Research On SAR Remote Sensing Imaging Mechanism And Inversion Of Typical Shallow Water Topography

Shallow water bathymetric data is an important foundation for scientific research and coastal management.Analysis of coastal topography and coastal zone environmental evolution is mostly based on shipboard fathoming methods,lacking extensive real-time data updates,especially for beach where the water depth is relatively shallow and the slope is relatively small.Bathymetric data gap still exists.Synthetic Aperture Radar(SAR)has become an emerging technology for detecting shallow water topography and monitoring coastal morphological changes,and can compensates the insufficiency of traditional bathymetry methods to a certain extent with its unique advantages.This thesis focuses on radar imaging mechanism and water depth inversion of shallow water topography using numerical simulation method and SAR remote sensing technology.We implemented a 3-dimensional(3-D)hydrodynamic model to simulated current filed influenced by two typical samples of shallow water topography.Then,a fully 2-dimensional(2-D)SAR remote sensing imaging model was developed by integrating the 3-D hydrodynamic model and a 2-D remote sensing radar model to analyze the image features and SAR imaging process of two typical shallow water topography,Lingding Channel in the Pearl River Estuary and Subei radial sandbank in South Yellow Sea.On this foundation,we selected Subei Shoal,a typical shallow sea area where sand waves and tidal inlets develop well,as the main study area and developed a 2-D inversion scheme suitable for this area.Using SAR images we completed the research on depth inversion and topography variation analysis in some typical areas of Subei Shoal.The main research contents and the results are as follows:(1)Based on previous researches,we selected sand wave(positive topography)and channel(negative topography)as two typical samples to discuss the interaction between underwater topography and current field.In order to simulate the current,we implemented a 3-D hydrodynamic model to analyze the distribution features of current velocity components,surface current and surface current gradient in different current fields and had a further understanding of SAR imgaging mechanism of underwater topography.(2)A fully 2-D SAR remote sensing imaging model of shallow water topography in the Pearl River Estuary was developed by adopting a 3-D hydrodynamic model and a toolkit for microwave scattering imaging oceanic surface(M4S).The 3-D hydrodynamic model was applied to real time simulate the temporal variations in the current due to the channel topography.Then,we simulated the SAR image based on the imaging model and compared it with the actual SAR image and the bathymetric data of this area.The results showed that the simulated SAR image was consistent with the actual one,indicating that the SAR remote sensing imaging model has the capability to image 2-D underwater topography under real environment conditions.It also laid the groundwork for the follow-up imaging simulation of Subei Shoal.(3)A similar SAR remote sensing imaging model was set up for Subei Shoal,which can real-time simulated the current filed and SAR images of typical underwater topography(sand waves and tidal inlets)in Subei Shoal.Two factors that can affect the SAR imaging of underwater topography were analyzed.Based on the SAR remote sensing imaging model,the influence of water depth and topographic slope on SAR image simulation were compared.The results confirmed that the radar cross-section is mainly modulated by variations in the topographic slope rather than changes in the overall water depth.(4)Taking into account the priori knowledge of Subei Shoal,we obtained the best fitting profile of the typical underwater topography in this area which contains two sand waves and a tidal inlet.Base on this profile function and the SAR remote sensing imaging model,we developed a 2-D underwater topography inversion method.Then,some exploratory researches were carried out to assess the 2-D water depth in Taipingsha area using two SAR images.(5)Using the measuring method of topography locations in SAR images,we analyzed the over-time migration of underwater topography in two typical regions of Subei Shoal by using time series remote sensing data.The results showed that the migration directions of these two regions are different,but the variation features represents a stable balance at a long time scale and a large space scale.