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Qualified Soundings From Interferometric Bathymetry

By Straud Armstrong • Yuhui Ai

Front of Topside Interface Unit (top) Back of Topside Interface Unit (bottom)
Phase-differencing bathymetric sonar (PDBS) systems are recognized as the most effective instruments for surveying in nearshore waters and riverways. PDBS system testing over the years has demonstrated that it is an effective technology for filling the bathymetry coverage gap between deeper waters (at depths greater than 20 meters) and the shoreline. The side scan sonar-derived bathymetry system has been improving in both functional capability and operational efficiency through the past decade.

These systems produce swaths with thousands of bathymetry sounding points and coregistered side scan acoustic imagery that spans more than 10 times the sonar altitude above the seafloor. For example, the new 455-kilohertz chirp-encoded side scan transmit signal from L-3 Communications Klein Associatesí (Salem, New Hampshire) HydroChart (HC)-3500 is sampled at a rate of 60 kilohertz. This generates up to 8,000 soundings across a swath spanning 50 to 200 meters, in water depths of 5 to 20 meters, respectively.

The latest bathymetry processing routine for Kleinís PDBS systems provides real-time values of signal-to-noise ratio (SNR), angular uncertainty and quality factor with each sounding, without binning or averaging the data. This is done by comparing the range, angle and SNR, along with a few environmental parameters, to an uncertainty model. These values support higher-accuracy surveys, with quantifiable uncertainties, prior to gridding. Filtering of data can be done on a more rational basis than using obscure factors of amplitude, angle or bottom detection to reduce time spent processing and render more useful, cost-effective, high-resolution survey data products.

The HC3500 generates both crisp side scan sonar imagery and qualified bathymetry data at a fixed resolution scale of approximately 2.5 centimeters throughout the sampled range. Data are collected across wide swaths for efficient and highly detailed coverage, even in shallow water (1 to 50 meters). This allows separation between the survey vessel and potential navigation hazards or interference with GPS. Thus, critical information is delivered for safe and efficient navigation routing, scour inspection around bridges, piers and other structures, as well as accurate accounting for volumes of dredge material, quantification of water resources and rapid environmental assessment of critical shallow-water habitats.

L-3 Klein introduced the predecessor to the HC3500, the HC5000 PDBS system, in 2010 (Sea Technology, June 2011) to address the inefficiencies of lead line methods, single-beam and multibeam technology, high costs of coastal lidar flights and unbridled data output from premature PDBS technology that affected shallow-water hydrographic survey operations. Through extensive testing and evaluation over the past three years, Kleinís HydroChart systems have proven to deliver quality 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 efficiency for surveying.

Meanwhile, the swath coverage of MBES systems typically spans three-to-five-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 fixed number of beams. In shallow waters, PDBS has demonstrated three to five times the efficiency 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 HC3500ís SNR, quality and uncertainty filters, which can be set during real-time data acquisition or post-survey processing.

During early stages of testing and evaluating PDBS in the 1990s, as with most advanced technology, various complicating factors were identified, 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, filtering and time-varying gain (TVG) modification, and formats 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). To continue this article please click here.

Straud Armstrong is technical sales manager for L-3 Communications Klein Associates. He specializes in interferometric sonar applications. He joined Klein after five years with Teledyne Benthos. He earned his masterís in geology and bachelorís in marine sciences from the University of South Carolina. He has a special interest in hydrographic mapping.

Yuhui Ai is a principal systems engineer with L-3 Communications Klein Associates. He specializes in sonar signal processing, underwater acoustics and sonar systems design. He has published more than 20 peer-reviewed papers in these fields.

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