High-Resolution Acoustic Multibeam Surveys for Bridge Assessment

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MULTIBEAM SURVEY CHALLENGES AROUND BRIDGESEven with a properly integrated multibeam survey vessel, there are some unique challenges presentedwhen trying to complete a high‐resolution multibeam survey around a bridge. Almost all high‐resolutionmultibeam surveys rely on real‐time kinematic (RTK) differential Global Positioning System (RTK DGPS)data for tracking precise vessel position and elevation during the survey. In most cases, the overheadbridge deck creates an intermittent obstruction to at least some part of the GPS constellation thatusually results in a short‐term degradation in the GPS position and elevation quality. The impacts of theshort‐term degradation in GPS quality can vary greatly depending on a number of factors (e.g., numberof available satellites, size and height of bridge, survey line orientation, etc.). Around bridges where theoverhead obstruction might be particularly extensive, there are other positioning techniques (e.g.,acoustic Doppler Velocity Log (DVL) bottom tracking, laser range/azimuth tracking, etc.) that can beused to augment the primary RTK DGPS.Some of the GPS quality impacts can be controlled by planning the survey during a time period whenthere is a favorable GPS constellation and also aligning the survey lines to minimize potentialobstruction impacts from the bridge. Usually a series of lines are run both perpendicular and parallel tothe main bridge alignment. When running perpendicular to the bridge there will generally be highqualityGPS data at the start and end of the line, with a short‐term drop in quality as the boat passesunder the bridge. When running parallel to the bridge, it is important to have a sufficient GPSconstellation visible on that side of the bridge to ensure adequate GPS quality. Many of these GPSquality concerns can be minimized by relying on the more robust GLONASS‐enabled GPS receivers thatare able to utilize the GLONASS constellation, effectively doubling the number of potential satellitesavailable at any time.During post‐processing, it is usually possible to interpolate over a short‐term degradation in GPS quality,particularly if higher‐quality GPS data brackets the brief lower‐quality period. This is often the case insurvey lines that pass directly underneath the bridge. Often, a short‐term loss of the most accurate“Fixed Narrow Lane” GPS solution will only have a noticeable impact on the computed elevation datathat are primarily used to provide the necessary vertical reference (i.e., tide or water‐level data) for thesurvey. In addition, though a 30cm drift may not be readily noticeable in the position data, it will bemore apparent in the elevation data due to the vertical offset in the sounding results. Because theelevation data values tend to change relatively slowly, it is usually a straightforward process to correctfor drift in the elevation data due to short‐term GPS issues. There are also some more advanced GPSprocessing tools that allow for both a forward and backward re‐processing of the raw GPS data that canprovide a more accurate and robust position and elevation solution. These techniques enable theentire acoustic survey to be re‐generated using the improved position and elevation data.The other primary challenge associated with multibeam surveys around bridges revolves around thelevel of data cleaning often required for the data that is acquired around the underwater structuralsupports. A multibeam echosounder typically does “bottom detection” based on either amplitude orphase measurements from each beam. Over the seafloor, the mutlibeam acoustic return signals arerelatively similar and the sonar can be tuned through gain and power settings to optimize theperformance of the bottom detection algorithms. However, when the acoustic signal encounters thehard vertical face of a structural support most multibeam systems have a more difficult time resolvingthe bottom detection process. Though multibeam systems are generally able to properly track thebottom as they approach and run along a structure, they often have difficulties re‐acquiring the truebottom as they maneuver away from the structure.4
The difficulties associated with bottom detection around the vertical bridge structures, usually results ina noisy acoustic dataset around these areas. Though some of these problems can be minimized bycareful attention to sonar tuning during data acquisition, it is inevitable that careful manual cleaning willbe required in the areas around most of the underwater supports. Because these challenging areas willhave been covered by multiple survey lines along multiple orientations, it is important that theindividual lines be edited carefully prior to examination within an area‐based editor. Because theposition tolerances are very tight in the areas around the vertical surfaces, if potential position issuescannot be resolved for a specific line, then that line should be excluded from the subsequent editingprocess. In the end, careful manual cleaning within an area–based editor will be required to createdatasets that accurately reflect the vertical structure.MULTIBEAM SURVEYS FOR UNDERWATER BRIDGE INSPECTIONSThe Federal Highway Administration’s (FHA) bridge inspection requirements specify that all bridges withany underwater supports must be inspected at no greater than a five year interval. Most existing bridgeinspection techniques are focused on structural assessment and rely primarily on diver visual orsometimes sector‐scanning sonar techniques. Many of these same inspection programs have alsoincluded some type of initial single‐beam fathometric survey requirement, though the re‐surveyrequirements seem to vary a great deal by state. Due to certain environmental conditions (e.g., deepwater, strong currents, and murky water), underwater inspections of some bridges can be difficult andsometimes dangerous. In addition, the existing structural assessment methods, like visual inspection orsector‐scanning sonar, are not well‐suited for making an accurate or complete quantitative assessmentof scour around all of the structural supports. Over the years, scour around the structural supports hasproven to be the primary cause for most major bridge failures. Though it will not replace the need forperiodic “hands‐on” structural evaluations through either diver visual methods or scanning sonartechniques, the multibeam survey approach offers a number of unique benefits as a bridge assessmenttool. The following section provides an overview of the methods and results for a recent bridgeinspection multibeam survey project.Representative Multibeam Bridge Inspection ProjectThe Maine Department of Transportation (ME DOT) has one of the more robust underwater bridgeinspection programs in the nation, as part of its effort to ensure the structural integrity of the state’sbridges and comply with the FHA’s bridge inspection requirements. ME DOT currently has a 12‐personprofessional dive team that conducts bridge inspections at intervals of no greater than five‐years foreach of the state’s bridges that rely on underwater structural support. Due to strong currents andrestricted visibility, underwater visual inspections of certain bridges ‐ including three that span theKennebec River in the towns of Bath and Richmond, Maine ‐ can be particularly difficult for divers tomanage. In light of these challenges and the safety risks posed for its divers, the ME DOT recently choseto use multibeam bathymetry to meet their underwater bridge inspection requirements.Substructure, Inc. of Portsmouth, NH was selected by ME DOT to conduct high‐resolution multibeamsurveys of the Bath and Richmond bridges, utilizing the company’s survey vessel Orion. Though thesebridges had been in place for many years, what was deemed to be the initial “baseline” surveys of theBath and Richmond bridges were conducted by Substructure in June 2009 during an unusually high flowperiod on the Kennebec River. In July 2010, Substructure conducted an additional baseline survey of theSheepscot River Railroad Bridge, and also returned to the Richmond Bridge to conduct a follow‐up“current‐conditions” survey (Figure 2). This re‐survey of the Richmond Bridge was completed to obtain5
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High-Resolution Acoustic Multibeam Surveys for Bridge Assessment

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