Bathymetric survey drones can be an easy solution in times when traditional bathymetric survey techniques are unworkable on bodies of water.
In some cases, dense seaweed can prevent the lowering of echo sounders in water, or complicated water access can rule out unmanned surface vehicles (USVs) entirely. In other cases, you may need to conduct multiple surveys of small ponds, lakes or trenches.
When an USV isn’t an option, bathymetric survey drones have the advantage of flight, providing surveyors with more data and a new view on how to map the land below the waterline.
Understanding Bathymetric Survey Techniques
Not to be confused with hydrography, "bathymetry" is a term originally used to refer to the ocean's depth relative to sea level, as explained by the National Ocean Service. However, over the years, bathymetry is widely used to refer to any type of underwater topographic survey.
In addition to surveying the ocean floor, bathymetry survey projects can include rivers, streams and lakes. There are multiple ways to conduct bathymetric surveys, including multi-beam and single-beam surveys, ADCPs, sub-bottom profilers, and Ecomapper Autonomous Underwater Vehicle.
With the expansion of what defines a bathymetric survey project, more technology has been developed to provide solutions to give surveyors access to water. “It is better to use a drone with an echo sounder for mapping, measuring and inspecting tasks and environmental monitoring if you are conducting bathymetric surveys of tailings dams and ponds,” says Alexey Dobrovolskiy, CTO at SPH Engineering, which provides software and integration services for unmanned systems. “Drones are also used in bathymetric surveys of ponds in open pits, mapping of sediments of oxidation and tailing ponds, and mapping water depths and sediments before dredging and cleaning of ponds, lakes, rivers, and canals.”
Additionally, he adds, “Drones can be used for river and lake bottom profiling for scientific investigations and environmental monitoring, underwater inspections for engineering works such as bridge or pipeline crossings, and for sludge volume measurements at waste stabilization ponds.”
How Bathymetric Survey Drones Compare to Traditional Techniques
Using a drone with an echo sounder in a bathymetric operation is not a complex task — maybe even simpler than most common photogrammetry missions. Included below is a sequence of steps you should follow to successfully conduct a bathymetric survey.
Before conducting a bathymetric drone survey in the field:
- Discuss with your client what types of data files they need to get with survey results (e.g., depth measurements in a CSV/NMEA-0183 data format or full echo sounder data in SEG-Y data format.
- Discuss with your client the preferred water survey method. This could be a continuous water profiling method (see figure 1) or measurements in points (see figure 2) if there is seaweed or something in the water. If measurements are made in points, the drone will fly to an intermediate waypoint, descend to a predefined altitude to put the echo sounder in the water, hover a couple of seconds, ascend and move to the next point. The entire process is fully automatic.
- Ask your client or define a maximum depth in the survey area to set a maximum depth range in the system settings.
- In the case of very small/narrow objects for survey (e.g., profiling of trenches), decide whether to use automatic missions or whether you will do the profiling in manual flights.
- Prepare a preliminary mission plan and routes using software. It is possible to do this in the field if there is access to mobile internet, but better to plan all the flights in a more comfortable place.
- Double-check and prepare all equipment.
Conducting a bathymetric drone survey in the field:
- Prepare all the equipment and drone for the flight.
- Attach a broadband SkyHub and an altimeter to the drone if they were not mounted previously.
- Connect an echo sounder to the drone.
- Check all equipment again, especially the level of batteries.
- Check the real situation and conditions of the survey, modifying mission plans if necessary.
- Check fail-safe settings. It is very important to disable automatic landing in case of low battery.
- Upload the route to the drone and fly the drone in automatic mode. Alternately, you can fly the drone manually if conditions are preventing an automatic mission.
- Repeat flights if necessary.
- After landing, download data from the drone to a laptop computer.
- If you have appropriate software on your laptop, quickly assess data.
In the office after a bathymetric survey drone flight:
- Process the acquired data using appropriate software (e.g., Hydromagic, Surfer, Reefmaster) and prepare a report for the client.
“For these operations, we recommend using standard commercially available drones like DJI M600 Pro, M210 or the new M300,” says Dobrovolskiy. “You can also use drones based on the open source Pixhawk autopilot. If you choose to use standard popular drones, the echo sounder will be just one more sensor for your multi-purpose bird. You can use it for photogrammetry, LIDAR, magnetometer and other missions, and configure for the bathymetric survey in 15 minutes if it is necessary.”
*Special acknowledgement goes to Alexey Dobrovolskiy, CTO at SPH Engineering, for his invaluable contributions to this article.