Excessive rainfall caused by Storms Ciara and Dennis earlier this year led to the wettest February since records began, leaving significant damage throughout many areas of the U.K. We recently worked with a national rail organization visiting flood affected areas of the country, inspecting bridges using high-resolution 3D sonars to locate areas of scour and prevent closures, to help keep Britain moving.
Efforts to extend the capability of underwater inspection, focusing on submerged infrastructure are gaining wider attention and uptake within the Asset Management sector. Traditional techniques of acquiring data such as ‘LiDAR’ are still in their infancy for use underwater.
It has been proven that static Mechanical Scanning Sonar (MSS) scans have accuracy approaching that of Terrestrial Laser Scanner (TLS). Traditionally, high accuracy LiDAR surveys are acquired by mounting the Laser scanner on a tripod in fixed locations. The scanning locations are positioned using land survey techniques, and the scans are registered to real world co-ordinates using LiDAR processing techniques. The application of this methodology underwater is widely accepted, where the sonar is mounted on a tripod or suspended from a crane. The model is then built from a series of scans from successive locations.
Subsea bathymetric surveys offshore employ acoustic sensors, Multibeam Echo Sounders (MBES), to obtain depth/range measurements, which in turn form terrain models. In the context of a typical bathymetric survey, these sensors are deployed on a survey vessel and acquired dynamically at a speed of around 2-5 knots. Georeferencing is governed by Inertial Navigation Systems (INS), so every point has a real-world position at the time of acquisition. GNSS post-processing techniques are then often deployed to correct for environmental or technical factors such as multipath or signal drop out, resulting in accuracy levels approaching that of traditional static deployment (2-5cm). This acquisition methodology has been transferred to MSS operation, as the MSS is a high resolution MBES. A Marine Laser Scanner is mounted on the vessel, which simultaneously acquires the terrestrial component of the model, negating the use of a separate land or UAV survey. Additionally, a second sonar, a standard MBES is used to image the subsea terrain.
Dynamic acquisition allows the inspection of environments in a matter of hours, whereas a similar static acquisition campaign would be prohibitive in either time or cost. Similarly, the use of physical diver inspections can be reduced by use of such surveys, their deployment can be informed by the identification of damaged infrastructure from the sonar dataset. Sediment mobility and seabed monitoring can also be readily performed, as successive models can be directly compared to each other.
Rapidly advancing sonar technology has resulted in accuracy levels that approach terrestrial LiDAR, and the deployment of MSS on a dynamic platform allows for the dramatic upscaling of high-resolution inspection surveys, when compared to static acquisition. We hope to continue to explore and develop the technology to inspect other submerged assets, improving survey efficiency and accuracy for our clients.