The Research Of Underwater Acoustic Daylight Imaging Depend On Noise By Rainfall

Acoustic imaging brings more visualized performance to detection.There are three main methods of acoustic imaging include active,passive,and in use of ambient noise.More precise shape and position information of target can be gotten in active imaging.Noise source could be localized in passive imaging.While imaging the target in the use of ambient noise has advantages such as covert,which can detect quiet targets.In this paper,several major physical processes of imaging with ambient noise is studied,the followings are principally included.1.Research of rain noiseThe noise sources distributed near the surface are dipole and satisfy the ray propagation model by comparison with several marine environment noise models for application in the problem imaging with ambient noise.Raindrops noise is dipole directivity and similar to ambient sea noise in frequency.The formation mechanism of and characteristics of the rain noise is studied by analyzing the sound signal of water droplets.The amplitude distribution of the initial impact sound and the un-impact sound,the distribution of waveform type,as well as the spectrum of various waveforms are obtained by detailed statistics and analysis on signals of water droplets.Time and number of un-impact sound appears are also statistic detailed.Acoustic signals of water droplets are simulated based on statistical data.Comparison for waveforms and spectrum of multiple droplets sound in experimental indicate the accuracy of the acoustic signal in simulation.2.Caculation of the scattered fieldThe principles and calculation process of several scattering algorithm are discussed detailed in the paper.Scattered field of spherical target in spherical wave is calculated in use of rigorous theoretical solution,and the scattered field of rigid sphere under distributed sources on surface is obtained according the result.The scattered field of spherical target in spherical wave and cylindrical target in a parallel plane wave are detailed calculated in use of wave superposition method,which is accuracy verified by compare with rigorous theoretical solution.A conclusion is reached that the indentation rate and the number of virtual point sources could affect the accuracy of wave superposition method.The scattered field of rigid sphere and cylinder are calculated according to Kirchhoff approximation,as well as the stabilization phase is considered in the calculation.Two cases are included in the results which are monostatic and bistatic.The acoustic field of the smooth surface in bistatic is calculated,also.The highlight model is developed to bistatic according to the calculation results and verified by experiments about scattered field of column.3.Design and simulation of acoustic imaging systemImaging system is designed according to the principle analogy imaging in light,parameters of the system were calculated based on the indexes.The formula is obtained from derivation of sound field near reflector focus area in the point source and parallel light.The experimental system performance is analyzed according to the formula which includes the intensity distribution at x-y and x-z plane.Differences between different frequencies point sources are shown in figures.The image of point source which deviates from the axis is calculated.The resolution of multi-point sources is in consider.With low frequency,the system could be scaled to get high resolution.The spherical target and the plate target are imaged in simulation with principle of mirror reflect.The spherical target is imaged in simulation as radiation of multi-point sources according to the wave superposition method.By instead of the plane surface with spherical surface,the imaging surface is improved which is verified in the simulation that the point sources with a large off-axis angle is visible on the imaging surface,and the off-axis aberration is reduced.4.Acoustic imaging experiments are implementedInstead of using plane array,a rotary cross array is placed at the focal point of the reflector.Performance of imaging system is tested with imaging of source.Plate target and triangle target located between the source and the reflector are imaged in experiments.The effectiveness of the imaging system under the active source imaging performance is shown.The presence of a target can bring difference in strength of image which is verified by experiment.