Advancements in Underwater Infrastructure Inspection with High Resolution Sonar and Software

Abstract: Waterfront Facilities Inspection is a key function of effective port asset management. Performing these inspections with engineer dive teams below the waterline, particularly in low- or no-visibility, is difficult, often relying on tactile inspection only. Several trends are making delivery of these services increasingly challenging for engineering firms, including increased resource demands and shortage of professional expertise. The loss of inspection expertise through retirements and lack of experience for new entrants threatens the quality of inspection reports below the waterline. Several developments in acoustic technology and image processing have promise to close this capability gap, including high-resolution multibeam imaging sonars that enable long linear lengths of port infrastructure to be scanned rapidly, and software that can quickly generate large-scale mosaics of port infrastructure below the waterline, at a resolution shown to clearly identify damaged and degraded infrastructure. This technology provides value to engineer dive teams by allowing pre-dive assessment that facilitates safe dive planning and improved operational efficiency. There is potential to integrate high-resolution sonar imagery with machine learning to assess condition of elements such as timber, steel, reinforced concrete, and prestressed concrete elements. Machine learning is already being used in high resolution drone photogrammetry for crack identification in pavements and concrete, and corrosion of steel structures (Lovelace, Ports 22). Fusing high-resolution photogrammetry above the waterline with sonar point cloud data below the waterline is also moving forward (Loftus, Ports 22). While point cloud data below the waterline cannot assist divers in identifying fine features in elements such as cracks and other damage, high resolution imaging sonar can. Work is already being done using machine learning to identify targets of interest for defense applications below the waterline detected with high resolution imaging sonar. These co-developments indicate that the classification and quantification of element degradation using sonar mosaics, optimized through machine learning, has potential to revolutionize below waterline inspection, while assisting engineering firms to close the supply/demand gap for their services.

How does this project / topic contribute to the advancement of the industry and profession?: Underwater inspection in low- or no-visibility conditions is difficult. The proposed topic describes Innovative Technologies available today to improve Structure Inspection and Rehabilitation of Facilities and Operations. The principal author provided input to the MOP 130 Working Group White Paper on advanced technologies. This paper proposes to do a deep dive into the technology and its contributions to improved workflows.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: Yes. Large scale mosaics of long linear lengths of port infrastructure can now be produced using high-resolution imaging multibeam sonar records that are post-processed with newly available software. The post-processing software does not require motion compensation or real-time kinematic positioning, which lowers the cost of generating these images while delivering value to the dive team’s workflow on all dimensions: cost, time, deliverable quality, and safety.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: There was much experimentation to identify the optimal mounting arrangement of the sonar to capture the best image data: grazing angle of the sonar beams with infrastructure, distance from sonar to the surface, vessel speed, and gain settings for the multibeam. Then several aspects of software settings during post-processing affect the quality of the mosaic images. Other challenges discussed are deployment under decking and around piles.

Who is the target audience for this paper?: This presentation will be of interest to Engineer Divers, and Port Engineers responsible for waterfront facility asset management.