Theory And Application Of Radar Imaging Of Shallow Water Bathymetry

The significance of coastal bathymetry and its changes cannot be overemphasized for the security of shipping,military,fishing,and other coastal and offshore operations.With increasing environmental pressures due to climate and population factors,the coastal geomorphological evolution in the future remains more uncertain and should be better understood.Hence,scientists and coastal managers need accurate bathymetric data to examine relevant theories and establish effective strategies.Compared with traditional shipboard in-situ instruments,remote-sensing techniques have the ability to sample a wide spatial and temporal range.SAR(synthetic aperture radar)sensors may be potentially a superb choice for monitoring coastal morphological changes because of their capability of observing the sea surface roughness under all-weather and day and night conditions.It is well known that radar sensors are capable of imaging of varying sea surface roughness due to divergent and convergent current flow over underwater sand waves.This manifestation can be simulated and explored through a flow model combined with a radar imaging model by explaining the interaction between bathymetry,currents and surface waves.This study focuses on 2-dimensional(2-d)radar imaging of bathymetric features in the shallow water using in-situ data,SAR observations and model simulations through a forward radar imaging system of underwater topography that integrates shallow water coastal models and the Radar Imaging Model(RIM)in the Yangtze Estuary.Complicated bright and dark contrasts across the current directions captured by SAR images due to complicated 2-d hydrodynamic process in the Yangtze Estuary were observed and analyzed.To further test the capacity of the radar imaging system,the impacts of bathymetry,wind fields and some radar paramters on simulated sea surface roughness were explored by maintaining some of the factors.Furthermore,the author designed an idealized model to examine the role of grid sizes on radar imaging of underwater sand ridges with distinct scales.The main contributions are as follows:(1)A fully 2-d radar imaging system of shallow water bathymetry in the Yangtze Estuary was set up for real-time simulating and capturing complicated 2-d bottom features.The bright-dark-bright features observed across the current direction in the SAR images cannot be explained by the 1-d radar imaging theory;and the 1-d flow model or radar imaging model restricts the performance of the radar imaging system to explain the complex bottom feature.In this study,2-d flow models were applied in the Yangtze Estuary to real-time simulate the complicated hydrodynamic processes and varying current fields due to changes in water depth in the estuarine area.A comparison of our results with direct in-situ current observations showed that the 2-D current patterns were adequately reproduced.The convergence and divergence of the current vector fields were calculated,and the results revealed the distinct presence of convergence and divergence bands in areas where the SAR observations indicated clear 2-d NRCS anomalies.By inputting the wind fields and known radar parameters,the simulated SAR images were consistent with the SAR measurements,which indicates the potential usefulness of the real-time radar imaging model of bathymetry.This provides the foundation of this study to explore the sensitivities of the imaging model to different parameters.(2)The main factors that can influence on the radar imaging system was quantitatively analyzed by maintaining some of the factors.Based on the radar imaging model,the impact of the water depth,wind speed,wind direction,radar look direction and some radar parameters on simulated radar backscatters were simulated and compared.The results showed that sea surface roughness is mainly affected by changes in the water slope rather than variations in the water depth.And relative low wind speeds for cross-wind radar look direction with up-wind or down-wind currents are more applicable for radar imaging of shallow water bathymetry.(3)The wave-breaking factor should be introduced into the SAR imaging of sea bottom topography mechanisms.By inputting the same current fields,we simulated and compared the results along the steep bottom slope of the underwater topography by different radar imaging models.Compared with the measured SAR data,simulated radar backscatters were underestimated by the two-scale model,while the RIM model can well present both the phase and the magnitude of the NRCSs,especially at large incidence angles.(4)The influence of grid resolution on the simulated SAR image was explored and analyzed.An idealized numerical model was set up in order to reduce the effect from non-bottom factors in a realistic environment to simulate and analyze the impact of grid resolution on radar imaging of shallow water bathymetry with respect to different scales of underwater sand waves.For small scale sand waves(<100 m)with grid spacing of 10 m in the imaging model,the results showed that the simulated NRCS variation can reach 0.43 dB,indicating that high-resolution SAR images are possible to detect bottom features on a small scale.Under same conditions of underwater topography,variations of sea surface roughness become larger as the increasing grid resolution.Compared to results with relative high resolution,the simulation results of low resolution cannot well present the shape and accurate position of the bottom feature.As the increasing grid resolution,simulated radar backscatter variations can gradually reach a stable phase,indicating that the grid spacing should be selected according to the ratio between the size of sand waves and model grids to fully present shallow water-induced changes in radar backscatters in the radar imaging theory of bottom topography features.Under more complicated real conditions,the grid resolution in the radar imaging system should also be selected according to the complexity of underwater topography and the resolution of the SAR image.