Research On Several Key Issues On The Processing Of Multibeam Backscatter Data

A multibeam echosounder,with its ability to simultaneously obtain bathymetry and backscatter information,has become an attractive choice for seafloor mapping.Acoustic seafloor classification from multibeam backscatter measurements is an attractive approach for mapping seafloor properties over a large area.Unfortunately,due to the complexity of the marine environment and the sonar system itself,some errors and noises are often present in multibeam backscatter data,which will have an important influence on the seafloor classification to use multibeam backscatter image inevitably.Among them,the errors caused by seabed topographic relief and specular echoes are the most significant.The former will cause inaccurate calculation of the beam incidence angle and the insonification area,while the latter will cause the nadir striped artefacts,which are extremely difficult to be removed.To this end,the calculating formula of the three-dimensional incidence angle and the insonification area are derived first.And then this thesis focuses on the method for removing the striped artefacts.The main work is as follows:(1)The calculation model of three-dimensional incidence angle is constructed based on the previous research.According to the multibeam point cloud positoning model and submarine DEM,the mounting angles of the transducer,vessel attitude,the sound ray bending,and submarine topography are considered at the same time to calculate the incidence angle in detail.This formula has the advantages of the high precision of calculation and the easiness of programming.(2)The calculation model of the insonification area is constructed,with the local inclination of the seabed.The uncertainty factors influencing the insonification area are first be analyzed.And then the computational formula for the insonification area is derived,based on the analysis of the influence of bottom angle on beam footprint area and sound pulse radiation area,respectively.Compared with the calculated area of the flat bottom hypothesis,the former is more in line with the actual fluctuation of the seabed.(3)A new method for the removal of multibeam backscatter artefacts in near-nadir region based on equal mean-variance fitting model is presented.The influencing factors and performance of this type of artifacts are first analyzed systematically.The reason for this approach is that the probability distributions of the backscatter intensities in the near-nadir region and the off-nadir regions should be similar,considering both the gradual change characteristics and the similar environmental factors.By fitting the local shape of the angular response curve,the striped artefacts are compressed and moved according to the relations between the mean and variance in the near-nadir and off-nadir region.The algorithm utilized the measure data of near-nadir region and retained the basic shape of response curve.The experimental results verify the high performance of the proposed method.