Research On Bottom Sound Scattering Strength Measurement Method And Reverberation Properties In Irregular Sea Area

There are a lot of scatterers in the ocean, so that when acoustic waves projected into this uneven media, it will have scattering. The sum total of the scattering contributions from all the scatters is called reverberation. Shore near the port in the gulf or in the sea area between the islands is very important. Reverberation in this irregular sea area has important infection by the shore and islands, and at the same time, the gradual deeply wedge shaped bottom also affect the reverberation rule. The irregular sea is the sea area with irregular horizontal and bottom boundary shapes and the complicated boundary conditions. Reverberation intensity in irregular sea has its own rule. The reverberation is one of the main interference factors for active sonar system, and the properties of reverberation in irregular sea area should be comprehended sufficiently when the problem about the detection with active sonar in irregular sea areas is researched. In shallow water, the bottom scattering strength is an important factor for bottom reverberation, and bottom scattering strength is an important parameter for bottom reverberation. With the development of the bistatic sonar system, research on bottom scattering strength with different incident grazing angle, scattered grazing angle and scattered azimuth angles is necessary. So the paper is focused on the investigating measurement method of three-dimensional bottom scattering strength and the properties of irregular sea area reverberation.A method for measured bottom three-dimensional scattering strength is given in the paper which can measure sector scattering area three-dimensional bottom scattering strength. T-receiving array which has narrow horizontal directivity and narrow vertical directivity receives scattering signal from the bottom scattering unit when transmitting transducer which has wide vertical directivity and narrow horizontal directivity transmits CW pulse signal. The three-dimensional scattering strength of the bottom scattering unit could be calculated with the transmission loss which could be got with incident path and scattering path, the source level of transmit acoustic source and the strength of scattering signal. In use of the technique of phase shift beam forming the acoustic axis could be pointed to another bottom scattering unit in scattering area, then the three-dimensional scattering strength of this scattering unit could be calculated. Further more the three-dimensional scattering strength of other units in scattering area would be known. The measurement method proposed in this paper is proved feasible because the result in theory accordance with the result of dealt data of examination on lake well. But from the result of dealt data of sea trial, the bottom scatter is anisotropy. By ray acoustics theory, the irregular sea monostatic reverberation intensity simulation model is established by using Lambert's law to calculate scattering strength of the bottom and lateral wall. The monostatic reverberation intensity of the simulation model is in consistent with the reverberation intensity of the experimental data. And by ray acoustics theory, the incline seafloor bistatic reverberation intensity simulation model is established by using three-dimensional scattering model to calculate bottom scattering strength. The bistatic reverberation intensity of the simulation model is in consistent with the reverberation intensity of the experimental data. The conclusion that the irregular sea area monostatic reverberation intensity is bigger than horizontal seafloor monostatic reverberation intensity and oblique seafloor monostatic reverberation intensity and the decaying speed of the irregular sea area reverberation intensity is faster than horizontal seafloor monostatic reverberation intensity and incline seafloor monostatic reverberation intensity would be got by theoretical calculation. For irregular sea area reverberation, the transmit pulse width and horizontal beam width of source make a big affection. The reverberation increases with the pulse width and beam width. The seafloor tilt angle and the open angle of two lateral walls mainly influence long-distance reverberation. The reverberation intensity decays with time faster with the tilt angle bigger and the open angle smaller. The tilt angle of two sides influences the short-distance reverberation intensity heavily.The simulating model of mulpi-path bottom reverberation signal, developed by means of the generating process of the bottom reverberation, based on the Ray theory, using the element scattering model, has been built. It plots out the scattering element by the bottom scattering coefficient correlation radius and it has a very definite physics sense. It can be concluded from the simulating reverberation signals that the instantaneous amplitude is submitted to Gauss distribution and the instantaneous phase is submitted to uniform distribution between (0,2Ï€). The comparison result of simulating reverberation signals of irregular sea area with experimental reverberation signals shows that vertical spatial correlation is stronger than horizontal spatial correlation. What is more, on the horizontal direction the spatial correlation of upright to the two lateral walls is stronger than the parallel's. Both vertical spatial correlation and horizontal spatial correlation decrease with reverberation signal frequency increase and the vertical spatial correlation changes more distinctness than horizontal.