Research Of The Bathymetry Inversion Method In Shallow Water With X-band Radar

The masters of the changes of the topography of shallow water in coastal areashave a significant impact on the near-shore activities. So researchers carry out a seriesof studies on the method to obtain bathymetry. Traditional methods obtaining depthinclude shipboard sonar detection, airborne laser detection, submersible vehicle probeand hyper-spectral image measurement. However, they are all high-cost andlow-efficient. What’s more, the result accuracy of some methods is affected by theclarity of the water. In order to overcome the shortcomings of traditional methods, theinversion method based on the X-band radar has been developed. In this paper, theinversion algorithm of the shallow water bathymetry and the sea surface currentvelocity is discussed based on the X-band radar image sequence.In shallow water, the local variations of bottom topography and the surface currentinfluence the propagation of the surface waves. The local bathymetry and currentvelocity can be inferred based on the changes of the propagation. Because the actual seacondition is complicated and the compared data are not accessible the simulated radarimage is used in this paper. At first, time-series of wave images are simulated based onthe linear wave theory. During the process of simulation, the water depth and currentvelocity is input. If the input value of the depth is equivalent, the simulated wave field ishomogeneous. If the input value of the depth is non-equivalent, the simulated wavefield is inhomogeneous. Assuming that the scattering cross section is first-orderapproximate to the wave height, the radar image sequence is simulated according to theimaging mechanism. The main considered modulation mechanism is the tiltmodulation and the shadowing modulation.Then for the homogenous wave field, the inversion algorithm of the depth andcurrent velocity is introduced and the inversion result is the average value of theselected region. The algorithm is based on the analysis of the three-dimensionalfrequency spectrum. In theory, the spectrum should be gathered near the dispersion relation surface. To make the spectrum fit the dispersion relation, the water depth andcurrent velocity can be extracted. In order to improve the inversion accuracy, themethod of choosing the spectrum coordinate sets is introduced. The inferred waterdepth and the input value are compared and21sets of data are in good agreement. Thiswork makes a good fundamental job for the inhomogeneous wave field.Then for the inhomogeneous, the inversion algorithm of the water depth andcurrent velocity is introduced. Assuming that the local region is homogeneous, the keyof the algorithm is how to get the three-dimensional spectrum of the local region. Thispaper adopts the method of frequency decomposition and directional decomposition.The local depth and velocity is obtained by analyzing the local spectrum. And finallythe spatial distribution of water depth and velocity of the whole wave field is obtained.The comparison of the inferred result and the input value are in good agreement. Thisverify the feasibility and effectiveness of the algorithm.The last part introduces the factors that should be considered during the realexperiment, including the weak nonlinearity of the waves, the nonlinearity of radarimaging mechanism, aliasing effect and the amplitude dispersion effect in shallowwater. Detection of the waves based on the shipboard X-band radar is briefly introducedas well.