Propagation And Imaging Techniques Of Vortex Acoustic Waves Under Ocean Turbulence

Ocean exploration and target imaging mainly use acoustic waves as information carriers.The application of orbital angular momentum in acoustic waves enables acoustic waves to obtain information capacity from a new dimension.The acoustic wave with orbital angular momentum presents a vortex structure,so it is called Vortex acoustic wave(VAW).When VAW irradiate a target,they form echoes with phase differences,thereby achieving two-dimensional staring imaging.However,VAW in the ocean are affected by turbulence,and studies have shown that weak turbulence in calm waters has a smaller impact on the intensity of VAW and a greater impact on their phase.This article aims to analyze the feasibility of underwater VAW imaging by studying the propagation characteristics and imaging effects of VAW under weak ocean turbulence effects,and to study VAW imaging methods with higher resolution performance.Firstly,the method of generating VAW using Uniform Circular Phased Array(UCPA)was introduced.On this basis,the echo model of VAW imaging based on UCPA and the processing process of Fourier transform imaging are studied.Secondly,COMSOL was used to analyze the propagation of VAW in ideal oceans.Based on the layered characteristics of ocean acoustic velocity,the effects of various parameters on the VAW were studied when there is a acoustic velocity gradient in the propagation domain and when the sound velocity is uniform.Thirdly,based on the Nikishov ocean power spectrum model,an inversion method is used to construct a random phase screen for ocean turbulence,and a propagation model of VAW in ocean turbulence is established.On this basis,the expression for the detection probability of the beam modulation topology mode in the spherical coordinate system was derived,and the effects of each parameter on the wavefront phase of the beam and the detection probability of the modulation topology mode were studied separately.Finally,an imaging model of VAW under ocean turbulence was established based on the ocean turbulence environment,and the influence of ocean turbulence parameters on the imaging performance of VAW was studied.Two imaging methods were proposed for discrete ship targets.One is an imaging method that uses Res-Unet neural network to perform phase correction on vortex echo data.The other is to use a cascaded two-dimensional PGA algorithm to compensate for azimuth phase error and range offset in imaging methods.The effectiveness of the two imaging methods was verified through simulation and comparative analysis.