From a single fan to a map of the seabed
A forward-looking multibeam such as the Oculus M750d paints a fan-shaped image of the seabed ahead of the drone, out to 120 metres. That is what most operators use it for: finding a target in murky water, steering towards it, and seeing what it is before the camera can. With that range, a single fan already covers a large patch of seabed.
Sonar Mosaic adds a new use for the same sonar. When you drive a survey, Blueye Cloud puts every ping in its place: it looks up the drone's position and heading at the moment of each ping in the dive log recorded alongside the sonar, draws the fan onto the map at that position, and blends the overlapping fans into one continuous, north-up image of the seabed. Where the navigation data alone is not precise enough, overlapping fans are aligned by image matching, so a boulder seen from two places along the track lands on the map as one boulder rather than two. The result is a map of a larger area than any single fan shows, made with the forward-looking multibeam you already carry rather than a dedicated side-scan sonar.
How it works
Sonar Mosaic is built from the files you already capture on every sonar dive. There is no separate processing software to install and no files to convert.
1. Scan with a forward-facing multibeam
Mount a forward-looking multibeam such as the Oculus M750d on your Blueye drone and record from the sonar view in the Blueye App. Each recording is saved on the drone as an .mbez file next to the dive's .bez log, which holds everything the mosaic needs to place the pings: position, heading, depth, altitude above the seabed and the sonar's tilt angle. How you drive the survey matters a great deal for the result, so we cover that in its own section below.
2. Upload to Blueye Cloud
Let the Blueye App sync the dive in the background, or drag the files onto the Upload page in Blueye Cloud. The multibeam recording is prepared for playback and listed with the rest of the dive's media, as it always has been.
3. Create a mosaic in a project
Mosaics live in Projects, next to the dives, missions, attachments and reports of the inspection they belong to. Open the project's Mosaics tab, click New mosaic, add multibeam recordings from the project's dives, and press Update. Blueye Cloud then:
- Splits each recording into stations: the stretches spent surveying the seabed. The transits between them, and any time spent in open water with the seabed out of range, are left out.
- Places every ping from the drone's position and heading in the dive log, using the sonar's tilt and the drone's altitude to turn the slanted sonar image into a flat view of the seabed.
- Aligns overlapping fans by image matching, taking out the small errors that remain in the navigation data.
- Blends the overlapping looks at each patch of seabed into one image, in a way that keeps targets crisp instead of averaging them away.
While you work you can change the palette (the same palettes as the multibeam player), the contrast and the resolution, and see the result immediately. The processing runs on your own computer's graphics card, right in the browser, so building and adjusting a mosaic needs an up-to-date version of Chrome, Edge or Safari on a reasonably recent laptop or desktop computer. Everyone else on the project, including the people you share it with, simply sees the finished mosaic on the map.
Placing the mosaic on the map. When the dive log carries a georeferenced position, the mosaic lands in the right place on the map by itself. That is the case with a DVL whose track starts from a GPS position, either your control device's or the Blueye GPS at the surface, or with a USBL. Without one, you drag and rotate each station into place over the satellite map and pin it there. Even with a positioning system, you can fine-tune the placement and rotation by hand to line the mosaic up perfectly with the map. Read more about how navigation and position tracking work underwater.
4. Share and export
A saved mosaic appears on every map in the project: the project map, the map of any dive opened from the project, and the project's share links. A customer opening your shared project sees the seabed under the dive tracks. See for yourself in our example inspection project, which has the Dora mosaic on its map. No sign-up required.
When you need the mosaic elsewhere, click Export and choose a destination:
- Download saves the mosaic as a high-resolution image, up to 8192 pixels on the longest side, together with a small file that records exactly where it sits on the map.
- Map layer turns the mosaic into a custom map layer for the project or for your whole organisation, so it sits under the dives on every map your team uses for that site.
Back underwater: the mosaic as a base map in the Blueye App
The real payoff comes when you return to the same site. Exported mosaics can be downloaded into the app from Manage custom maps in the map settings: tap From Blueye Cloud to list the map layers of your organisation and its projects, and download the ones you need. Once downloaded, the mosaic works without an internet connection and is drawn under the drone on the navigation map. You can steer straight to the anchor block, the pipe joint or the wreck you found last time, and plan the camera inspection before you leave the surface.
Getting good results: scan in one direction
A mosaic can only be as good as the survey behind it, and one rule matters more than any other. Sonar return depends on the angle you look from: the same boulder lights up from one direction and nearly disappears from another, and its acoustic shadow always falls away from the sonar. If you survey an area by driving back and forth, the overlapping strips are imaged from opposite directions, the shadows point opposite ways, and the mosaic turns soft and smeared where they meet. Scan in one direction, not back and forth.
A few more habits make a visible difference:
- Keep a steady altitude. A constant height above the seabed keeps the fans consistent from ping to ping. Auto-altitude holds it for you, and needs a DVL or an altimeter on the drone.
- Tilt the sonar down. Around 20° down, at an altitude of about 20 % of the sonar range, images the whole fan usefully, even on soft sediment that returns little at shallow angles.
- Use high frequency when the range allows. On the Oculus M750d, high frequency (1.2 MHz) resolves finer detail than low frequency (750 kHz), and its 40 metre range is a good balance between coverage and detail for mapping.
- Overlap your lanes. The outermost beams are the weakest part of every fan, so let neighbouring lanes overlap a little.
- Turn outside the area. Make turns beyond the edge of the survey area, so the fans that image its edges are recorded on a straight, steady run.
Plan the survey in Blueye Cloud
The Multibeam Survey instruction in the Blueye Cloud mission planner does this geometry for you. Draw the area, pick your sonar model, frequency, range and tilt, and choose how much neighbouring lanes should overlap. The planner works out the altitude, the width of the strip the sonar images on the seabed, the lane spacing, and how far past the edge the drone should travel before it turns, and it draws the imaged swath under every lane so you can check the coverage before you get wet. With an Oculus M750d at 20 metres range, for example, that is an altitude of 4 metres, a swath of about 36 metres and lanes 32 metres apart. Export the mission to the Blueye App and the drone flies the lanes at the planned altitude and spacing.
Today the drone flies a survey as a classic lawnmower pattern, with every other lane run in the opposite direction. That covers the area completely, but for a mosaic it means neighbouring lanes look at their shared edge from opposite sides. When you drive a survey by hand you can already avoid this: stop the sonar recording at the end of each lane, drive back to the start of the next one, and record again on the same heading as before.
Next: surveys flown one way
Making that automatic is our next step. We are improving the mission executor on the drone so a multibeam survey can be flown one way, either by reversing back after each pass so the drone and sonar keep facing the same direction, or by stopping the recording at the end of a lane, moving back, and recording again on the next forward pass. The goal is a workflow where you plan the survey in Blueye Cloud, run it as a mission, and get recordings that stitch into a crisp mosaic without any extra effort at the controls.
Availability
Sonar Mosaic is part of Projects in the Blueye Cloud Pro plan, alongside AI inspection reports, live streaming and custom map layers. Because it works on the files you already upload, multibeam recordings already in Blueye Cloud can become mosaics too: add the dives to a project, open the Mosaics tab and press Update. Building and adjusting a mosaic needs an up-to-date version of Chrome, Edge or Safari on a reasonably recent laptop or desktop computer; the finished mosaic can be viewed in any modern browser, and in the Blueye App once it is exported as a map layer.
Sign up at cloud.blueye.no to try Sonar Mosaic on your own recordings, or explore the Dora mosaic in our example inspection project first.
