Research On Underwater Acoustic Corner Reflector Properties

Underwater corner reflector is an underwater concave target including multiplescattering wave, its computation method and the properties of the acoustic scattering fieldare important.The subjects investigated in this dissertation are elastic scattering ofsubmerged elastic plate, computation method of the acoustic scattering field of concavetargets, properties of the acoustic scattering field of corner reflector, method of simulatingseafloor reverberation, and properties of successive pulses reverberation.The frequency dispersion equation of Lamb wave is derived for submerged elasticplate, and the phase velocity and group velocity dispersion curves are obtained by numericalcalculation method. It is found that the phase velocity is greater or less than the groupvelocity at different frequency-thickness products. The energy propagation speed of wave isgroup velocity, so the arrival time of elastic scattering wave is determined by group velocityof Lamb wave in plate and the speed of sound in water. Experimental results show thatelastic scattering wave is ahead of or behind the edge wave in echoes of elastic steel plate.The experiment results confirm validity of the theoretical analysis results.A shooting and bouncing beams method is proposed, which is able to predict theacoustic scattering field when the sound beam is reflected on the concave target surfacemore than once. Incident sound field is divided some sound beams. The direction andtransmission loss of every sound beam is calculated based on geometrical acoustic method,and the acoustic scattering field of the last reflection beam’s planar element is calculatedbased on physical acoustic method. The superposition of all beam’s scattering field is thetarget’s scattering field. As a high-frequency approximate numerical method, the shootingand bouncing beams method can be used to calculated acoustic scattering field includingmultiple scattering. The comparison with both the results of calculation and experiment hasbeen found satisfactory, thus confirming the validity of the proposed method.The properties of corner reflector backward acoustic scattering are studied. Targetstrength of dihedral corner reflector, square trihedral corner reflector, circular trihedralcorner reflector, triangular trihedral corner reflector, and with angle error corner reflectorare computed by the "shooting and bouncing acoustic beams" method. It is found thatcorner reflector has high target strength at wide target azimuth angle, and the change chartof corner reflector target strength with target azimuth angle is quickly changed with theincrease of the angle error, and the range of high backward wave in one octant of circular trihedral corner reflector is wider than square or triangular trihedral corner reflector’s. Thenacoustic-path is calculated, it is found multi-scattering echoes of corner reflector can bedescribed by highlight model. The highlight position of dihedral corner reflector is thevertex of dihedral angle, and the highlight position of trihedral corner reflector is the vertexof trihedral angle. Compared the acoustic backscattering fields of corner reflector withdifferent frequency, we find that the change chart of corner reflector normalized targetstrength with target azimuth angle remain unchanged when ka>60.Taking the seafloor reverberation as a studied object, a method of simulatingreverberation is proposed based on time-space discretization to increase the computingspeed. The reverberation is the linear response of scatters to the emitting signal, and can bedescribed by a linear network to obtain the impulse response function. By the timediscretization of the impulse response function and regional division of the seafloor, thesummation of scatters is translated into the total number of time slice, and the computingspeed is increased. The results of numerical simulation show that the proposed method caneffectively reduce the computational complexity, and the performance is well in agreementwith the theoretical analysis.The data of bottom target detection show that there are non-correlated componentsbetween successive pulses reverberation. Combined with the spectrum of reverberation,some influencing factors are analyzed. According to the law of large numbers, an integratedcancellation method for suppressing the non-correlated components is proposed. The resultsof lake experiment show that the non-correlated components are suppressed, and theuniform background of reverberation is usually conducive to target signal detection.