The calibration of shallow water multibeam echo-sounding systems
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Multibeam echo-sounders have been developed to gather bathymetric and acoustic data for more efficient and more exact mapping of the oceans. This gain in efficiency does not come without drawbacks. Indeed, the finer the resolution of remote sensing instruments, the harder they are to calibrate. This is the case for multibeam echo-sounding systems (MBES). We are no longer dealing with sounding lines where the bathymetry must be interpolated between them to engender consistent representations of the seafloor. We now need to match together strips (swaths) of totally ensonified seabed. As a consequence, misalignment and time lag problems emerge as artifacts in the bathymetry from adjacent or overlapping swaths, particularly when operating in shallow water. More importantly, one must still verify that bathymetric data meet the accuracy requirements.
This Master of Engineering report summarizes the system integration involved with MBES and identifies the various sources of error pertaining to shallow water surveys (100m and less). A systematic method for the calibration of shallow water MBES is proposed and presented as a set of field procedures. The procedures aim at detecting, quantifying and correcting systematic instrumental and installation errors. Hence, calibrating for variations of the speed of sound in the water column, which is natural in origin, is not addressed in this document.
Calibration data, acquired by the Canadian Hydrographic Service using Simrad EM1000 and EM100 multibeam echo-sounders, have been used to demonstrate the proposed techniques. The method uses a post-processing system that allows the visualization of swaths in plan views and cross-sections, and requires the ability to perform digital manipulation of datasets and statistical analysis. To this end, the HIPS post-processing package (Universal System Ltd) has been used.
Hydrographers using MBES must still certify the acquired bathymetric data comply with their national accuracy standards or with the ones defined by the International Hydrographic Organization (IHO). With an adequate calibration, hydrographers will ensure the sounding system operates without the loss of accuracy that results from systematic biases. When the sound speed profile of the water column is well known, accurate and consistent depth measurements will be obtained. A post-calibration quality assessment procedure, herein called the ”performance check”, is presented. The procedure employs the existing “Reference Surface Methodology”, developed by the Ocean Mapping Group of the University fo New Brunswick, along with other statistical tools to assess the overall accuracy of the calibrated system. The output of the performance check can be used to certify that the data meet accuracy requirements.