sonar lets me make that choice.
Choosing the Right Sonar for Your Shaft
You've got two main types: mechanical scanning sonar and multibeam imaging sonar. The classic is the Kongsberg Mesotech MS1000. It spins a single transducer and builds a 2D slice. Slow, but rock solid—I've used one for years. For real-time video-like images, you want a multibeam like the Teledyne BlueView M900 or a Tritech Gemini. Those give you a picture you can watch live, great for guiding an ROV. You trade range for resolution. A 1.2 MHz head gives you crisp imagery of small objects but you lose range. A 675 kHz head reaches farther but you'll miss the fine detail. In a 30-foot-diameter shaft, high frequency is fine. In a 1,000-foot tunnel, you need more range, so you drop the frequency. I usually mount the sonar on a retractable pole or a small ROV. In a narrow shaft, lower the head on the crane. In a tunnel, put it on a skid or a wheeled ROV. Stabilize it—motion ruins the scan.
Running the Survey: A Path to Clean Data
Start with the drawings. A shaft isn't just a cylinder; it has gratings, pipes, ladders, valves. You need to know what's there so you don't interpret a ladder as a crack. Mark your reference points on the rim, then tie your sonar head to that grid. I always lay out a simple coordinate system on the top flange—north, south, east, west—and call out those coordinates as I scan. That way I can say, "check the 2 o'clock position at a depth of 37 feet," not just "over there." Lower slowly, stop, scan. Overlap each scan by at least 25 percent. If you're using an ROV, fly a grid pattern—east-west then north-south. Log everything to a hard drive; don't trust a single screen grab. And watch for air bubbles—they show up as ghosts and can cover up real features. Once I spent an hour trying to identify a "crack" in a pump chamber wall on the screen. Turned out it was a shadow from a zip tie hanging off a cable. The lesson: take a good sharpie to the structure before you flood it, or just keep a note of where you left anything.
Making Sense of the Image
Learn to read shadows. A convex shape like a rock on the bottom throws a dark shadow. A recess like a missing bolt shows as a bright echo with no shadow. You'll see the weld lines, the rivets, the concrete texture. Anything that looks different from the surrounding pattern deserves a second look. Most sonar software has built-in measuring tools. I've measured a 12-inch void behind a liner with an MS1000. But I'd give or take an inch. The acoustic pulse spreads and reflects off multiple points, so the edges may be fuzzy. Always report your measurement uncertainty—I usually say "±2 inches" for typical scans. If you see serious damage, you'll need a diver or a camera to confirm. Sonar isn't a substitute; it's a filter. It tells you what to inspect close-up.
Frequently Asked Questions
Can sonar see cracks in concrete underwater? It can show features that look like cracks, but it's not a photo. A crack wider than about 1/8 of an inch at close range might show up as a dark line. But the echo from a rough surface can fool you. I always follow up with a camera or a diver if the crack matters. How long does a typical sonar survey take? Depends on the plant and the setup. A single shaft of 20 feet can be scanned in half an hour, including rigging. A 500-foot tunnel with an ROV will take a full day. The interpretation takes longer than the scan. Do you need a permit or confined space entry for sonar inspection? You still need to follow plant confined space and work-at-height rules for the surface area. The sonar head itself doesn't need a permit. But if the shaft is a confined space, you need the paperwork for anyone around the opening. It's a safety issue. What's the difference between an imaging sonar and a profiling sonar? An imaging sonar gives you a picture, like a TV image. A profiling sonar gives you a single distance reading—like a tape measure—and you move it around to build a map. Both are useful. I use both. For inspecting a wall, I want imaging. For measuring a conduit, profiling gives me exact dimensions.