Underwater Measurements via a Compact, Pulsed Time-of-Flight LiDAR Technology
The Airborne Bathymetric Scanner (ABS) is making waves in the world of geospatial data acquisition. This compact sensor, carried by an Unmanned Aerial Vehicle (UAV), uses a Nd:YAG laser as a light source with a 35 kHz repetition rate for topographic and bathymetric measurements.
Laser scanners, tachymeters, and laser trackers have long relied on light properties for terrestrial geospatial data acquisition. The ABS, through clever optical design, significantly reduces solar influence and improves performance in underwater environments.
The extinction coefficient of coastal water is assumed to be 1.0 m, ocean water 0.15 m-1, and clear water 0.05 m. Using a common conversion relation, the visibility range equals 1.7/μ. The solar power received by the detector can be estimated using a relation that depends on the spectral sensitivity range of the receiver, the solar irradiance, the detector area, and the focal length of the receiving lens.
The ABS captures the complete time-resolved signal, allowing it to detect objects that would normally be hidden behind stray light and achieve measurement ranges that significantly exceed the visibility range. The attenuation factor spans four orders of magnitude and goes down to 10, serving as a rough estimate for a detection threshold of the compact airborne bathymetric system.
Performance Comparison
Resolution and Accuracy
Underwater laser scanning, typically using LiDAR systems, delivers extremely high-resolution 3D models of the seafloor, with centimeter-level accuracy in both the horizontal and vertical dimensions. However, laser systems are limited by water clarity; turbidity, suspended particles, and biological growth can significantly reduce effective range and data quality.
Multibeam sonar systems (MBES) offer lateral resolutions of 5–10 cm and depth accuracies around 1 cm under optimal (shallow, clear water) conditions. Side-scan sonar provides high-contrast imagery of surface features and can detect objects as small as 5–10 cm, but does not provide true bathymetric (depth) data unless combined with MBES. Sonar performance degrades less in turbid water compared to lasers, making it more versatile in a wider range of environmental conditions.
Range and Depth
Effective range is typically limited to a few tens of meters in clear water for laser systems, with performance dropping sharply as clarity decreases. Deployment is often restricted to ROVs or static platforms close to the target. Sonar, on the other hand, can operate effectively at depths from a few meters to several hundred meters.
Data Processing and Integration
Laser scanning produces dense point clouds suitable for creating digital twins and detailed structural analyses. Sonar provides geo-referenced relief maps of the seafloor, ideal for bathymetric mapping and large-scale feature detection.
Applicability by Industry
| Application | Laser Scanning Strengths | Sonar Strengths | Key Limitations | |------------------------------|----------------------------------------------------------|---------------------------------------------------------|---------------------------------------------------------| | Offshore Wind Energy | High-resolution inspection of foundations, cables, scour | Large-area bathymetry, turbine placement surveys | Laser limited by water clarity, range | | Oil & Gas Industry | Crack/pipeline inspection, platform integrity | Pipeline route surveys, seabed morphology | Laser unsuitable for turbid/deep environments | | Coastal Mapping | Fine-scale topographic mapping in clear, shallow water | Broad-area, all-weather mapping, change detection | Laser hampered by turbidity, waves | | Port Security | Detailed inspection of underwater structures, targets | Wide-area monitoring, anomaly detection | Laser less effective for rapid, large-scale surveys |
Practical Considerations
- Sonar systems are more robust in variable water conditions. Laser systems excel only in clear, shallow, and calm waters.
- Laser systems are often deployed on ROVs or fixed platforms for close-range inspection. Sonar systems (especially MBES) are typically vessel-mounted, enabling efficient large-area coverage.
- Both technologies require significant expertise, but sonar is more mature and widely adopted. Laser systems are advancing but remain niche due to environmental limitations and higher per-survey costs in challenging conditions.
Conclusion
Laser scanning is unmatched for high-resolution, close-range 3D mapping and inspection in clear, shallow water. Sonar-based systems are the industry standard for bathymetric data acquisition across a broad range of depths and water conditions, offering reliable, large-area coverage essential for offshore energy, coastal mapping, and security applications.
Combined use of both technologies can provide comprehensive datasets—sonar for broad coverage and laser for detailed interrogation of areas of interest. The choice between underwater laser scanning and sonar depends on the specific requirements of resolution, environmental conditions, and survey scale. For most industrial applications, sonar remains the primary tool, while laser scanning fills a critical niche for high-detail, close-range inspection where conditions permit.
The final device weighs 2.5 kg without battery and without GNSS/IMU, allowing it to be easily carried by UAVs with less than 25 kg takeoff mass. Two demonstrators have been designed and built: a compact UAV-borne bathymetry scanner for shallow waters and a laser scanner designed for direct underwater application for high-resolution measurement of technical underwater infrastructure.
- The Airborne Bathymetric Scanner (ABS) is revolutionizing the global data acquisition within geospatial science and environmental-science, offering remarkable performance improvements in underwater environments.
- In the realm of finance, wealth-management firms and personal-finance advisors are recognizing the significance of investing in groundbreaking technology such as the ABS, considering its potential impact on various industries like business, real-estate, and energy.
- The compact design of the ABS, suitable for home-and-garden storage, also caters to manifold lifestyle preferences, inviting discussions about how technology can seamlessly blend with personal pursuits.
- The sports industry may embrace the ABS for monitoring underwater infrastructure at Olympic pools and other athletic facilities, enhancing safety and performance.
- The travel sector can benefit from the ABS data in creating precise maps for maritime navigation, improving boating experiences and promoting coastal tourism.
- The ABS's application in climate-change mitigation could extend from measuring changes in seafloor topography due to melting ice caps, to evaluating coastline erosion's impact on real-estate development, thereby affecting urban planning and personal-finance decisions.
- Manufacturing industries seeking to reduce their environmental footprint may incorporate the ABS technology in environmental-science research, enabling studies on the ecological effects of human activities on water bodies and oceans.
- The energy sector could leverage the ABS for ensuring safe and effective maintenance of offshore renewable energy installations, such as wind turbines and underwater cables.
- As the integration of technology further penetrates our daily lives, the ABS exemplifies a product that fulfills several contemporary requirements spanning multiple industries, underscoring its relevance in the emerging technological landscape.
