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Phase Differencing Bathymetric Sonar
Phase Differencing Bathymetric Sonar (PDBS) systems, also referred to as interferometric sonar, use the measurement of phase at each of several receive elements to determine the angle from which the acoustic return originates (see Figure 1).

 

 Black and white picture showing how phase is measured at each of several receive elements, which are spaced at precisely known distances.  Additional pictures showing how the angle from which the return originated is calculated using differences in the phase as measured at each element.  

Figure 1 – Phase is measured at each of several receive elements which are spaced at precisely known distances. The angle from which the return originated is calculated using differences in the phase as measured at each element (graphic from Hiller, T.M., and K. Lewis. 2004. Getting the Most out of High Resolution Wide Swath Sonar Data. Proceedings of the 14th International Symposium of the Hydrographic Society. The Hydrographic Society Special Publication Number 53, Paper 8)

 

Once the phase of the acoustic return has been precisely measured, the differences between the phase at each of the receive elements are used to calculate the angle (θ) from which the return originated using a technique similar to that shown below:

θ = αn – αn+1
where α = atan (I/Q)
I = phase1 and Q = quadrature1
and n = interferometric receiver element

This angle of origin, in combination with range calculated from the two way travel time, provides a discrete location on the seafloor. This sampling is done thousands of times per acoustic ping, providing a cross-track profile of bathymetry. A number of consecutive profiles are then combined to build a three dimensional model of the seafloor.

Currently, the Office of Coast Survey (OCS), in collaboration with the University of New Hampshire (UNH), is studying several commercially available PDBS systems and has recently procured a PDBS system for operational testing. The studies indicate that they provide high quality data, while significantly increasing efficiency and safety of operations in near shore and very shallow water areas. The gains in efficiency and safety are attributable to the wider swath width that interferometric sonars produce relative to shallow water multibeam systems in areas shoaler than ~20m. This increased swath width appears to enable hydrographers to survey more than twice the shallow water area per given time and allow vessel operators to stand further off of hazardous features and the shoreline while acquiring a denser dataset than is possible with technologies in use today (Figure 2).

   Figure 2a) Color 3-D picture and the line transect showing safe limit (i.e. depths) of near-shore data collection using a shallow water multibeam system.

Figure 2a) Data acquired with shallow water multibeam while running a line as close to shore as safely feasilble.

 

   Figure 2b) Color 3-D picture and line transect showing data collected using tan interferometric sonar system running the same line as shown in Figure 2a.
Figure 2b) Data acquired with PDBS sonar running same line as in Figure 2a.  The PDBS system achieved a 20 m greater lateral inshore coverage providing a much more complete representation of the slope.  This enables the surveyor to either stand further off from dangerous features while obtaining the same amount of data or acquire coverage over a larger portion of shallow water areas and features.   Both systems were mounted at approximately 2m below the surface.  The artifacts present in the PDBS data are inherent to the technology which yields the lowest data density at nadir, the artifacts are on the order of 10cm in amplitude.
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