Underwater Inspection of Bridge Substructures Using Imaging Technology June 14, 2018 Sponsored by Federal Highway Administration Office of Infrastructure FHWA-HIF-18-049 FOREWORD With the growth in the range and capabilities of sonar technology, many states are investigating its use for underwater bridge substructure inspection. FHWA provides overall guidance for these inspections and recognized the need for a comprehensive evaluation of the capabilities of sonar technologies for this important activity. In this study, a range of sonar technologies were evaluated in a variety of underwater conditions to evaluate how these technologies can be effectively used in conjunction with or independently of divers. It will be useful for bridge personnel responsible for inspecting bridge foundations. The study described in this report was conducted at the Federal Highway Administration (FHWA) Turner-Fairbank Highway Research Center (TFHRC) J. Sterling Jones Hydraulics Laboratory. The state transportation departments that contributed funding for this study were California, Missouri, North Dakota, South Carolina, Texas, and Wisconsin. Cheryl Allen Richter, P.E., Ph.D. Director, Office of Infrastructure Research and Development Notice This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. This report does not constitute a standard, specification, or regulation. The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ names appear in this report only because they are considered essential to the objective of the document. They are included for informational purposes only and are not intended to reflect a preference, approval, or endorsement of any one product or entity. Quality Assurance Statement The Federal Highway Administration (FHWA) provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement. TECHNICAL DOCUMENTATION PAGE 1. Report No. 2. Government Accession No. 3. Recipient’s Catalog No. FHWA-HIF-18-049 4. Title and Subtitle 5. Report Date Underwater Inspection of Bridge Substructures Using Imaging Technology June 14, 2018 6. Performing Organization Code 7. Author(s) 8. Performing Organization Report No. Jerry Shen, Roy Forsyth, and Roger Kilgore 9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) GENEX SYSTEMS, LLC 2 Eaton Street, Suite 603 11. Contract or Grant No. Hampton, VA 23669 DTFH61-11-D00010-T-5009 12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered Office of Infrastructure Research and Development Pooled Fund Study Report Federal Highway Administration October 2011 – July 2016 6300 Georgetown Pike 14. Sponsoring Agency Code McLean, VA 22101-2296 15. Supplementary Notes The Contracting Officer’s Technical Representative (COR) was Kornel Kerenyi (HRDI-50). Thomas Everett, Ian Friedland, Sheila Duwadi, and Gary Moss reviewed interim project documents. Dr. Bret Webb (University of South Alabama) provided technical review of the document. 16. Abstract As sonar technologies improve, interest in applying these tools for underwater bridge inspection is increasing. While the capabilities, standards, and processes are well-established for bathymetric surveying, sonar use for identifying important characteristics relevant for underwater bridge inspection is evolving. FHWA, working with several states, developed and implemented a series of field tests to evaluate the capabilities of sonar in underwater bridge inspection. This report provides an overview of sonar basics, the types of 2D and 3D sonar technologies available, and factors affecting the reliability of sonar data. The report also describes the field testing and results. The field testing evaluation focused on: 1) identification of relevant underwater features, 2) performance in adverse environments, 3) data collection/reporting time, 4) equipment costs, and 5) personnel requirements. Findings demonstrated that sonar technology is capable of identifying larger scale characteristics of interest such as scour holes, debris, and moderate to large size voids and protrusions. It is more limited in identifying small scale cracks or features hidden by marine growth. The results showed that sonar is particularly effective in adverse conditions where limitations on diver bottom time exist, such as deep water, and conditions where diver safety or mobility is of particular concern including swift currents or turbid water. In all environments, including those with adverse conditions, sonar technology is useful for identifying macro features quickly. These technologies can be used to inspect underwater structural elements where divers cannot work effectively, to guide divers for a closer look, or to provide independent inspection insights. Sonar can also be effective in covering large areas quickly. The study led to two broad conclusions. First, sonar inspections have not demonstrated the ability to identify some smaller scale elements of substructure condition that may be important in assessing the bridge and recommending maintenance. Second, sonar technologies offer significant opportunities for improving underwater bridge inspections, especially in adverse environments or to inspect extensive areas. The study provided recommendations that support the expansion of sonar technologies for bridge inspections: 1) develop guidance or benchmarking information for the use of sonar technologies in underwater bridge inspection, 2) develop a program of training for bridge inspectors in the use of acoustic technologies, and 3) encourage the use of sonar technologies to improve underwater bridge inspection. 17. Key Words 18. Distribution Statement acoustic imaging, bridge inspection, condition No restrictions. assessment, defect documentation, deterioration evaluation, diving, scour, sonar, underwater imaging, underwater inspection 19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price Unclassified Unclassified 153 Form DOT F 1700.7 (8-72) Reproduction of completed page authorized TABLE OF CONTENTS Chapter 1. Background and Objectives ...........................................................................................1 Guidance Development for Underwater Bridge Inspection .......................................................2 Diver Qualifications and Certification .......................................................................................3 Underwater Imaging ..................................................................................................................4 Chapter 2. Sonar Basics ...................................................................................................................7 How Sonar Works ......................................................................................................................7 Sound Speed and Angle .............................................................................................................9 Return Echoes ..........................................................................................................................10 Georeferencing .........................................................................................................................13 Chapter 3. Sonar Technologies ......................................................................................................15 Three-Dimensional Sonar Systems ..........................................................................................15 Fathometers/Echosounders (Single-Beam) ........................................................................15 Geophysical Sub-Bottom Profilers ....................................................................................17 Multibeam Swath and Mechanical Scanning Sonar ..........................................................18 Real-Time Multibeam Sonar..............................................................................................19 Two-Dimensional Imaging Sonar ............................................................................................20 Side-Scan Sonar .................................................................................................................20 Sector-Scanning Sonar .......................................................................................................24 Lens-Based Multibeam Sonar ............................................................................................25 Chapter 4. Sonar Reliability ...........................................................................................................27 Variations in the Speed of Sound.............................................................................................27 Transducer Head Movement ....................................................................................................27 Interference and Noise .............................................................................................................28 Acoustic Multipath...................................................................................................................29