Sea Technology

JUN 2013

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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soundings in shallow water (from shore to depths of more than 25 meters). With regard to deeper waters, multibeam echosounder (MBES) technology is proven in its capability. Recently, systems have been applied in waters as shallow as 10 to 20 meters. However, in shallow waters, the limitation of narrow swath coverage (60 to 70 degrees, for accurate soundings) precludes its cost effciency for surveying. Meanwhile, the swath coverage of MBES systems typically spans three-to-fve-times altitude, requiring line spacing of 15 to 50 meters, or in the case of some MBES systems that generate swath spans to 120 degrees, the surveyor is left with sparse beam spacing, as they are still constrained by a certain fxed number of beams. In shallow waters, PDBS has demonstrated three to fve times the effciency of MBES. PDBS supports a sounding coverage that spans 100 to 240 meters in minimal time on the water, while exceeding the same minimum survey requirements set by the International Hydrographic Organization (IHO) for special order bathymetry total propagated vertical and horizontal uncertainties. Time can be saved off the water as well, given the HC3500's SNR, quality and uncertainty flters, which can be set during realtime data acquisition or post-survey processing. for data logging, as well as the need to register data output with various third-party survey software standards. Once these improvements were made, data quality could be analyzed, mostly by comparison with nearshore MBES or intertidal lidar. At this stage, it became clear that PDBS technology not only warranted further consideration, but supported capability not previously available for surveying large areas with great detail, either in shallow waters or in low-altitude vehicles for high-resolution mapping in greater water depths (such as with AUV, ROV and deep-tow survey systems). Development During early stages of testing and evaluating PDBS in the 1990s, as with most advanced technology, various complicating factors were identifed, many of which involved hardware and software improvements similar to those required for MBES to be useful. These changes included electronic noise reduction, improvement of sonar and motion data timing relative to GPS (through 1 pulse per second or network time protocol), precise logging of source timestamps, development of graphical user interfaces for ease of operation, averaging, fltering and time-varying gain (TVG) modifcation, and formats (Top) Illustration depicting coverage achievable with a multibeam echosounder (MBES), with its swath coverage in yellow, versus that of a PDBS (HydroChart 3500), which ensonifes the seafoor, extending below the second vessel to the pier. (Middle) L-3 Klein SonarPro software showing a real-time survey environment. Note the full swath coverage with both side scan and bathymetry spanning 150 meters to each bank over a maximum depth of 18 meters. (Bottom) A SonarPro real-time survey environment. Real-time across-track display and control settings are shown at the top, with real-time adjustable display attributes and depth control, followed by the data quality, signal to noise ratio and uncertainty flter settings (from left to right). In the lower left is the real-time survey display showing signal quality and two perspectives for bathymetry waterfall, side scan waterfall, coverage map and a target image. In the lower right is a set of Klein PDBS survey data processed and displayed in HYPACK Inc.'s (Middletown, Connecticut) Hypack 2013. 36 st / June 2013 www.sea-technology.com

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