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Calibration and quality control of multibeam hydrographic surveys; some case studies

Calibration and quality control of multibeam hydrographic surveys; some case studies

Hydrographic Journal 124(Pages 11-19

Calibration and verification of multibeam echo sounder systems can become a bottleneck as the accuracy of the positioning methods is being improved and the resolution of the measured seafloor model gets steadily higher. Therefore, in order to utilise the potential precision of such systems today, their verification should be offered attention. Some case studies will demonstrate how the correlation between the calibration parameters and how the relief of the seafloor, influences the precision of field calibration. In shallow water areas the precision of the calibration parameters can be sensitive to errors in the eccentricity data for the transducer. Therefore, if a multibeam system is calibrated in shallow water, it should not be used in deeper water areas without verifying the calibration. Some case studies demonstrate how the eccentricity data of the transducer effects the calibration parameters. Field calibration of multibeam echo sounders usually utilises the relation between horizontal movement and the corresponding change of the seafloor depth. This calibration methodology requires seafloor with some relief if all the static offset parameters of the transducer are to be computed. In the North Sea, for example, there are huge flat areas and calibration in these areas seems difficult, but some case studies demonstrate how pockmarks, i.e. depressions in the seafloor, can make satisfactory relief for the computation of the static offset parameters. In dual head multibeam systems the transducers are tilted so that the beams cover the one half side of the sector usually measured in single head systems. This asymmetry makes that high correlation between roll offset and errors in the measured sound speed can occur. The quality of multibeam measurements can be controlled from crossing lines, i.e. some extra survey lines across the mapped area can be utilised in quality evaluations. An application of this method to a one week mapping project in the North Sea demonstrates how corrections for the tidal water can be controlled and how the precision of the calibration parameters can be derived. However, in order to run this kind of quality assessment in an efficient way, the required computations should be offered in an automated procedure. The auto-calibration tool (SeaCal) in the data acquisition system SIS from Kongsberg Maritime AS is used in the present investigations. SeaCal, which is protected by patents, is developed by Norwegian Defence Research Establishment (FFI).

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Accession: 030391898

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