| Most of the energy of very low frequency underwater sound in shallow water will be transmitted into the seabed and propagate to the distance in the form of seabed seismic wave.The seabed seismic wave has fast propagation speed,slow attenuation and contains ship information,which can be used for underwater target detection.In this context,this paper studies the following aspects:(1)According to the characteristic equation of Scholte wave and the displacement formula of particle,the dispersion characteristics of Scholte wave velocity in different shallow sea environments,the displacement and trajectory of particle are calculated by numerical simulation.The results show that the smaller the depth of seawater layer and the harder the seabed medium,the wider the dispersion interval of Scholte wave velocity.The horizontal displacement of the particles in the seawater layer and the seabed layer at the interface is discontinuous,and the vertical displacement is continuous.The vibration trajectory of the particle in the seawater layer excited by the Scholte wave is a forward flat ellipse.After a certain distance from the interface,the trajectory becomes a forward vertical ellipse.The particle vibration trajectory in the seabed excited by the Scholte wave is a forward vertical ellipse.At a certain distance from the interface in the seabed,the particle vibration trajectory is up and down,and then the particle vibration trajectory in the seabed is a reverse vertical ellipse.(2)Based on the finite element analysis software COMSOL,the acoustic-vibration propagation characteristics of point source in shallow water environment with sound velocity gradient are studied,which has potential application value for the development of very low frequency acoustic-ground joint detection technology for shallow water targets.The results show that the sound velocity gradient in the Bohai Sea in winter has little effect on the underwater acoustic and seabed seismic waves at the interface.With the increase of the sound source depth,the underwater acoustic propagation loss at the interface and the displacement propagation loss of the seismic wave decrease,and the increase of the sound source frequency has a similar law.The seismic wave at the interface of sediment seabed is mainly horizontal displacement,and the seismic wave at the interface of basalt seabed is mainly vertical displacement.When the seabed medium is layered,the underwater acoustic propagation loss at the seawater-seabed interface and the displacement propagation loss of seismic waves are reduced.Channels that are not conducive to underwater acoustic propagation appear in the seawater layer,and surface waves exist at the interface between the layers in the seabed.(3)The cylindrical shell is the main structure of the underwater vehicle.Studying its acoustic and vibration propagation characteristics has certain guiding significance for the design and development of the underwater vehicle.Based on the COMSOL,the modal of the cylindrical shell and the underwater acoustic and seabed seismic waves radiated by the cylindrical shell under low frequency excitation are analyzed.The results show that the modal shape of the cylindrical shell is the extrusion and tension on the two ends or sides;the modal frequency increases with the increase of the thickness,the decrease of the radius and the decrease of the length of the cylindrical shell.The modal frequency decreases when the cylindrical shell is underwater.The seabed seismic wave excited by cylindrical shell vibration is mainly vertical vibration.At the interface,the change of the length of the cylindrical shell mainly affects the underwater acoustic and seabed seismic waves in the direction of the end face,and the change of the radius of the cylindrical shell mainly affects the underwater acoustic and seabed seismic waves in the side direction.The increase of the submerged depth of the cylindrical shell and the increase of the excitation force will enhance the underwater acoustic and seabed seismic waves at the interface.The frequency of the excitation force changes,and the sound pressure directivity at the interface and the displacement directivity of the seismic wave change significantly. |
