Bathymetric LiDAR vs SONAR: Which Technology Should You Choose for Your Underwater Surveys?

In Bathymetry, two technologies are often compared: Bathymetric LiDAR and SONAR

Each method offers unique advantages depending on the type of environment to be surveyed, the water depth, and the specific project requirements.
In this article, we will explore these two technologies, their respective advantages, limitations, and help you determine which is best suited for your underwater surveys.

1. How Bathymetric LiDAR Works

The Bathymetric LiDAR (Light Detection and Ranging) uses light pulses, typically in the green spectrum, to penetrate water and measure the depth of seabeds. This technology has the advantage of being airborne, mounted on an aerial drone or an aircraft, allowing it to fly over specific areas and collect data without the need for direct contact with the water.

Benefits of Bathymetric LiDAR:

• Shallow Water Surveys: Airborne bathymetric LiDAR is particularly effective in shallow waters (up to 15 to 20 metres depending on turbidity, measured in Secchi*, a unit of turbidity). In such conditions, green LiDAR can penetrate the water and take measurements where a SONAR-equipped boat would not be able to reach.
• Inaccessible Areas: LiDAR is ideal for coastal zones, rivers, estuaries, or any other region difficult for a boat to access. Thanks to drones, mapping becomes possible in complex and cluttered environments.
• Rapid Data Collection: Due to its aerial deployment, data collection can be extremely fast, especially for small to medium-sized areas, without the need for ferry costs or launching a vessel (as required with SONAR).

Drawbacks of Bathymetric LiDAR:

• Limited Depth: Even under optimal conditions, bathymetric LiDAR is limited to depths of around 15 to 20 metres. Beyond this, the green light is absorbed by the water.
• Sensitivity to Turbidity: The turbidity of the water directly affects the LiDAR's ability to penetrate the water. In murky waters or those with sediments, the effective depth can be further reduced.

2. How SONAR Works

SONAR (Sound Navigation and Ranging) relies on acoustic waves to measure depth. It is mounted on boats or underwater vehicles and sends sound waves to the water's bottom. These waves are then reflected back by the seabed or objects, allowing for the determination of depth and underwater topography.

 

Benefits of SONAR:

• Deep Water Surveys: SONAR excels in underwater surveys ranging from several tens to a few hundred metres. Unlike LiDAR, sound propagates effectively in water, allowing for accurate measurements even in areas where light cannot penetrate.
• Suitable for Large Areas: SONAR is better suited for large-scale surveys, such as deep-sea or ocean floor mapping.
• Reduced Sensitivity to Turbidity: Unlike LiDAR, SONAR is largely unaffected by water clarity, enabling its use in turbid environments without compromising accuracy. However, it remains susceptible to interference from suspended sediment, which can impact the quality of acoustic returns.

Drawbacks of Bathymetric SONAR:

• Shallow Waters: SONAR faces challenges when conducting surveys in shallow waters or near coastlines, where boats cannot navigate due to limited depth. This is where bathymetric LiDAR becomes particularly advantageous.
• Longer Deployment Time: Deploying a SONAR-equipped boat requires more time for setup and data collection compared to a drone or aircraft fitted with bathymetric LiDAR.
• Higher Cost for Small Areas: In small or shallow areas, the deployment cost of SONAR can be prohibitive, especially when compared to a LiDAR-equipped drone.

3. Technology Comparison: LiDAR vs SONAR

Aspect	Bathymetric LiDAR	SONAR
Max. depth	Up to 15-20 metres, depending on the turbidity.	Up to several hundred metres, depending on the sonar configuration.
Typ. area	Shallow waters, coastal zones, rivers, and areas inaccessible to boats.	Suited for great depths and large-scale marine mapping.
Speed	Rapid data collection thanks to aerial deployment (drone).	Slower data collection, dependent on the boat and underwater movements.
Turbidity	Affected by water clarity. Less effective in turbid waters.	Unaffected by turbidity, can operate in highly turbid waters.
Deployment	Aerial: Drones or planes flying over the area.	Marine: Requires a boat or underwater vehicle.
Costs	Less expensive for small areas in shallow waters.	More expensive, but suited for deep-water or large-scale projects.

4. Which Technology Should You Choose?

The choice between Bathymetric LiDAR and SONAR primarily depends on the water depth, environmental conditions, and the specific project requirements.

• If your project involves surveys in shallow waters, coastal areas, or locations difficult for a boat to access, then bathymetric LiDAR is the ideal technology. It enables you to capture accurate data quickly and at a lower cost.
• If you need to map deeper areas or are working in environments with highly turbid water, SONAR is a better option, as it performs more effectively for these types of surveys.

In many cases, the two technologies can be complementary. LiDAR can be used for surface surveys in shallow waters, coastal areas, and perimeters, while SONAR can take over for deeper surveys, ensuring a comprehensive and accurate coverage. The output files from both systems are then aligned in the same coordinate system.

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Conclusion

Bathymetric LiDAR and SONAR are two powerful technologies, each with their own place in the field of underwater surveys.
LiDAR excels in aerial surveys of shallow waters and inaccessible areas, while SONAR is superior for deep-water surveys. Depending on your specific needs, selecting the right technology will allow you to optimise your underwater surveys and ensure full coverage of your study area.

If you would like to learn more about LiDAR technology or request a quote for your underwater surveys, feel free to contact us.