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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS016 Oral Presentation 2340
A New Method for the Mosaic of Multibeam Bathymetry Data
Mr. Fanlin Yang
Key Laboratory of Foundational Geo-information Shandong University of Science &
Technology IASPEI
Cuiyu Sun, Xiangwei Zhao, Xinzhou Wang
Multibeam sonar system (MBES) usually has a wide swath. Refraction artifacts and Roll residuals are
usually presented in multibeam survey and more obvious in flat seafloor and shallow water
environment. They are very serious for the edge beams of swath and very trivial at nadir, so the mosaic
for neighboring swaths is difficult because of these errors. Roll residuals are obvious on orthogonal
plane, so they can be solved easily. However, in some situations, especially in the estuary, sound speed
changes rapidly. SSP can not be accurately obtained while surveying. Sound refraction artifacts can
seriously degrade the quality of final result. A postprocessing method for the removal of roll system
residuals and sound refraction artifacts must be researched as early as possible. A new method for
solving this problem is presented by this paper. Because of the limit of mounting method, the
transducer can not be placed in ideal position. A patch test may not solve this problem perfectly, too. If
some swath data in a flat seafloor are projected along orthogonal direction, the shape of data on
projection plane likes V. When we change the roll parameter, the shape of artifacts will vary as roll
parameter modulates. When the V is disappeared, this roll parameter in this time is that we need. Then
sound refraction is removed by one kind of postprocessing method based on equivalent SSP theory.
According to former research, a real SSP can be displaced by a new SSP under the some prerequisites.
We present a new SSP model. It has three water layers. The two upper layers are constant sound speed
gradient layers, and the third layer has same sound speed gradient and sound speed with the deepest
layer of measured SSP. The unknown parameters include surficial sound speed c0, the sound speed c1
in first and second layer interface, the bottom depth z1 of first layer and the bottom depth z2 of second
layer. In order to simplify the algorithm and retain the basic shape of raw profile of each ping, we
correct the raw date on the basis of their raw positions and depths. We assume that the original water
column only has one layer and the sound speed is 1500m/s. Travel time and arrival angle of each beam
can be recalculated according to the raw accrosstrack distance and depth, instead of raw record. So c0
is 1500m/s. The total depths of first and second layer are known. If we assume that the two upper
layers have the same thickness, the unknown parameter will only leave one, i.e. c1. Then a search
algorithm is adopted to obtain the sound speed c1 in first and second layer interface. The spatial
position of each beam will be recalculated according to the new SSP. To avoid a local optimization, the
depth at nadir must control the final result. In order to limit the impact of terrain, multibeam data must
be divided into many sub-regions. Each ping in sub-regions is projected along track direction. However,
a discontinuity will occur at the edges of sub-regions. In order to solve this problem, the parameters of
each sub-region will be only owned by the central ping. All other pings will be calculated through the
interpolation between sub-regions. Finally, the redundant beams for two neighboring swaths are
removed. After all procedures have been finished, the fine mosaic can be obtained. The method is
verified by the simulated and real data and has been applied in processing a large area data.
Keywords: multibeam sonar, roll residual, refraction