Test and Evaluation of Interferometric Sonar Technology

Sea Technology, Mar 2007 by Gostnell, Caleb, Yoos, L T

Integrating Phase Differencing Bathymetric Sonar into Suite of Tools Used for Nautical Charting Hydrographic Surveying

The National Oceanic and Atmospheric Adminislnition (NOAA) spends a large portion of its overall nautical charting hydrographie survey effort obtaining bathymetrie data in waters shoaler than 21) meters. Not only does it take more time and effort to survey in these depths, but these regions are also frequenily the niosi dangerous areas in which surveys are conducted. In the near-shore waters of Alaska, both visible and submerged rocks are prevalent, and currents can be strong. In the shallow and turbid waters of the Gulf of Mexico, submerged pipeline terminations and other obstructions rising to within a tew feet of the surface are common. Waters of these depths are considered navigational!}' significant and must he surveyed in an accurate and methodical fashion.

While multi-beam echosounders (MBHS) are known to provide very accurate bathymetrie information and are used throughout many of the world's hydrographie offices, the data acquisition capability is typically limited Io three to five times the water depth when usine a standard single head system. This does not become a major limiting factor until working in waters shoaler than IO to 15m where it can become difficult to efficiently attain full bottom coverage. In many of these areas, water turbidity or resolution requirements preclude the use of lidar. and there are few alternatives lor obtaining bathymetry in an efficient manner.

Interferometric sonar systems are one tool thai may be capable ol" significantly improving the safety and efficieney of hydrographie survey operations in shoal waters. Interferometers, also referred to as phase differeneing bathymetrie sonar I PDBS) systems, provide high-resolution, wide-swath bathymetry in shallow water with swaths of IO lo 15 times the instrument altitude -a significant improvement over MBEvS in similar depth waters.

While the bathymetrie data from phase differencing sonar systems has been historically of suspect quality, recent advances in electronics and phase deconvolution techniques and algorithms have markedly improved their precision and reliability. These improvements. combined with NOAA's ongoing conversion to surface-based nautical charting hydrographic survey deliverables. make the use of PDBS a potentially beneficial tool for NOAA's nautical charting survey program.

Methods:

The goal of these tests was to ascertain me current state of interferometric technology to determine if it would he advisable al lhis time for NOAA's Office of Coast Survey to integrate PDBS into the suite of tools used to acquire nautical charting hydrographie survey data.

Data were acquired with MBES, SSS and each of three commercially available PDBS systems over a period of four weeks during the summer of 2005. All data were acquired aboard the NOAA S/V Bay Hydrographcr in and around lhe mouth of the Patuxent River in Chesapeake Bay. Md. Four study sites were developed Io test specific capabilities of PDBS systems: target detection, shallow slope resolution, vertical feature resolution and potential efficiency gains.

MBES data were fully processed within Caris HIPS/SIPS software package, while PDBS data were processed within each vendor's proprietary or recommended software package. All data had vessel motion, sound speed and water-level correctors applied. Data were then imported into IVS 3D's Redermaus data visualization package using similar conversion parameters for comparison and evaluation. All grids were created at one meter resolution using a weighted moving average and a weight diameter of three. PDBS data were then compared lo MBES and SSS data covering similar regions and features.

Results: Target Detection

Although all targets were resolved by both MBES and PDBS. the largest target, with a known height of 1.04 meters, was used for analysis. After binning, the target had a vertical presence of 0.34 meters in the MBES data and O.18 meters in the PDBS data. While a portion of this height difference is due to smearing during binning, the point data from the PDBS did not tend to provide as dense a sampling of soundings on the targets as MBES did. and those soundings did not necessarily represent the shoal depths. While there were differenees in the heights of the targets between datasets. other small features, sueh as 0.2 meters amplitude oyster beds, were similarly modeled by both technologies, indicating that at least a portion of the problem may reside in the data filtering techniques applied and not necessarily with the data or technology itself. Over relatively flat areas, large objects, and areas of more gradual or consistent change this did not seem to be an issue and in general the surfaces varied little between MBHS and PDBS.

Resolving Slopes

PDBS technology appears quite adept at yielding quality data on sloped surfaces, the wide swath of bathymetry enabled significant additional dala Io he acquired along the representative slope than was feasible with MBHS. On the shoreward most line the PDBS provided, an inshore swath four to six limes as wide as that yielded from the MBRS. The result was that the PDBS data extended 20 meters laterally hcyond the MBIiS data. Addiliunally. the PDBS systems provided data to two meters water depth, a significant improvement over the MBES system. As the sonar head was mounted approximately two meters below the surface, it is anticipated that bringing the head closer to the surface would yield additional data.