Sonar or camera? Specifying for water you cannot see through

The most common disappointment in this category is a vehicle that performed beautifully in the demonstration and is useless in the customer's own basin. Almost always the cause is the same: it was specified on camera quality for water where cameras do not work.

The threshold that matters

There is no exact number, but as a working rule: below roughly a metre of visibility, camera specification stops being the deciding factor. You can add resolution, dynamic range and lighting indefinitely and none of it resolves suspended sediment. Light scatters off the particles; more light scatters more.

At that point the honest choice is acoustic. Sound propagates through water that light cannot, which is why sonar exists and why it is not an optional accessory in silted basins, rivers, outfalls or working harbours.

What sonar actually gives you

Not a picture in the photographic sense, and it is worth managing expectations internally about that. What it gives you is structure, geometry and range - where things are, how big, how far, and whether there is something in front of the vehicle.

• Navigation. Not driving into things you cannot see is the first and most underrated benefit.
• Search. Covering an area systematically and detecting objects against a seabed - the primary tool for search and recovery.
• Large structure survey. Understanding the shape of a quay wall or a foundation at a scale a camera can only sample.
• Getting the camera close enough to be useful. Frequently the real workflow: sonar to find and approach, camera for the last metre.

The combination is usually the answer

Framing it as sonar or camera is generally the wrong frame. In poor visibility the productive pattern is acoustic for navigation and detection, optical for close inspection once you are on the target.

That has a practical consequence for platform choice: the vehicle needs to be able to carry a sonar, and to power and integrate it. Some platforms have a payload rail and a documented interface; others are closed and adding capability later means a different vehicle. This is worth settling before purchase, not after.

Video · Cerulean SonarOmniscan 450 FS Imaging Sonar for the BlueROV2

Types, briefly

• Imaging sonar - a real-time forward-looking picture. The general-purpose choice for navigation and inspection in poor visibility.
• Scanning sonar - builds an image by sweeping. Higher detail, slower, best from a stationary vehicle.
• Echosounders and altimeters - a single accurate distance. Deceptively useful for holding height above a seabed or standing off a wall.
• Acoustic positioning - where the vehicle is, which is a different problem from what it can see, and the one that makes surveys repeatable.

The other lever: lighting and camera technique

Before concluding that a site needs sonar, it is worth checking that the optical setup is not simply wrong. Lights mounted close to and parallel with the camera axis illuminate the particles between camera and subject and produce exactly the washed-out result people blame on the water.

Separating the lights from the camera axis, working closer, and reducing output rather than increasing it all help. In genuinely marginal visibility - one to two metres - that alone sometimes recovers a usable image. Below that, no technique saves it.

Where you put the lights matters

Where this bites hardest

Ports and inland infrastructure. Quay walls, lock gates, intakes, outfalls and river crossings are frequently in the worst water anyone works in, and they are also the structures where inspection frequency matters most. If you are specifying for that environment and sonar is not in the conversation, the specification is probably going to disappoint.